Download Seasonal and Interannual Variability of Pelagic Zooplankton

Seasonal and Interannual Variability of Pelagic Zooplankton Community
Structure and Secondary Production in Lake Bosumtwi Impact Crater,
Ghana
By
Peter Osam Sanful (B.Sc)
A Thesis submitted to the Department of Theoretical and Applied Biology, Kwame
Nkrumah University of Science and Technology in partial fulfillment of the
requirements for the award of
DOCTOR OF PHILOSOPHY
Faculty of Biosciences
College of Science
© 2008 Department of Theoretical and Applied Biology
DECLARATION
I hereby claim sole authorship of this thesis. It is a true copy inclusive of any revisions as
required by my examiners. To the best of my knowledge, it is an original contribution to
knowledge without prior publication of any aspect by another person nor does it contain
material submitted or accepted for the award of any other degree in any other University,
except where due acknowledgement has been made in the text. I agree to the lending of this
thesis to individuals and institutions for the purposes of scholarly research.
Peter Osam Sanful
PG7669104
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Signature
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Date
Prof. Emmanuel Frempong
(Supervisor)
……………………….
Signature
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Date
Dr. Samuel Aikins
(Co-supervisor)
………………………..
Signature
…………………..
Date
Dr. P. K Baidoo
(Head of Department)
………………………….
Signature
ii
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Date
ABSTRACT
The seasonal and interannual variability of the community structure, biomass and production
of zooplankton taxa in the pelagic zone of Lake Bosumtwi were investigated from August
2004 to October, 2008. Field studies occurred biweekly at a fixed central station throughout
2005 and 2006. Diurnal vertical distribution of zooplankton was assessed in relation to
vertical gradients in light, temperature, dissolved oxygen, food and predation risk. Diel
vertical migration was also studied on three dates corresponding to the seasonal deep mixing,
weak mixing and the stratified period which was also characterized by shifts in the maximum
depth distribution of chlorophyll a concentration. The zooplankton community was species
poor and comprised nine taxa namely, a dominant sole copepod, the cyclopoid Mesocyclops
bosumtwii, which is a novel species and perhaps endemic to the lake. The other species
included the sole cladoceran, Moina micrura, and six rotifers, Brachionus calyciflorus,
Brachionus dimidiatus, Keratella cochlearis, Filinia pejleri, Filinia camascela and Hexarthra
intermedia. The principal invertebrate predator was the larvae of the dipteran Chaoborus
ceratopogones which possessed fairly stable population sizes in 2005 due to low pelagic fish
predation pressure inferred from the low pelagic fish abundance. Adults and developmental
stages of M. bosumtwii constituted 68 % on average of monthly total zooplankton abundance
and together with H. intermedia contributed over 80 % of annual total community abundance.
There was a weak and insignificant statistical correlation between chlorophyll a concentration
and total herbivore abundance (R2=0.10, P > 0.05). Similarly, correlation analyses between
grazeable phytoplankton biomass (GPB) and the biomass of major herbivores revealed that
maximum herbivore biomass occurred between low to high GPB and that predation was
likely holding down herbivore populations below their carrying capacities based on GPB.
Total zooplankton abundance was higher during the mixing season relative to the stratified.
Stage-specific species length-dry weight relationships were developed for adults and
copepodites of the cyclopoid copepod, M. bosumtwii, adults of Moina micrura and larval
instars and pupae of C.ceratopogones. There was no significant difference in length-dry
weight relationships between the stratified and mixing seasons for Chaoborus instars (slope:
P=0.78, n = 4; intercepts: P= 0.36, n = 4) and all other taxa except differences in intercepts
(slopes: P= 0.43, n = 4, intercepts: P < 0.05, n =11). Mean total annual community biomass
and production were 11.19 gdwm-3 and 1.59 gdwm-3y-1 respectively with M. bosumtwii and
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Chaoborus both contributing about 99 %. Mean community biomass was 0.5 gdwm-3 whereas
mean production recorded was 0.92 gdwm-3y-1. Likewise, M. bosumtwii formed 68 % of
annual community production but the contribution to community biomass was shared almost
equally with Chaoborus in successive years. Mean annual community P/B ratio (biomass
turnover rates) measured was 23.40, which is lower than those reported for most tropical
lakes. Community P/B ratio was highest during the deep mixing period and Chaoborus
contributed the least to community P/B ratio. Maximum epilimnetic chlorophyll a
concentration was about 25µg/L whereas levels reached ~ 99 µg/L in the metalimnion during
the seasonal deep water chlorophyll a maximum from April to July of each year. Over 90 %
of each taxon with the exception of copepod nauplii (80 %) occurred in the epilimnion
throughout the year except during the deep seasonal mixing when a broader vertical
distribution occurred, nonetheless, high epilimnetic densities of taxa were maintained. The
zooplankton in Lake Bosumtwi have individually distinct vertical distribution patterns.
Vertical migration was apparent in some taxa and developmental stages of M. bosumtwii, but
only Chaoborus migrated en masse, ascending into surface waters at night and descending
into deeper and darker depths below 20 m during the day. The zooplankton community
responds appreciably to climatic changes regarding wind generated water column mixing and
precipitation. Seasonal increases in community abundance are interrupted by short term
fluctuations in abundance. The intensity of mixing and the onset of seasonal maximum
precipitation both influenced short term, seasonal and interannual differences in community
abundance. The stable patterns in zooplankton vertical distribution may enhance effective
predictions about community structure and dynamics of zooplankton in the lake. The effect of
seasonal rainfall and wind generated lake mixing as prime physical factors influencing
zooplankton community abundance, biomass, production and vertical distribution
characterizes the zooplankton communities of many tropical lakes. A large proportion of
zooplankton production may be unutilized at higher consumer levels and much of this may
enter detrital pathways to feed a potentially rich microbial food web, but potentially,
production is mostly lost to the deeper anoxic waters of Lake Bosumtwi. Understanding of
the ecological role of zooplankton in the functioning of lake ecosystems will effectively
enhance management of fisheries and additionally inform policy and decision making
regarding the sustainable use and management of lake ecosystems and their resources.
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TABLE OF CONTENTS
DECLARATION .........................................................................................................................ii
ABSTRACT ............................................................................................................................... iii
TABLE OF CONTENTS ............................................................................................................ v
LIST OF TABLES ..................................................................................................................... ix
LIST OF FIGURES ................................................................................................................... xi
ACKNOWLEDGEMENT ....................................................................................................... xiv
CHAPTER 1: INTRODUCTION .............................................................................................. 1
1.0 Background and Conceptual framework .......................................................................................... 1
1.1 Rationale for Research...................................................................................................................... 3
1.1.1 Trends and scale of zooplankton research: tropical and African context .................................. 3
1.1.2 Ghanaian Context ...................................................................................................................... 6
1.2 Thesis Overview ............................................................................................................................... 8
1.2.1 Research Objectives................................................................................................................... 9
1.2.2 Preview of Thesis Chapters ....................................................................................................... 9
CHAPTER 2: LITERATURE REVIEW & THEORY ......................................................... 11
2.0 Regulation of tropical seasonality and impact on freshwater ecosystems ...................................... 11
2.1 Zooplankton community structure and associated regulators......................................................... 12
2.2 The Trophic Cascade Hypothesis (TCH) ....................................................................................... 15
2.3 Temperate and tropical scenarios of zooplankton community structure ........................................ 16
2.4 Zooplankton-Phytoplankton Relationship ...................................................................................... 18
2.4.1 Chlorophyll a concentration (surrogate of phytoplankton abundance) ................................... 19
2.5 Energy Budgets: biomass and production of zooplankton ............................................................. 20
2.5.1 Effect of nutrition on zooplankton biomass and production.................................................... 21
2.5.2 Measurement of biomass and production ................................................................................ 24
2.5.3 Limitations to measurement of secondary production ............................................................. 27
2.5.4 Production-Biomass Ratios (biomass turnover rates) .............................................................. 28
2.6 Vertical distribution of zooplankton: character and seasonality..................................................... 29
2.6.1 Diel Vertical Migration (DVM)............................................................................................... 32
2.6.2 Synthesis of tropical aspects of vertical distribution studies ................................................... 33
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CHAPTER 3: MATERIALS & METHODS .......................................................................... 35
3.0 Study Site: Lake Bosumtwi ............................................................................................................ 35
3.1 Morphometry (Origin and Relief) .................................................................................................. 35
3.2 Climate, Phenology and Vegetation Characteristics....................................................................... 37
3.3 Limnological Characteristics .......................................................................................................... 38
3.4 Palaeolimnological and palaeoenvironmental importance ............................................................. 39
3.5 Field Sampling ................................................................................................................................ 41
3.5.1 Community Structure, Biomass and Production...................................................................... 41
3.5.2 Vertical Distribution ................................................................................................................ 42
3.5.2.1 Diel Vertical Migration studies......................................................................................... 44
3.5.3 Chlorophyll a concentration .................................................................................................... 45
3.5.4 Measurement of temperature and oxygen profiles................................................................... 45
3.5.5 Laboratory Procedures ............................................................................................................. 45
3.5.5.1 Taxonomic analyses.......................................................................................................... 45
3.5.5.2 Measurement of Species Diversity ................................................................................... 46
3.5.5.3 Determination of abundance ............................................................................................. 46
3.5.5.3.1 Subsampling and enumeration procedures: abundance ............................................. 46
3.5.5.3.2 Vertical distribution and DVM .................................................................................. 48
3.5.5.3.3 Calculation of abundance .......................................................................................... 49
3.5.5.4 Chemical analyses of Chl a concentration ........................................................................ 49
3.5.6 Statistical analyses for community structure ........................................................................... 49
3.5.7 Data analyses for vertical distribution ..................................................................................... 50
3.6 Measurement of biomass and production ....................................................................................... 51
3.6.1 Evaluation of samples .............................................................................................................. 51
3.6.2 Development of length-weight regressions ............................................................................. 51
3.6.2.1 Determination of individual dry weights .......................................................................... 51
3.6.2.2 Determination of rotifer dry weights ................................................................................ 55
3.6.3 Derivation of population parameters ....................................................................................... 56
3.6.4 Routine body length measurements ......................................................................................... 59
3.6.5 Computation of secondary production..................................................................................... 60
3.6.6 Calculation of Production-Biomass (P/B) ratios ..................................................................... 61
3.6.7 Statistical analyses ................................................................................................................... 61
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CHAPTER 4: RESULTS .......................................................................................................... 63
4.1 Species composition, community diversity and trophic structure .................................................. 63
4.2 Patterns in community abundance .................................................................................................. 68
4.2.1 Interannual patterns of community abundance ........................................................................ 70
4.3 Trends in abundance of each taxon................................................................................................. 71
4.3.1 Copepod- Mesocyclops bosumtwii........................................................................................... 71
4.3.2 Rotifers .................................................................................................................................... 72
4.3.3 Cladocera-Moina micrura ....................................................................................................... 74
4.3.4 Phantom midge larvae- Chaoborus ceratopogones ................................................................. 74
4.4 Relationship among herbivorous zooplankton, Depth of Mixed Layer (DML) and
Chlorophyll a concentration. .......................................................................................................... 76
4.5 Relationship between individual herbivore species and Chlorophyll a concentration ................... 78
4.6 Interactions between predators and herbivores............................................................................... 81
4.7 Regression model, statistics and general zooplankton biometrics .................................................. 82
4.7.1 Zooplankton Biometrics .......................................................................................................... 85
4.7.1.1 Copepod body length and weight distribution .................................................................. 85
4.7.1.2 Chaoborus body length and weight distribution............................................................... 86
4.8 Biomass and production of dominant taxa ..................................................................................... 88
4.8.1 Patterns of community biomass and production ...................................................................... 88
4.8.2 Trends in copepod biomass and production ............................................................................ 89
4.8.3 Trends in Chaoborus biomass and production ........................................................................ 93
4.8.3.1 Patterns in Chaoborus biomass and production ............................................................... 95
4.8.4 Relationship between herbivore biomass and Chl a concentration. ........................................ 97
4.8.5 Correlation Analyses: herbivore biomass versus grazeable phytoplankton biomass
(GBP). ................................................................................................................................... 100
4.8.6 Production-Biomass (P/B) Ratios .......................................................................................... 103
4.9 Indicators of Habitat Quality: vertical gradients in light, temperature, dissolved
oxygen, food availability and predation risk ................................................................................ 108
4.9.1 Patterns of vertical distribution of zooplankton and responses to seasonal alterations in
habitat quality parameters ..................................................................................................... 111
4.9.1.1 Copepods ........................................................................................................................ 111
4.9.1.2 Cladocera ........................................................................................................................ 114
4.9.1.3 Rotifers ........................................................................................................................... 115
4.9.2 Epilimnetic proportion of each zooplankton species ............................................................. 116
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4.9.3 Diel Vertical Migration (DVM) pattern of zooplankton........................................................ 118
CHAPTER 5: DISCUSSION ................................................................................................. 122
5.1 Species richness and community diversity ................................................................................... 122
5.1.1 Effect of biotic processes on diversity ................................................................................... 123
5.2 Patterns of zooplankton abundance .............................................................................................. 125
5.2.1 Patterns of overall community abundance ............................................................................. 127
5.2.2 Rarity of rotifers in Lake Bosumtwi ...................................................................................... 129
5.2.3 Community dominance of predators...................................................................................... 130
5.3 Relationship between Chl a concentration and total herbivore abundance .................................. 132
5.4 Length-weight relationships ......................................................................................................... 132
5.4.1 Length-weight relationships of Chaoborus instars ................................................................ 134
5.5 Patterns of biomass and production .............................................................................................. 136
5.5.1 Overall community patterns................................................................................................... 136
5.5.2 Biomass and production patterns of copepods ...................................................................... 138
5.5.3 Biomass and production patterns of Chaoborus .................................................................... 140
5.5.4 Biomass and production patterns of Cladocera and Rotifera................................................. 141
5.6 P/B ratios (biomass turnover rates)............................................................................................... 143
5.7 Variation in habitat quality and vertical distribution of zooplankton ........................................... 145
5.7.1 Percent zooplankton in epilimnion ........................................................................................ 147
5.8 Patterns of Diel Vertical Migration (DVM) of zooplankton ........................................................ 148
5.8.1 DVM during stratified periods............................................................................................... 148
5.8.2 DVM during circulation period ............................................................................................. 149
CHAPTER 6: CONCLUSIONS & RECOMMENDATIONS ............................................ 151
6.1 Research Overview and General Comments ................................................................................ 151
6.2 Major findings and Conclusions ................................................................................................... 152
6.2.1 Community structure and dynamics: ..................................................................................... 152
6.2.2 Biomass and Production: ....................................................................................................... 153
6.2.3 Vertical distribution and migration ........................................................................................ 154
6.3 Implications of Research for Ecosystem Resource Management ................................................. 155
6.3.1 Implications of Research for Fisheries Management............................................................. 157
6.4 Recommendations and future research directions. ....................................................................... 157
REFERENCES ........................................................................................................................ 160
APPENDIX .............................................................................................................................. 187
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LIST OF TABLES
Table 3.0 : Development times of individual developmental stages of M. a.
aequatorialis from Lake Malawi. Source: (Irvine and Waya, 1999). ...................... 57
Table 3.1: Development times of individual tropical Chaoborus instars from Lake
Lanao, Philippines. Source: (Lewis, 1979). Mean values calculated with
permission .................................................................................................................. 57
Table 4.0: Interannual species diversity indices of the zooplankton community………..68
Table 4.1: Pairwise Spearman Rank correlation coefficients for patterns of
interannual seasonal correspondence (2005 versus 2006) in individual
zooplankton abundance and total zooplankton abundance at 5% significance
level.. .......................................................................................................................... 74
Table 4.2: Regression statistics for various length-weight relationships of individual
Chaoborus instars during the stratified period. Regression equations used for
routine calculations are in bold. ................................................................................. 83
Table 4.3: Regression statistics for various length-weight relationships of nonovigerous adult female copepod and adult non-ovigerous M.Micrura during the
stratified season.......................................................................................................... 83
Table 4.4: Regression statistics for various length-weight relationships of individual
Chaoborus instars and pupae except instar 1 during the mixing period. ................... 83
Table 4.5: Regression statistics for various length-weight relationships of individual
developmental stages of M. bosumtwii during the stratified period. ......................... 84
Table 4.6: Monthly mean values of selected biometrics of all zooplankton taxa and
developmental stages. SEM is the standard error of the monthly means. Range
represents the range of monthly means recorded, Wt (weight) and L is the total
body length of organisms. .......................................................................................... 86
Table 4.7: Characteristic Head Capsule Lengths (HCL) of individual Chaoborus
instars used as the primary discriminant among instars............................................. 87
Table 4.8: Summary statistics of biomass, production, P/B ratios of dominant
zooplankton taxa and developmental stages in 2005. Mass is given as
mgdwm-3 and production as mgdwm-3day-1………………..……………...105
Table 4.9: Summary statistics of biomass, production, P/B ratios of dominant
zooplankton taxa and developmental stages in 2006. Mass is given as
mgdwm-3 and production as mgdwm-3day-1…………..…………………... 106
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Table 4.10: Mean annual biomass, total production, annual P/B ratios of dominant
zooplankton in Lake Bosumtwi in 2005 and 2006...………………………107
Table 5.0: Comparison of mean lengths (µm) and weights (µg) of Mesocyclops sp in
Lake Bosumtwi (Bos.) Ghana, Lake Nkuruba (Nku.) Uganda, Lake Awasa
Ethiopia and Lake Ontario (Ont.) Canada-USA. Fourth And fifth copepodite
stages in lake Awasa are integrated in to 3 general size classes.Values in bold
are estimated from data given by Culver et al (1985) for Lake Ontario. Source
of Lake Nkuruba data is Kizito (1998) .................................................................... 134
Table 5.1: Annual P/B Ratio of the zooplankton community (including Chaoborus)
of Lake Bosumtwi compared with several tropical lakes and the mean of
temperate lakes (IBP). Mean daily biomass is given as mgdwm-3 whereas total
annual production is given as gdwm-3day-1. Data for the first 7 lakes were
extracted from Mengestou and Fernando (1991) and Winberg et al., 1972. Lakes
containing Chaoborus are in bold ............................................................................ 143
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LIST OF FIGURES
Figure 3 (a): Map of Ghana showing the geographical location of Lake Bosumtwi. . …36
Figure 3 (b): Bathymetry of Lake Bosumtwi showing the central sampling station
(CS) and the onshore docking site Abono……………............................................ 36
Figure 4 (a): Adult copepod males, adult copepod females of Mesocyclops
bosumtwii and the cladocerann Moina micrura in Lake Bosumtwi at 100 X
magnification – Reference organisms are indicated by arrows ................................. 64
Figure 4 (b): Photomicrograph representation of copepodites and nauplii of Mesocyclops
bosumtwii and the rotifers Hexarthra intermedia and Brachionus calyciflorus in Lake
Bosumtwi at 100X magnification. Reference organisms are indicated by arrows….65
Figure 4 (c): Photomicrograph representation of the rotifers Filinia pejleri, Filinia
camascela and Brachionus dimidiatus in Lake Bosumtwi at 100 X magnification.
Reference organisms are indicated by arrows…………………..……………………..66
Figure 4 (d): The rotifer Keratella cochlearis in Lake Bosumtwi at 100 X magnification.
Organism is amongst the rare zooplankton in the Lake. ……………………………67
Figure 4.1 Percent contribution of each zooplankton taxon to monthly total zooplankton
abundance…………………………………………...................................................69
Figure 4.2: Annual percent composition of each zooplankton taxon and copepod
developmental stages to total zooplankton abundance. ............................................. 69
Figure 4.3: Patterns in abundance of total zooplankton, adults and developmental
stages of M. bosumtwii (copepods), M .micrura (cladocera), H.intermedia,
B.calyciflorus and Chaoborus in individuals per cubic meter of water in
successive years. ........................................................................................................ 73
Figure 4.4: Abundance of B.dimidiatus, Keratella cochlearis, Filinia pejleri and
Filinia camascela in thousands of individuals per cubic meter of water. ................. 75
Figure 4.5: Effect of variation in Depth of Mixed Layer on Chl a. and total
herbivore abundance. DML (half-filled circles). Chl a (green) and total
herbivore abundance (solid circles). .......................................................................... 76
Figure 4.6: Responses of individual herbivore species and total herbivore abundance
to Chl a, seasonal variation in Chl a and relationship between total herbivore
and predatore abundance. .......................................................................................... 79
Figure 4.7: Linear correlation between Chl a and total herbivore abundance. ............... 80
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Figure 4.8: Generalized curvilinear (power function) regression model describing the
the growth pattern of zooplankton and the relationship between zooplankton mean
body length (independent or predictor variable) and the mean body weight
(dependent or response variable). Where a and b are fitted constants of the
regression………………………………………………………………….…….......82
Figure 4.9: Percent contribution of each zooplankton group to total annual
zooplankton biomass and production in 2005 and 2006). Brachionus
calyciflorus and Hexarthra intermedia are not represented in the production
estimates given ........................................................................................................... 88
Figure 4.10: Percent contribution of adult copepods and developmental stages to
total copepod biomass and production in successive years. ...................................... 90
Figure 4.11: Interannual patterns of variation in biomass and production of nauplii
and adult males of M. bosumtwii, as well as total copepodite, total copepod and
total zooplankton biomass and production in successive years. ................................ 91
Figure 4.12: Interannual patterns of variation in biomass and production of
individual copepodite stages and adult females of the copepod M. bosumtwii in
successive years. ........................................................................................................ 92
Figure 4.13: Biomass of each Chaoborus instar in successive years .............................. 93
Figure 4.14: Interannual differences in proportion of total production contributed by
each larval instar of Chaoborus. ................................................................................ 94
Figure 4.15: Interannual patterns of variation in biomass and production of
individual Chaoborus instar stages and biomass of pupae in successive years......... 95
Figure 4.16: Interannual patterns of variation in biomass and Chl a of the major
herbivores; B.calyciflorus, H.intermedia including production of M.micrura
respectively in successive years. ................................................................................ 98
Figure 4.17: Temporal patterns of the effect of variation in thickness of mixed
depth on seasonal total biomass and production of zooplankton............................... 99
Figure 4.18: Linear correlation between grazeable phytoplankton biomass and 2-week
lagged individual herbivore biomass. Copepodites and nauplii are included for
speculative analyses…………………………………………………………….............100
Figure 4.19: Patterns of herbivore biomass and grazeable phytoplankton biomass.
Grazeable phytoplankton biomass (dotted-lines) and individual herbivore
biomass (solid lines)……………………………………………………………......102
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Figure 4.20: Variation of Secchi depth and vertical distribution of temperature and
dissolved Oxygen (DO) during the stratified and interannual mixing seasons of
2005 and 2006. Temperature and oxygen profiles for the stratified season were
taken in May 2006 whereas that of the mixing periods were taken in August of
each year. ................................................................................................................. 108
Figure 4.21: Depth distribution of monthly Chl a (µg/L) in 2005 and 2006 showing
deep water microbial maximum from April to July each year. Lower frequency
of measurements in 2005 is attributed to logistical constraints. .............................. 110
Figure 4.22:Patterns of vertical distribution of Chaoborus during 2005 and 2006 ....... 111
Figure 4.23: Patterns of vertical distribution of copepod nauplii (top panel) and
copepodite (lower panel) developmental stages respectively. Seasonal
circulation periods are illustrated with the arrows. .................................................. 112
Figure 4.24: Patterns of vertical distribution of adult copepods (top panel) and M.
micrura (lower panel) respectively .......................................................................... 114
Figure 4.25: Patterns of vertical distribution of B.calyciflorus in 2005 and 2006 ......... 115
Figure 4.26: Patterns of vertical distribution H.intermedia in 2005 and 2006............... 116
Figure 4.27: Annual mean percent of each species and copepod developmental
stages occuring in the epilimnion of Lake Bosumtwi.............................................. 117
Figure 4.28: Patterns of DVM of zooplankton including temperature and dissolved
oxygen profiles during the onset of the strong seasonal mixing on 29th July
2005. Chaoborus was not detected in the water column (surface-30m) at 12
noon. ........................................................................................................................ 118
Figure 4.29: Patterns of DVM of copepod nauplii, copepodites, adult copepods, M.
micrura, Chaoborus and H.intermedia including temperature profiles on 26th
may, 2006. B. calyciflorus was absent from the water column. Night
temperature profiles were not taken due to lack of battery power for CTD.
Dissolved oxygen profiles were not included due to their similarity with
temperature profiles. ................................................................................................ 120
Figure 4.30: Patterns of DVM of copepod nauplii, copepodites, adults, Chaoborus,
B. calyciflorus, H. intermedia recorded on 19th December, 2006. .......................... 121
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ACKNOWLEDGEMENT
It has been a memorable journey of many significant landmarks. In this journey I have
had large portions of both despair and exhilarating experiences, albeit God’s great love
and faithfulness has anchored me through it all by His favour and Grace
I pour my heartfelt gratitude to my family, who have been my perpetual source of
strength and encouragement. Their tremendous sacrifices have shaped my life from
childhood and also guided me through this journey. To P.O. Sanful (Snr) of blessed
memory, Theresa, Sis Emma, Nana Koom, Maame Grace, Nana Amanko, Jojo,
Bentumba, Uncle Kobina Topp and Uncle Kodwo Nyansa, I say thank you folks.
I am eternally indebted to my fiancée and best friend Dr. Pearl Adu-Nyako. She and her
family have been through it all with me. Her encouragement, emotional and spiritual
support in addition to the genuine loving care held me through these tough times. God
richly bless you.
To the research team; my supervisors, Prof. E. Frempong and Dr. S. Aikins, associate
supervisors, Prof. R. E Hecky (Biology Department and Large Lakes Observatory,
University of Minnesota, USA) and Prof. R.I Hall (University of Waterloo, Canada) and
colleagues Francis E. Awortwi and Dr. Megan Puchniak; I say, you have been awesome.
Thank you for enduring my enthusiasm. Your encouragement and concern for both my
academic and personal welfare is much appreciated. You did not waver in your
conviction that I could do it and I sincerely hope that, that confidence you reposed in me
has been duly honoured. I am also grateful to Prof. S. Danuor (Physics Department,
KNUST) and Mr. Fredua Agyeman for their assistance with field work including John
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Bilson and Yaw Manu my field assistants with whom I spent several stormy nights on the
lake.
Major funding for fieldwork and logistics was provided by the Natural Sciences and
Engineering Research Council of Canada (NSERC). As well, KNUST Staff Development
Programme provided me with a Ph.D studentship and partial grant for field work. Also, I
am eternally grateful to the Department of Foreign Affairs and International Trade
(DFAIT) Canada, for funding my 1 year research activities in Canada that enabled me to
complete the desired analyses of my field samples and advance my thesis write up at the
University of Waterloo, Ontario, Canada. Much appreciation goes to the Swedish
Institute for funding my 6 month thesis research and writing of scientific papers at MidSweden University, Campus Sundsvall, Sweden. The crucial consecutive International
Society for Limnology (SIL) Tonolli Memorial Awards in 2006 and 2007 are deeply
appreciated. Special thanks go to a personal friend and scientist, Prof. William Michael
Lewis (former SIL General Secretary/Treasurer) who without prior personal knowledge
showed keen interest in my research work and personal welfare and was instrumental in
those two SIL Awards.
I thank my other scientist friends, some of whom I am yet to meet personally; Prof. Dr.
W.D Taylor (University of Waterloo - who perceptively advised on sampling, thesis
content and for reviewing the thesis), Prof. H. J Dumont (Ghent University, Belgiumwho provided taxonomic assistance to my work), Prof. Iskandar Mirabdulayev
(Institute of Zoology, Uzbek Academy of Sciences, Uzbekistan - with whom I coauthored descriptions of Mesocyclops bosumtwii), Dr. Ellie Prepas (Lakehead
University, Canada), Dr. Claudio Tudorancea (University of Waterloo) and Prof. Nils
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Ekelund, my supervisor at Mid-Sweden University. Thank you Prof. D.A Livingstone
(Duke University, USA) and Dr. Piet Verburg (Centre for Physical Limnology, Santa
Barbara, Georgia, USA) for introducing me to practical limnology.
To my wonderful compatriots; Mizpah Asase, Kwame Ansah Duodu, Dennis Yar
Dekugmen, Emmanuel Osei-Tutu, Daniel Hayford, Tyhra Carolyn Kumase,
Christopher Antwi, Owusu-Manu DeGraft and George Rockson, you guys have been
great. Thanks a lot for everything not forgetting that we have been in this together by
our corporate determination and resilience.
To the lovely guys at University of Waterloo Aquatic Ecology Group (UWAEG)
especially, Greg Silsbe, Peter Njuru, Mangaliso Gondwe, Sairah Malkin, Ann
Balasubramaniam, Vicky Jackson, Adam, Jen, Ted Ozersky and Mohammed. Thanks a
lot for your kindness and assistance with academic and social welfare during my time
with you at the Department.
To my best friends in Canada, Michael Kani, Musonda Chisanga and Ekow Andoh; I
cherish the many exciting moments we had and remember how you guys will stop at
nothing to make my stay in Canada enjoyable. I deeply appreciate your brotherly love
and kind hearts.
To all who in diverse ways contributed to my research work, I say a big THANKS.
When I cast my mind back to where I began and examine where I stand now, I can only
say that I have indeed come a long way by the Special Grace of the Almighty God.
To Him be Glory, Honour, Power and Praise now and forever more.
xvi
CHAPTER 1: INTRODUCTION
1.0 Background and Conceptual framework
Zooplankton constitute an important component of lake ecosystems and their unique
central position in food webs provide the ecological machinery for the processing and
transfer of energy and matter from lower trophic levels (algal biomass) to higher
trophic levels such as fish. Understanding of their production dynamics and the factors
that affect their production and abundance, as well as their critical linkages with other
ecosystem elements, are vital to optimizing resource use and enhancing sustainable
lake ecosystem management. Zooplankton in freshwater ecosystems are dominated by
four major invertebrate groups, in terms of biomass, numbers and nutrient cycling;
protists that are composed of protozoa and heterotrophic flagellates, rotifers and two
subclasses of the Crustacea- the Copepoda and Cladocera (Wetzel, 2001). A few
representatives of coelenterates, larval trematode flatworms, gastrotrichs, mites and
larval stages of insect and fish also may occur ephemerally among the true
zooplankton (Wetzel, 2001). These major zooplankton groups exhibit wide variations
in morphology (micro-macro), physiology, life history, behaviour and adaptations to
changing environments. They are also widely distributed among lakes of varying
trophic status (Gannon and Stemberger, 1978). Such differences result in distinct
structural and functional ecological diversity among the zooplankton. Members of the
Copepoda, which consist of three subgroups - Cyclopoida, Calanoida and
Harparticoida, are bisexual and are characterized by a relatively longer life history as
opposed to the shorter parthenogenetic habit of the Cladocera and Rotifera that enable
them to optimize favourable conditions by ensuring rapid reproduction to offset losses
1
to various mortality factors and mechanisms. Sexual reproduction occurs in the
parthenogenetic zooplankton, but in most cases it is a strategy to survive adverse
environmental conditions (Wetzel, 2001). Because zooplankton play a critical role in
lake ecosystem functioning by serving as purveyors of matter and energy from
primary producers to higher consumer levels (mostly fish), factors that limit
zooplankton productivity have direct repercussions on fish productivity (Gosline,
1971; Kerfoot and Lynch, 1987) and many ecosystem processes. Knowledge of the
important connection of zooplankton to fish productivity is universal and optimal
supply of zooplankton is critical to the survival of post larval fish (Fernando, 1994).
Reproductive failure in Cypriniformes resulting from limited supply of zooplankton
for the diet of free-living fish larvae has been documented (Gosline, 1971). Infact, fish
and zooplankton have been shown to evolutionarily synchronize their reproductive
strategies so that fish larvae will have access to the required zooplankton ration during
growth and development (Gosline, 1971; Kerfoot and Lynch, 1987), inorder to
enhance recruitment into the fishery. Additionally “the magnitude of young-of-theyear (YOY) fish stocks is determined by match and mismatch with its zooplankton
food supply” (George and Harris 1985 cited in (Kalff, 2002)). Furthermore, the
generally short life history of zooplankton makes them suitable assessors of ecosystem
health due to their ability to integrate acute and chronic environmental stress over
short periods (Gannon and Stemberger, 1978).
Zooplankton also play an important role in the cycling of nutrients within aquatic
ecosystems via feeding and excretion pathways (Peters and Rigler, 1973: Lehman,
1980; Urabe, 1993). Their activities often include a spatial dimension via vertical
transport of nutrients along the water column through their vertical migration
2
behaviour (e.g. Williamson, et al., 1996) and horizontally via onshore-offshore
migratory behaviour (e.g Burks et al., 2002). These important ecosystem activities of
zooplankton are influenced by seasonal and interannual climatic fluctuations
especially in temperate and subarctic lakes (e.g Burns and Mitchell, 1980; Primicerio,
2000). Tropical lakes also exhibit seasonal and interannual climatic variations but with
a relatively reduced amplitude (e.g Twombly, 1983; Kurki, 1996; Isumbisho et al.,
2006). Food web interactions - mainly interspecific competition and predation and
availability of food resources also regulate zooplankton structure and function in lake
ecosystems (e.g Brooks and Dodson, 1965; Hall et al., 1976; Carpenter and Kitchell,
1993). Variations in these factors impose considerable constraints on the optimal
performance of zooplankton in freshwater ecosystems.
1.1 Rationale for Research
1.1.1 Trends and scale of zooplankton research: tropical and African context
In the tropics, especially in tropical Africa, there is a long history of study of the
taxonomy, systematics and biogeography of zooplankton in freshwater ecosystems
(e.g Fernando et al., 1990; Dumont, 1994 b; Green, 1967, 1976). In contrast,
information on zooplankton community structure and function is much more limited.
There has however been a gradual progression of research in zooplankton community
structure and their associated regulatory factors and mechanisms in different regional
contexts. Much of this research has tended to concentrate in East Africa (virtually
Africa’s Lake District), where the Rift Valley harbours some of the largest freshwater
ecosystems in the world, including the three transnational Great Lakes, Lake
Tanganyika, Lake Malawi and Lake Victoria (e.g Burgis, 1971, 1973; Twombly,
3
1983; Hecky, 1984; Hecky and Kling, 1987; Hecky, 1991). Thus, in terms of scale of
research and not necessarily the distribution of freshwater ecosystems, there is a
regional discontinuity of zooplankton research in African freshwater ecosystems. The
attraction of the East African lake series to freshwater research is in part due to the
vast fisheries resource base that commands greater socio-economic impact compared
to systems in other parts of Africa. Albeit, continent wide knowledge of African lake
ecosystems is essential for understanding of the various factors that regulate ecosystem
structure and function in the tropics and how human impacts could be identified and
managed in effective sustainable ways.
Aspects of zooplankton community structure and factors that regulate them have been
studied by some workers in the tropics, notably Green (1967; 1976; 1985), Burgis
(1971; 1973), Lewis (1979), Fernando (1980a, 1980b), Mavuti and Litterick (1981),
Twombly (1983), Fernando et al., (1987), Mengestou and Fernando (1991), Dumont
(1994 a), Kurki (1996), Isumbisho et al (2006). Information on population dynamics
and community abundance is inadequate to effectively elucidate the functionality of
tropical zooplankton communities. Analyses of biological communities based on
energy budgets provide the best means of assessing the impact of environmental
stressors on communities and ecosystems (Krebs, 1994). The trophic-dynamic
approach is therefore relevant, if a better understanding of tropical zooplankton
communities is to be achieved. Such information is fundamental in any trophic
analyses.
Assessment of energy budgets of zooplankton communities is a rather laborious task
and also requires advanced instrumentation. This problem particularly constrains
4
African-based
efforts
at bridging the biomass-production information gap.
Consequently, studies have been conducted on aspects of biomass and production of
zooplankton in only a few African lakes (Burgis, 1971; 1974; Vareschi and Jacobs,
1984; Hecky, 1984; Mengestou and Fernando, 1991; Irvine and Waya, 1999). Usually,
when biomass and production are determined, rotifers and Chaoborus, both of which
are integral members of many tropical zooplankton communities (Fernando et al.,
1990; Mengestou et al., 1991) are largely excluded, though, it has long been
established that Chaoborus is the principal invertebrate predator present in tropical
systems (Fernando et al., 1990). However, the energetic aspects of this influential role
of Chaoborus have been poorly examined. The role of Chaoborus in the functioning
of zooplankton communities has been assessed in relation to its general ecology
(McGowan, 1974), predation rates (Moore and Gilbert, 1987), nutrition (Hare and
Carter 1987; Moore et al., 1994; Lewis, 1977) feeding strategies (Pastorok, 1981;
Riessen et al., 1984) and energetics (Cressa and Lewis, 1986; Jäger and Walz, 2003).
But, few of these studies apply to tropical lakes (McGowan, 1974; Hare and Carter,
1987; Cressa and Lewis, 1986). Similarly, rotifers have been shown to contribute
substantially to energy flux by their high intrinsic turnover rates (Makarewicz and
Likens, 1979; Pace and Orcutt, 1981; Wetzel, 2001; Pallaez-Rodriguez and
Matsumura-Tundisi, 2002), and in certain instances may exceed biomass and turnover
rates of macrozooplankton (Makarewicz and Likens, 1979). Consequently there is a
dearth of information on biomass and production of rotifers and Chaoborus species in
tropical and most importantly African freshwater systems.
Patterns of vertical distribution and migration of zooplankton in tropical lakes remain
poorly understood, but they are important responses to changing habitat quality
5
conditions and contribute significantly to understanding zooplankton community
structure and dynamics. This has also been poorly resolved. Most available data on
vertical distribution in tropical and African lake systems are from shallow equatorial
and sub-equatorial lakes that lack permanent stratification of the water column
(Burgis, 1973; Talling (1964), Ganf and Horne (1975), Seaman et al., (1978) all cited
in Mavuti (1992)). Consequently, absence of mass vertical migration by zooplankton
in these systems has been highlighted by Mavuti (1992), and in tropical systems,
dissolved oxygen tolerances, food availability and predation risk have been suggested
to be the dominant controlling forces of patterns of zooplankton vertical distribution
(Kizito, 1998). Information on the vertical distribution and migration patterns of
zooplankton in deep tropical lakes, with persistent annual stratification, and seasonal
circulation is seriously lacking. Kizito (1998) has documented a continuous 16 month
bi-weekly observation on the vertical distribution of crustacean zooplankton and
Chaoborus in crater lakes in Western Uganda. But rotifers were largely excluded from
his study. Kizito (1998) also studied Diel Vertical Migration (DVM) of zooplankton
and Chaoborus but with sampling frequency of < 3 months in both lakes Nkuruba and
Nyahira in Western Uganda. Elucidation of the effects of continuous, year-round
predation by Chaoborus and fish on zooplankton community structure and dynamics,
including vertical distribution patterns of zooplankton is important to the
understanding overall functioning of tropical freshwater ecosystems.
1.1.2 Ghanaian Context
In Ghana, much of the research on zooplankton has occurred on Lake Volta with a
large proportion of that in the early years of its formation as part of post-impoundment
ecological impact assessment programmes (Proszynska, 1966 cited in Viner, 1970b;
6
Petr, 1970; Viner, 1970a, 1970b; Obeng-Asamoa, 1977). Nonetheless, with respect to
the depth of information required to fully understand the structure and functioning of
zooplankton in Lake Volta, the available information is grossly inadequate. Research
coverage on the limnology of other freshwater ecosystems in Ghana, mostly
reservoirs, has significantly advanced during the past decade, with emphasis on
pollution and chemical-based water-quality assessments (e.g Ansa-Asare and Asante,
1998; Bosque-Hamilton et al., 2004). Information on zooplankton is mostly qualitative
and limited to the level of genera (Obeng-Asamoa, 1977). Proszynska (1966 cited in
Viner (1970b)), provided information on quantitative measurements of Copepoda and
Cladocera in Lake Volta, whereas Viner (1970b) measured density-dependent vertical
distribution patterns of zooplankton in relation to thermal stratification regime of the
lake. His measurements were based on single monthly samples in March and April of
1966. He classified the zooplankton according to the major Cladocera, Copepoda and
Rotifera groups and genera and identified the Copepoda as the dominant group.
Frempong (1995) has also reviewed limnological research in Ghana and the only
available substantial information on zooplankton among the myriad of freshwater
ecosystems he assessed were from Lake Volta. This information was based on
taxonomic descriptions made by Obeng (1969 cited in Frempong (1995)), also at the
genus level only. Qualitative data on zooplankton in few reservoirs and fish ponds
have also been reported in undergraduate and graduate dissertations at the Biology
Department of the Kwame Nkrumah University of Science and Technology.
Clearly, there is a paucity of information on zooplankton in freshwater ecosystems in
Ghana which then potentially limits our understanding of how these systems function.
Detailed information on community structure which includes species richness and
7
diversity, numerical abundance, species interactions, productivity and succession is
seriously lacking. Furthermore, the functional role of zooplankton in aquatic
ecosystems is poorly understood, due to lack of information on the energy budgets
(biomass and production) of zooplankton and associated environmental regulators and
mechanisms. Such information is fundamental to understanding ecosystem dynamics.
In addition, the adaptation mechanisms of zooplankton which involves morphological,
life-history and behavioural responses to alterations in habitat quality due to
environmental and anthropogenic stressors are ill-resolved. Because zooplankton are
central to the production base in lake ecosystems, evaluation of fish productivity
without thorough knowledge of the zooplankton dynamics is incomplete and
considerably conjectural. Finally, the available quantitative measurements (Viner,
1970b) are based on short-term studies lacking any seasonal dimension. Knowledge of
the phenology and interannual variation in zooplankton dynamics is essential to
understanding the impact of changing global and local climate conditions on
freshwater ecosystem health and productivity in space and time. Unfortunately, data
on phenology and interannual aspects of zooplankton dynamics in Ghanaian
freshwater ecosystems remains non-existent.
1.2 Thesis Overview
This research which included two continuous years of sampling (Jan. 2005 to Dec.
2006) the lake at roughly biweekly intervals was driven by two specific goals; firstly,
to improve knowledge of the seasonal and interannual variability of pelagic
zooplankton community composition and energy budgets in tropical and African lake
ecosystems, and secondly, to use the only natural lake in Ghana (Lake Bosumtwi) as a
model system to elucidate qualitative and quantitative features of its zooplankton
8
community in order to improve knowledge and understanding of the structure,
function and temporal change of zooplankton communities in Ghanaian freshwater
(inland) ecosystems (rivers, lakes, reservoirs and wetlands). The knowledge acquired
will be useful for diagnosis and solution of problems concerning the integrity,
management and sustainable use of aquatic resources as well as serving as important
bioassessment and biocriteria tools for water resource planning and decision making.
1.2.1 Research Objectives
To realize these research goals, specific objectives were outlined to adequately address
the relevant research questions. The specific objectives were as follows;
1: To determine the taxonomic composition, community structure and factors
controlling changes in zooplankton abundance in Lake Bosumtwi.
2: To determine the magnitude of zooplankton biomass and production for dominant
zooplankton and factors affecting biomass production.
3: To determine the vertical distribution patterns of zooplankton in Lake Bosumtwi.
4: Identification of all the major physico-chemical and biotic factors influencing
zooplankton community structure, biomass, production and vertical distribution and to
assess the overall patterns of seasonal and interannual variation of zooplankton
community dynamics.
1.2.2 Preview of Thesis Chapters
Chapter 2 provides background literature and review of relevant theories and concepts
employed in this research highlighting their limitations. General as well as specific
literature pertaining to both temperate and tropical scenarios of zooplankton
9
community structure and function are concisely presented. Regulation of tropical
seasonality, the driving force behind tropical ecosystem dynamics is examined.
Chapter 3 describes the methodological issues and approaches to the research
questions and objectives and justifies the choice of Lake Bosumtwi as a unique lake in
tropical Africa with striking international climatic, palaeolimnological and
palaeoenvironmental importance. A brief chronology of the development of diverse
hypothesis about the origin of the lake and evidence to support a conclusive origin
and summary of the information generated by research on various aspects of the lake
are discussed.
Chapters 4 and 5 deal with the results and their discussion. The discussion of the
research results from the various objectives focuses on the interpretation of results,
that lead to major new findings and confirmation of the extant knowledge and
understanding of how tropical zooplankton communities function. Where applicable,
characteristics of temperate and tropical zooplankton communities are compared.
Chapter 6 focuses on the major conclusions of the research and the implications of the
research for policy and effective ecosystem management. The importance of the
research results to policy development and decision making to enhance the operational
scientific framework for evaluation of various uses of freshwater ecosystem resources,
by environmental regulatory institutions such as the Ghana Environmental Protection
Agency (EPA) and the Water Resources Commission (WRC) is discussed. Lastly
areas of future research are identified and recommendations presented.
10
CHAPTER 2: LITERATURE REVIEW & THEORY
2.0 Regulation of tropical seasonality and impact on freshwater ecosystems
Seasonality is an important driving force in the structure and functioning of freshwater
ecosystems. The effect of changing seasons is reflected in the timing of biological
events that ultimately determine the structure and function of biological communities
(Krebs, 1994). Seasonality in the tropics is characterized by two major periods of
alternating dry and rainy seasons, controlled by an atmospheric boundary; the InterTropical Convergence Zone (ITCZ). The migratory dynamics of the ITCZ determines
seasonal precipitation and wind patterns resulting in a “hydrologically controlled
tropical seasonality” (Talling and Lemoalle, 1998). Major responses of tropical lakes
to this climatically driven hydrological seasonality are observed in patterns of water
levels and river discharge (Talling and Lemoalle, 1998) whilst the effect of wind
patterns are seen in hydrodynamic changes affecting water column properties such as
thermal stratification and vertical distribution of chemical constituents and biota
(Saunders and Lewis, 1988 a). Strong wind energy in association with reduced air
temperatures, depresses the thermocline and causes a deepening of the mixed layer,
inducing hypolimnetic entrainment of nutrient-rich water to fertilize the mixolimnion
which stimulates algal and microzooplankton production (Beadle, 1981; Hecky and
Fee, 1981; Hecky and Kling, 1987; Sarmento et al., 2006; Isumbisho et al., 2006).
These changes in nutrient supply to the phytoplankton and its direct effect on
herbivore zooplankton populations cascade to higher trophic levels leading to
significant changes in abundance of species populations and subsequently on the
overall zooplankton community biomass and production (Twombly, 1983; Isumbisho
et al., 2006). The seasonal patterns of zooplankton communities in tropical lake
11
systems have been shown to respond dynamically to these climate-driven
hydrodynamic changes (Isumbisho et al., 2006). Apart from the regular seasonal cycle,
intra-season short term variations in population size have also been documented, and
these are known to be caused by sporadic climatic changes affecting the water column
dynamics (Lewis, 1979; Kizito, 1998; Twombly,1983; Kalk, 1979; Degnbol and
Mapila, 1982).These transitory changes of the water column can confer opportunistic
demographic advantages to populations with short generation times, enabling them to
exploit the favourable conditions in order to stabilize their population sizes (Green et
al., 1976). Zooplankton populations in the tropics are also known to exhibit marked
interannual variations. Differences in the timing, scale and periodicity of population
abundance are known to exist and replication of patterns of annual variations is muted
by aseasonal and seasonal fluctuations in population size (Twombly, 1983; Gras et al.,
1967). They conclude that annual cycles in tropical zooplankton populations are
dynamically variant and reflect continuously changing climatic processes.
2.1 Zooplankton community structure and associated regulators
Community structure is determined by both physical and biological drivers that
influence species composition and diversity, species interactions and succession.
These influences have been the subject of many extensive investigations in temperate
lakes (e.g. Brooks, 1969; Makarewicz and Likens, 1979; Balcer et al., 1984), but to a
lesser extent in tropical environments (Green, 1967; 1976; 1985; Burgis 1971; 1973;
Lewis, 1979; Fernando, 1980a, 1980b; Fernando et al., 1987, Dumont, 1994a;
Masundire, 1997). Studies of zooplankton community structure have also driven
comparative studies among different freshwater ecosystems (Kerfoot and Lynch,
1987; Lehman, 1988). These have also enhanced understanding of general ecological
12
processes (Brooks and Dodson 1965; Patalas, 1971; Hall et al., 1976; Carpenter and
Kitchell, 1984; 1993). The body of knowledge that has accrued from both descriptive
and quantitative investigations of zooplankton community structure has been
invaluable to the advancement of both the theoretical and applied aspects of freshwater
ecology. Studies of zooplankton community structure have been conducted within the
framework of biogeography, community dynamics, whole lake metabolism, fishery
management, bioassessment and monitoring.
A key step to achieving the ultimate goals of ecological research within the various
frameworks is acquiring the basic knowledge of the number of species (species
richness) and diversity within the scope of study. Species diversity, or biodiversity,
constitutes a fundamental aspect of zooplankton community structure and is useful for
evaluating overall ecosystem attributes and functionality. Various ecological theories
have been proposed to explain the forces that shape species diversity in nature (Fisher,
1960, Paine, 1966, Connell and Orias, 1964, Chesson and Case, 1986). Local species
diversity is shaped by interactions among species and with the physical environment
which determine the success of immigrant species (Shurin, 2000). Integration of
several of these theories has produced the Stability-Time Hypothesis (Sanders, 1968)
which predicts greater species diversity over time in more stable environments. Based
on these models, tropical freshwater systems are expected to harbour a greater number
of species relative to systems in the temperate zone, but the impoverishment of
tropical limnetic zooplankton communities has been described as a paradox
(Hutchinson, 1961; Dumont, 1980; Fernando, 1980a, 1980b; Dussart et al., 1984;
Mitchell, 1986; Fernando et al., 1987; Kerfoot and Lynch, 1987). Pelagic zooplankton
communities of have “simplified food webs” (Dumont, 1994 a). Major effects of fish
13
and invertebrate predation are the restrictions imposed on the potential body size
spectra of prey (Soranno et al., 1993). Planktivorous fishes in tropical lakes have
developed very efficient mechanisms for feeding on zooplankton (Fernando, 1994).
Their uninterrupted breeding cycles reduces population variability and maintains a
high predation pressure on zooplankton consequently altering food web structure in
many lakes in tropical Africa (Dumont, 1986 a; Ligtvoet et al., 1989; Fernando, 1994).
In some instances, outright exclusion of herbivore zooplankton species (Isumbisho et
al., 2006) and delimitation of previously widespread group like the Branchiopoda to
fishless lakes have occurred (Kerfoot and Lynch, 1987).
In contrast to the limnetic environment, the littoral habitat is more complex and
possesses greater species richness and consequently elevated biodiversity (Fernando et
al., 1990, Dumont, 1994 a, Fernando, 1994). Diminution of species richness in
zooplankton communities in the tropics are known to be caused principally by life
history traits that enhance year round fish predation (Zaret, 1980; Lowe-McConnell,
1987; Lazzaro, 1987), invertebrate predation, absence of pelagic fishes (Kerfoot and
Sih, 1987; Fernando et al., 1990) and low zooplankton production (Fernando, 1994).
Predation by pelagic fishes appears to increase zooplankton species richness by
reversing competitive exclusion through exploitative means and preventing certain
species from being selected against (Fernando et al., 1990b). Lehman (1988) has
proposed that constraints on survival strategies by adverse conditions, osmoregulatory
difficulties and narrow evolutionary diversification are other factors contributing to
low species diversity of pelagic freshwater zooplankton communities.
14
Several empirically-based mathematical models have been formulated to measure
species diversity. Examples are the Shannon-Weiner index (H) and that proposed by
Simpson (1949).
Physico-chemical factors and other biological phenomena are also responsible for
structuring zooplankton communities both in space and time. Examples of these forces
shaping zooplankton communities are biogeography and dispersal (Carter et al., 1980;
Shurin, 2000; Cottenie et al., 2003; Havel and Shurin, 2004), water chemistry (Tessier
and Horwitz, 1990), foodweb interactions (Hrbacek et al., 1961; Brooks and Dodson,
1965; Lynch, 1979). Lewis (1979) suggests that a comprehensive synthesis of
community structure should be based on a thorough understanding of the physicochemical
environment,
community energetics
and
evolutionary
adaptations.
Competition and predation have also been shown to be the proximal factors
controlling community structure (Kerfoot and Sih, 1987) and they are evolutionary
products of adaptive energetic strategies (Lewis, 1979).
2.2 The Trophic Cascade Hypothesis (TCH)
Insights into mechanisms of zooplankton community regulation by food web
interactions have been acquired through many experiments, notably whole-lake
manipulations out of which The Trophic-Cascade Hypothesis (TCH) in lakes,
proposed by Carpenter and Kitchell (1993) emerged. The TCH provides a key
synthesis of the interactive effect of nutrient, primary productivity and food web
dynamics and states that “nutrient input sets the potential productivity of lakes and
that deviations from the potential are due to food web effects” (Carpenter et.al., 1985).
Predation through size-selective feeding proves to be the principal driving force of the
15
TCH (Hrbacek et al., 1961; Brooks and Dodson, 1965) and the magnitude of its
cascading effect is expressed differentially at each level along the trophic gradient (de
Bernardi, 1981). Size-selective predation determines the size spectra of zooplankton,
particularly herbivores, and influences individual grazing and nutrient renewal rates
(Peters and Rigler,1973; Peters,1975; Hall et al., 1976, Bartell and Kitchell,1978 )
which are major contributions to the limnetic phytoplankton nutrient budget (Kitchell
et al., 1979; Lehman, 1980), especially during lake stratification (Bruce et al., 2006).
Size structure and composition of planktivores are piscivore controlled (Tonn and
Magnuson, 1982). Likewise planktivores and invertebrates affect the herbivore
community structure which in turn regulates the phytoplankton assemblage (Brooks
and Dodson, 1965; Sommer, 1989). Response indices of the zooplankton community
structure to these regulatory factors include predator-avoidance mechanisms such as
diel vertical migration (Zaret and Suffern, 1976; Stich and Lampert, 1981, 1984;
Gliwicz, 1986; Dini and Carpenter, 1988; Lampert, 1989) changes in total zooplankton
biomass (Soranno et al., 1993), alteration in relative contribution of individual taxa to
overall community biomass (Soranno et al., 1993), effect on size structure (Hall et al.,
1976) and alterations in demographic characteristics ( Hall, 1964; Wright, 1965).
2.3 Temperate and tropical scenarios of zooplankton community structure
Comparative assessment of the factors that control zooplankton community structure
in temperate and especially in tropical lake ecosystems, which is a primary objective
of this investigation, is of fundamental interest in community ecology. Temperate lake
ecosystems are marked by pronounced seasonal cycles of strongly alternating abiotic
and biotic controls (Lehman, 1980). Changes in the physical environment elicit
variable structural and functional responses in zooplankton communities. Populations
16
of individual species undergo regular, cyclic changes whilst abundances vary widely
in amplitude over brief seasonal periods (Pennak, 1949; 1955). These dynamic
characteristics contrast sharply with tropical systems which diverge from their
temperate counterparts in having high but limited range of annual irradiance and
predictable photoperiod (Lewis, 1974, 1987; Talling and Lemoalle, 1998) permitting
sustained high rates of primary productivity (Melack, 1979a; Hecky and Fee, 1981)
and uninterrupted zooplankton reproduction (Gras et al., 1967; Hart, 1981).
Furthermore, species composition remains relatively unchanged and zooplankton
biomass and production vary little over an annual cycle (Burgis 1971, 1974; Twombly,
1983).These observations led early workers to deem tropical zooplankton communities
to be characteristically aseasonal. Such conclusions largely resulted from short term
studies and anecdotal data acquisition procedures. Limited endogenous research
capacity to sustain long term observations and to keep the growth of the science in
phase with temperate trends compounded the problem. But with many data emerging
from intensive quantitative and qualitative investigations in the tropics, the
longstanding view about tropical zooplankton communities being constant is being
progressively displaced by a new paradigm that identifies inherent seasonal variability
of tropical freshwater ecosystems. This assertion is supported by several lines of
evidence from work conducted in the following tropical lakes; Lake Lanao (Lewis,
1979), Lake Malawi (Twombly, 1983), Lake Tanganyika (Narita et al., 1986) Lake
Valencia (Saunders and Lewis, 1988 a; b), Lake Tanganyika (Mulimbwa, 1988, 1991),
Lake Naivasha (Mavuti, 1990) Lake Nkuruba and Lake Nyahira (Kizito, 1998), Lake
Malawi (Irvine and Waya, 1999) and Lake Awasa, (Mengestou, 1989).
17
2.4 Zooplankton-Phytoplankton Relationship
The sustenance of zooplankton organisms depends primarily on the quantity and
quality of a reliable food supply (e.g. Persson, 2007). Though zooplankton exploit
various food sources, phytoplankton remains the principal source of quality nutrition
for zooplankton. Interactions between zooplankton and phytoplankton occur
predominantly at the herbivore interface and cascade to higher trophic levels. Grazing
by herbivores affect phytoplankton abundance and community composition (Gliwicz,
1977; Lynch and Shapiro, 1981; Reynolds, 1984a) and size structure through speciesspecific size selective feeding (Naddafi et al., 2007). The impact of size-selective
feeding may be visible and pronounced during phytoplankton seasonal succession
(Lampert et al., 1986; Sommer et al., 1986; Vanni and Temte, 1990). Additionally,
grazer effects are reflected by variations in the proportion of edible to inedible algae
(Sterner, 1989). However, large grazers have greater impact on the phytoplankton
assemblage than small sized zooplankton (Lynch and Shapiro, 1981; Schoenberg and
Carlson, 1984) through their superior feeding mechanisms which is an outcome of the
Size Efficiency Hypothesis (Hall et al., 1976). According to Burns (1968), Reynolds
(1984a) and Peters and Downing (1984), feeding rate and food selectivity by herbivore
zooplankton are strongly size dependent and have overall ecosystem implications
(Berquist et al., 1985). Available data suggests that tropical phytoplankton
communities also undergo seasonal variations (e.g Talling, 1966; Awortwi, 2009.) and
similar to their zooplankton counterparts the amplitude, scale and periodicity are lower
compared to temperate plankton communities (Lewis, 1978). Tropical phytoplankton
communities seem to be dominated by the Cyanophyta (Green, 1972; Burgis, 1973;
Lewis, 1973) and they nutritionally inhibit the growth and reproduction of herbivores
18
(Haney, 1987). Filamentous species such as Anabaena and Oscillatoria reduce
filtration efficiency of cladocerans (Gliwicz, 1980; Haney, 1987) whilst others
produce toxins (Arnold, 1971). However, calanoid copepods and to a lesser extent the
cyclopoid, have been found to efficiently utilize the Cyanophyta and this may
influence their dominance in tropical lake ecosystems. Isumbisho et al., (2006) in
comparing the relative trophic efficiencies of Lake Kivu zooplankton to those of lakes
Tanganyika and Malawi found the absence of calanoids in Lake Kivu to account for its
lower pelagic zooplankton production. This suggests that many common rotifer and
cladoceran herbivore species, notably Moina micrura, may be food limited due to their
high requirements for edible portions of algal food supply making them inferior
competitors for edible algae (Lehman, 1988; Romanovsky, 1984). Seasonal growth of
phytoplankton in response to climate driven nutrient supply mechanisms will likely
enable such inferior herbivore competitors to offset food limitation by exploiting the
increased fractions of smaller sized edible phytoplankton.
2.4.1 Chlorophyll a concentration (surrogate of phytoplankton abundance)
Due to the difficulty posed by qualitative work on phytoplankton communities
generally (Lewis, 1978), chlorophyll a concentration (Chl a.) is often used as a
surrogate of phytoplankton abundance and biomass in many community and water
quality analyses (Dillon and Rigler, 1974; Desortova, 1981; Isumbisho et al., 2006).
Chlorophyll a is the predominant form of phytoplankton chlorophyll pigment and can
act as a useful proxy to quantify relative amounts of algae by widely used fluorometric
and spectrophotometric techniques.
19
2.5 Energy Budgets: biomass and production of zooplankton
Bioenergetic approaches to understanding dynamic aspects of community and
ecosystem functioning are comparatively more useful tools for analyzing zooplankton
communities (Bottrell et al., 1976; Downing and Rigler, 1984; Cressa and Lewis,
1986). Inferences on relative importance of species in a community that are solely
based on numerical abundance as an index of growth and success of populations
potentially limit resolution of energy budgets and their associated controlling factors
within communities. Among the variables that could potentially serve as ecosystem
“currencies”, energy has been considered as the most instructive by ecologists
(Lindeman, 1942; Krebs, 1994). Biomass, defined as the total dry weight of organic
material per unit area present during a given period is one measure of energy accrual
whereas secondary productions describing the rate at which consumers accumulate
organic material constitutes another. Biomass is therefore the product of the
population density and the total organic dry weight of the population at one point in
time. Analyses of communities based on energy budgets and flux represent the best
means of evaluating the effect of environmental stressors on communities and
ecosystems (Krebs, 1994) because all the functional components of the community are
evaluated by one common denominator. Additionally, measurement of secondary
production is important for fishery management (Waters, 1977; Williams et al., 1977;
Irvine and Waya, 1999) and has been suggested to highly correlate fish production
(Hanson and Leggett, 1982). It has also been recognized that development of
freshwater aquaculture could adequately be driven by application of knowledge
obtained from secondary production processes (Johnson, 1974). Despite the valuable
information obtained from secondary production, its accurate measurement remains a
20
daunting task considering the myriad of variables that directly and indirectly affect the
process. Downing and Rigler (1984) in their seminal overview of the potential factors
that influence secondary production processes recognized these challenges and
stimulated investigations focusing on the leading determinants of secondary
production in freshwater ecosystems particularly the zooplankton and benthos. The
factors they identified as potential regulators of secondary production include
population characteristics such as biomass (Makarewicz and Likens, 1979; Lewis,
1979; Lewis and Saunders, 1988) life history (Waters, 1977) body size (Zelinka,
1977), taxonomic differences and trophic status (Makarewicz and Likens, 1979;
Nauwerck et al., 1980), physicochemical factors mainly temperature and oxygen
(Edmondson, 1974; Brylinski, 1980; Selin and Hakkari, 1982), food web interactions
and its effect on community dynamics (Brooks and Dodson, 1965; Hall et al., 1976;
Carpenter and Kitchell, 1993; Kitchell and Carpenter, 1993) and mechanisms of food
acquisition and nutrition. These factors have been shown to take preeminence in
regulating production and consequently dictating the energy budgets of zooplankton
communities. This is attested by the associated rigorous investigations that have
characterized the past two decades.
2.5.1 Effect of nutrition on zooplankton biomass and production
Many experiments on mechanisms of zooplankton feeding have hinged on the
herbivores because of their direct contact with primary production and their role as
conduits in primary energy transfer to upper trophic levels. Specifically, feeding
efficiency has been shown to be governed by behavioural responses to mechanical
constraints on feeding apparatus and food characteristics including type, configuration
and quantity (Starkweather, 1980 a; Gilbert and Starkweather, 1977). Variations in
21
these feeding characteristics may result in differences among herbivore species and
may permit niche segregation and promote coexistence of even related species within
the same habitat (Wetzel, 2001). Complex relationships exist among different
herbivore species in relation to filtering, feeding/ingestion and assimilation rates.
Suspension feeding rotifers have varying degrees of ingestion rates, food density and
selectivity (Gilbert and Starkweather, 1977). Cladoceran filtering rate vary with body
size, food characteristics and temperature (Peters and Downing, 1984; Wetzel, 2001)
whilst food concentration and body size may dominate in the copepod situation
(Wetzel, 2001). Generally, food composition, size, density and selectivity regulate
ingestion rates among the herbivorous zooplankton. Ingestion rate is however a poor
predictor of assimilation rates because not all food ingested is assimilated due to the
variable food types and qualities (Wetzel, 2001). It has been documented that when
cyanobacteria and large filamentous and colonial algae dominate phytoplankton
communities, ingestion rates, assimilation efficiency as well as growth, survivorship
and reproductive rates of herbivorous zooplankton decline significantly (Arnold, 1971;
Haney, 1987). Consequently shifts in seasonal algal community composition may
exert a major influence on the seasonal variation in zooplankton biomass and
production (Bergquist et al., 1985; Bergquist and Carpenter, 1986; Wetzel, 2001).
Analysis of the effect of food quality and quantity on zooplankton biomass and
production has focused on the role of threshold food concentration in maintaining
positive population growth (Lehman, 1988; Wetzel, 2001). Food quality-concentration
and nutritive balance of essential growth substances in an animal’s diet- is vital for
optimum growth (Edmondson 1974; Nauwerck et al., 1980) and a positive relationship
between primary production and secondary production has been documented (Dermott
22
et al., 1977; Makarewicz and Likens, 1979). Consequently seasonal alterations in
primary production controlling variables such as light intensity and nutrients
influences levels of secondary production (Hall et al., 1970). Similarly, herbivore
biomass has also been shown to have a positive relationship with phytoplankton
biomass (Hanson and Peters, 1984; Yan, 1986) though Saunders and Lewis (1988 b),
found no statistical relationship between the two variables. The common feature of
majority of the observed relationship between the variables points to much
unexplained variation in herbivore biomass or production. This discrepancy may be
attributed to peculiarities of ecosystem complexities regarding differences in
environmental factors and biotic interactions of predation and competition.
Though bacteria and detritus may represent alternative food sources for zooplankton,
their assimilation efficiencies are relatively lower compared to algae (Saunders, 1969;
Ojala et al., 1995). But in lakes with low phytoplankton production, bacteria and
heterotrophic protozoa may provide an important supplementary energy source to
zooplankton (Saunders, 1969; Jansson et al., 2000; Karlsson et al., 2002). It is now
well established that cyanobacteria affect zooplankton nutrition in four principal ways;
poor nutrition resulting in energy deficits incurred from low ingestion rates (Schindler,
1971; Arnold, 1971; Porter and McDonough, 1984) and consequently reduced growth
and reproduction (Haney, 1987), toxins released affect filtering efficiency (Lampert,
1981; Ostrofsky et al., 1983). However in general, herbivores are known to have
relatively higher productivities than detritivores or carnivores (Waters, 1977;
Jonasson, 1978) and calanoid copepods have been suggested to possess greater
assimilation abilities for cyanobacteria and thus may be an important trophic link in
tropical food webs where cyanobacteria they dominate the algal communities (Haney,
23
1987). Adequate supply of high quality food will permit growth rates to offset
respiratory losses and consequently enhance trophic efficiency of energy transfer
across food webs (Sterner and Elser, 2002; Hessen, 1992).
Trophic efficiency (TE) is the percentage of energy transferred from one trophic level
to another. TE varies significantly among rotifers, cladocerans and copepods. Earlier
studies undermined the importance of rotifers in ecosystem functioning (Brooks, 1969;
Porter, 1977) mainly due to inaccurate biomass and production measurements
(Schindler, 1972). However, many evidence suggest that rotifers may be relatively
more important compared to the Cladocera and Copepoda in having high biomass
turnover times and rates (Schindler, 1972; Makarewicz and Likens, 1979) and should
be an integral component in analyses of the functioning of zooplankton communities
and overall ecosystem metabolism.
2.5.2 Measurement of biomass and production
The productivity of a population is measured by the cumulative growth of each
individual in the population (Wetzel, 2001) including biomass of reproductive
products. This is determined as net productivity in excess of respiration and
maintenance losses. Accurate assessment of the productivity should involve
knowledge of the distribution of the population, the different stages of development,
age distribution and the generation times (Wetzel, 2001). Most secondary production
measurements are aptly described as estimates due to limitations imposed by the
population characteristics such as emigration and immigration and variation of
population characteristics among different species and seasonal aspects of population
dynamics (Edmondson and Winberg, 1971; Edmondson, 1974; Bottrell et al., 1976;
24
Downing and Rigler, 1984). The workers also ascribe methodological limitations and
sampling intervals to measured deviations from the true population productivities. The
initial step towards calculating the production of a population is the determination of
the weight of individual organisms.
Several methods have been used to measure the weights of zooplankton. These are
length-wet weight regressions (Lewis, 1979; Mengestou and Fernando, 1991), dry
weight approximated from biovolume measurements (Ruttner-Kolisko, 1977) and
direct dry weight measurements (Burgis, 1974; Dumont et al., 1975; Dumont and
Balvay, 1979; Culver et al., 1985; Masundire, 1994; Irvine and Waya, 1999).
Physiological methods such as carbon content as an index of dry weight (Burgis,
1971) are also reported in the literature. With the exception of direct dry weight
measurements which has now been well established as the most reliable and
recommended method, all other methods are subject to considerable error margins that
magnify in the final calculations of production and reduce the level of accuracy.
Dumont et al (1975) have noted that variations in the water content of organisms and
their corresponding differences in “volume/surface ratio” may contribute to erroneous
results on fresh weights. Dry weights also enable effective comparison of production
among various within-ecosystem functional components as well as among lakes of
different trophic status. This is plausible for meso- and macrozooplankton assemblages
but not tractable for rotifers (Ruttner-Kolisko, 1977). Suggestions for determination of
rotifer dry weights have been directed towards dry weights approximated from
biovolume measurements using morphological shapes of rotifers fitted to regular
geometrical formulae (Ruttner-Kolisko, 1977). The limited number of direct dry
weight measurements in the literature is an attestation of the laborious nature of the
25
exercise. Replication of the method for individual organisms for every sample is
hugely impracticable. To overcome this obstacle, species specific size-based lengthweight regressions are developed to estimate dry weight of individual organisms and
developmental stages. This permits many samples to be analyzed for corresponding
dry weights once the specific lengths are known.
The literature contains a diverse set of methods for the final calculation of secondary
production. The choice of a particular model for production of a particular population
under study depends on some assumptions that should be made about the behaviour of
that population (Downing and Rigler, 1984). However, two simplified models that
describe the two major categories of the population growth characteristics in natural
populations are frequently used to estimate production. The Growth Increment
Summation (GIS) method (Winberg, 1971) and the Instantaneous Growth Rate (IGR)
Ricker method. Growing populations could either have recognizable cohort structure
or continuously growing population whose generations overlap and cohorts are not
clearly distinguishable. This latter situation typically describes the characteristics of
zooplankton populations. The GIS model is most effective when population growth is
linear (Downing and Rigler, 1984) and is described by the following relation;
P = N (W max- W min)
D
Where (P) is the average production in a particular size class or stage per unit time,
(N) is the average population density at time (t)
Wmax (maximum) and Wmin
(minimum) weight of the population during a given interval and (D) is the
development time or duration of the population. Population herein refers to age class
or developmental stage. This method assumes uniform age distribution and linear
26
growth increment throughout the life cycle of a population and can be used to
approximate the production of a continuously growing population that has no
recognizable cohort structure. All growth increments for each developmental stage
within a specified interval are summed up to obtain the production of the entire
population. Adult production is determined as the number of eggs produced during a
given time period. Additionally, the method assumes zero mortality within the time
interval because mortality is difficult to quantify in natural populations as well as
development time which is mostly determined through laboratory simulation
experiments of natural conditions. The IGR assumes individual exponential growth
rate within all size classes (Downing and Rigler, 1984) and is most ideal for steady
state populations. The IGR is also described by the expression;
G = 1 ln (Wmax)
D Wmin
In this growth model, G is defined as the instantaneous growth rate constant and is
calculated as the exponential rate of increase for a population that has constant age
distribution, (D) is the development time at each life stage, (Wmax) the maximum
weight of each size class and (Wmin) the minimum weight within the size class or life
stage The daily average production (P) of the population is derived by the product of
the sum of biomass increments (B) and (g) of each size class or stage;
P = ∑giBi
Similarly, the IGR is accurate when populations exhibit an exponential growth rate.
2.5.3 Limitations to measurement of secondary production
The difficulty associated with the measurement of secondary production is a reflection
of the dearth of information on biomass and production generally in the tropics and
27
particularly in African lake ecosystems. Few studies on zooplankton dry weight,
biomass and production have been conducted predominantly on crustacean
components as opposed to the frequency of investigations on population densities and
community structure for Lake Awasa (Mengestou and Fernando, 1991), Lake George
(Burgis, 1971, 1974) Lake Kariba (Masundire, 1994), Sri Lanka (Amarasinghe et al.,
1997), Lake Malawi (Irvine and Waya, 1999), Lake Lanao (Lewis, 1979) Lake
Nkuruba and Lake Nyahira (Kizito, 1998). Unfortunately rotifers have largely been
excluded from these studies. (Makarewicz and Likens, 1979) have however shown that
delimitation of rotifers in any functional analyses of communities is a disincentive to
elucidation of the energetics and relative importance of different populations to
community dynamics. Makarewicz and Likens (1979) indicated that rotifers possess
high biomass turnover rates and production and contribute substantially to the total
energy flux. Other workers have also demonstrated that biomass and production of
microzooplankton which includes rotifers and protists may sometimes exceed the
contribution of meso- and macrozooplankton to total zooplankton biomass and
production (Carrick et al., 1992; Weisse, 1997; Taylor and Johansson, 1991).
2.5.4 Production-Biomass Ratios (biomass turnover rates)
Productivity and biomass can vary among populations in different lakes ecosystems
and in different seasons within the same lake. The relationship between production (P)
and biomass (B) has commonly been expressed as a ratio (P/B) and is a useful index of
the trophic status and productivity of different lake ecosystems and facilitates the
comparison of different ecosystems with regards to gradients in productivity and
elucidation of the factors that control population growth (Wetzel, 2001). It is also
useful for assessing the fit of populations under changing environmental conditions
28
and disturbances (Wetzel, 2001). P/B is a weighted mean value of the rates of biomass
accumulation of all individuals in a population (Benke, 1993). Factors that affect P/B
ratio include temperature, body size, biomass and food availability (Nauwerck et al.,
1980).
Environmental factors such as temperature and oxygen are known to affect secondary
production processes. The effect of temperature is directly related to the mechanics of
food acquisition in relation to feeding rate and to the physiological processes of
ingestion and assimilation rates. Temperature has been shown to largely determine the
rates of biomass accumulation (Shuter and Ing, 1997), whereas the potential amount of
biomass that can be accrued depends on the food supply and individual body size
(Wetzel, 2001). However, the effect of temperature may be of lesser importance in
tropical environments due to the relatively narrow temperature range and low
amplitude of temperature variation over the seasonal and annual cycles.
2.6 Vertical distribution of zooplankton: character and seasonality
In thermally stratified lakes of temperate and tropical regions, vertical gradients in
light, temperature, oxygen, food availability, interspecific competition and predation
risk provide different habitat qualities over depth for zooplankton. Consequently,
zooplankton actively distribute themselves among various depth habitats in response
to varying degrees of alterations in habitat quality (Primicerio, 2000; Lampert, 2005).
Changes in vertical distribution can vary hourly, daily, seasonally and annually, with
respect to its timing and amplitude (Williamson et al., 1996; Primicerio, 2000). Diel
vertical migration- the daily upward and downward movement by some zooplankton taxa- is
evidence of habitat shift by zooplankton when favourable conditions in a local habitat
29
decline over repeated, regular intervals within each 24 hr period (Lampert, 2005) and
can be considered to be a response to daily changes in habitat conditions with depth.
Various factors may operate to drive differences in habitat selection among
zooplankton. Differences in competitive ability among competing prey species, often
with a strong size- dependent component (Hall et al., 1976), result in spatial
segregation and influences the pattern of vertical distribution (Primicerio, 2000). Sizedependent predation by vertebrates (principally fish) and invertebrate predators may
force prey species into tolerance of suboptimal depths (Primicerio, 2000) and allow an
inferior competitor to occupy the vacated habitat (Primicerio, 2000). The depth
distribution of light may also enhance escape of prey species from predators in
hypolimnetic refuges of deeper, darker depth habitats (Gliwicz, 1986; Lampert, 1993).
Competition and predation thus complement each other in shaping the character of
distribution among competing prey species.
One of the selective advantages of vertical distribution is that it promotes co-existence
of competitors within a water column when they select different depth habitats of
varying food availability and predation risk (Liebold, 1990; Primicerio, 2000). This is
particularly important for high latitude subarctic lakes which possess short ice-free
seasons that do not favour succession of zooplankton (Primicerio, 2000) relative to
temperate lakes with a longer growing season and distinct successional patterns among
zooplankton (e.g. Burns and Mitchell, 1980).
Abiotic, physico-chemical factors influence the vertical distribution of zooplankton.
Vertical gradients in light create a hypolimnetic refuge for prey species to escape from
visually-feeding predators (Gliwicz, 1986; Lampert, 1993). Similarly, temperature is
30
related to enhanced feeding abilities in warmer, food-rich surface waters versus
efficient metabolism in low temperature deeper waters which may lead to increased
reproductive potential (McLaren, 1974; Enright, 1977). Burns and Mitchell (1980)
have further demonstrated that vertical distribution of zooplankton can be related to
both the depths at which the thermocline and hypolimnetic anoxia occur. Seasonal
aspects of these abiotic factors further result in differences in the pattern of vertical
distribution.
A broader dispersion of zooplankton throughout the water column during lake mixing
and marked spatial segregation during lake stratification has been documented (e.g
Gophen, 1979; Burns and Mitchell, 1980). However, in subtropical and shallow
equatorial poly holomictic lakes, diurnal stratification reduce the thermal gradient
between mixed layer and deeper waters and mixing throughout the annual cycle and
mutes vertical distribution patterns of zooplankton (Talling (1964), Ganf and Horne
(1975), Seaman et al., (1978) all cited in Mavuti, 1992). Such situations provide an
interesting aspect of community dynamics in comparison to the limnological
behaviour of monomictic and dimictic lakes. Patterns of vertical distributions of food
resources can also affect the nature of zooplankton vertical distribution. Liebold
(1990) showed that herbivorous Daphnia can alter its habitat selection by responding
differentially to changing food conditions in both the epilimnion and hypolimnion.
Usually food concentrations and temperature (rapid pace of development) in the
epilimnion are expected to be greater than the hypolimnion. Thus, the epilimnion
becomes the preferred habitat by most zooplanktonic organisms. However, the
increased risks of predation from visual predators in the epilimnion may offset the
advantage gained by eplimnetic residence. Large and conspicuous zooplankton are
31
therefore faced with a trade-off between optimum epilimnetic conditions and
migration to hypolimnetic refuges of sub-optimal conditions (e.g Lampert, 2005). The
typical difference in eplimnetic and hypolimnetic food conditions are challenged by
observations that deep-water, hypolimnetic algal maxima exist in certain oligotrophic
and mesotrophic lakes and that, these algal maxima are higher than epilimnetic algal
concentrations (Fee, 1976; Barbiero et al., 2001). This situation has the potential to
influence the pattern of vertical distribution during periods when food concentrations
shift between the epilimnion and hypolimnion.
2.6.1 Diel Vertical Migration (DVM)
Most zooplankton migrate vertically through the water column on a daily basis. They
leave the warmer, higher irradiance and often more food-rich epilimnion during the
day to reside in the often colder, darker and poorly resourced hypolimnion. Several
reasons, drawn from a myriad of empirical investigations, have been put forward to
explain this observation. Yet, no ultimate causes for this phenomenon have been
found. The factors identified as important causes of diel vertical migration (DVM)
include; intensity of visual predation (e.g Zaret and Suffern, 1976; Stich and Lampert,
1981; Liebold, 1990), light intensity (e.g Hart and Allanson, 1976; Gophen, 1979;
Andersen and Nival 1991), food levels (Gliwicz and Pijanowska, 1988; Liebold, 1990)
and oxygen concentrations (Burgis et al., 1973).
DVM can be a fixed or an induced behaviour (Pijanowska, 1993) and several
hypotheses have been formulated to explain the adaptive significance of DVM (e.g
Enright and Honegger, 1977). Principal among them are: the metabolic-demographic
advantage, which predicts a physiological-reproductive benefit by organisms during
32
diurnal hypolimnetic residence (McLaren, 1974; Enright, 1977; Enright and Honegger,
1977), photo-damage hypothesis, that solar radiation may be injurious to plankton
(Williamson et al., 1994), the predation hypothesis, that vertical migration minimizes
predation risk (Zaret and Suffern, 1976; Stich and Lampert, 1981) and recently the
hunger-satiation hypothesis, that states that vertical movements of zooplankton are
driven by hunger and satiation (Pearre, 2003). Among these hypotheses, predation risk
is the most examined, and seems to commonly overlap with the other factors to
explain overall patterns of vertical migration in different lake systems. There are also
cost implications of DVM on most migrating zooplankton. A primary cost in DVM is
the low food availability and reduced temperatures (Williamson et al., 1996) in deep
waters, which further reduces both reproductive potential and development rate.
Additionally, diversion of energy of growth and reproduction into swimming for
extensive migrations and reduction in feeding time are inevitable costs of DVM, which
further reduces population growth rate of migrating zooplankton (Dodson, 1990).
These vertical distributions and “migrations influence the population and community
ecology of zooplankton, the trophic dynamics of aquatic food webs and the vertical
transport of nutrients in the water column” (Williamson et al., 1996).
2.6.2 Synthesis of tropical aspects of vertical distribution studies
Vertical distribution and migration have been studied on different scales in tropical
freshwater ecosystems. Tropical systems seem to follow subarctic and temperate
systems in the trend of phenology of vertical distribution patterns. In tropical systems
the uninterrupted annual cycle permits continuous growth and reproduction of
zooplankton organisms. Organisms therefore have little option, but to adapt and
coexist with competitors and predators. Moreover, persistent stratification especially in
33
deep lakes (e.g Tanganyika, Malawi), and shallow lakes with regular diurnal mixing
over the annual cycle, e.g George (Uganda), Naivasha (Kenya), will undoubtedly
enhance understanding of zooplankton community dynamics, and provide interesting
perspectives on vertical distribution patterns in tropical systems.
Most available data on vertical distribution in tropical and African lake systems have
been drawn from studies of shallow equatorial and subequatorial lakes that lack
permanent stratification of the water column (Burgis, 1973; Talling (1964), Ganf and
Horne (1975), Seaman et al., (1978) all cited in Mavuti, 1992). Consequently, absence
of mass vertical migration by zooplankton in these systems has been highlighted
(Mavuti, 1992) and in tropical systems, dissolved oxygen tolerances, food availability
and predation risk have been suggested to be the dominant controlling forces of
patterns of zooplankton vertical distribution (Kizito, 1998). In temperate lakes
however, temperature, predation, dissolved oxygen and food availability take preeminence as factors regulating vertical distribution of zooplankton (Zaret and Suffern,
1976; Burns and Mitchell, 1980; Stich and Lampert, 1981).
34
CHAPTER 3: MATERIALS & METHODS
3.0 Study Site: Lake Bosumtwi
3.1 Morphometry (Origin and Relief)
Lake Bosumtwi sits in a 1.07 Myr old meteorite impact crater (e.g Koeberl et al.,
1997a). The crater has a rim-to-rim diameter of 10.5 (± 0.5) km and the lake’s
diameter is 8 to 8.5 km across the lake surface (e.g Koeberl, et al., 2007). Lake
Bosumtwi is 78 m deep and centered at 06°30' N and 01°25' W in the Ashanti Region
of Ghana, West Africa, about 32 km from Kumasi the regional capital. The origin of
the lake was a subject of controversy for many years and many hypotheses regarding
its origin were put forward. The predominant view was a volcanic origin, but Ferguson
(1902) opposed this view whilst Kitson (1916) proposed a subsidence feature.
Rohleder (1936) explained the formation of the crater based on endogenic explosive
mechanisms. Maclaren (1931) was the first to postulate an impact origin. Many recent
evidences have confirmed the impact origin of the lake particularly Koeberl et al.,
(2007).
The basin is hydrologically closed (e.g Turner et al., 1996 a). The contribution of
inflowing streams to the total annual water budget is < 20 % (Turner et al., 1996 b).
The crater is steep sided, with the highest point of the crater rim being 469 m above
msl. The steep sides are broken by a terrace (Jones et al., 1981) at about the same
elevation as the lowest point on the rim (210 m). The crater has a surface area of 106
km² with 52 km² currently occupied by the 78 m deep lake (Turner et al., 1996 a). The
lake is maintained by precipitation on its surface and the surrounding small watershed
35
through few small streams originating from the crater walls and water levels are
modified by losses via evaporation (Beuning et al., 2003).
Lake
Bosumtwi
Figure 3 (a): Map of Ghana showing the geographical location of Lake Bosumtwi
N
CS
Source: Google.com
0
2
4 km
Figure 3 (b): Bathymetry of Lake Bosumtwi showing the central sampling station (CS)
and the onshore docking site Abono.
36
3.2 Climate, Phenology and Vegetation Characteristics
Lake Bosumtwi is affected by two major air masses; the West African monsoon off
the Atlantic Ocean, which brings precipitation to the region during much of the year,
and the dry, northeasterly flow (Harmattan winds) that penetrate into West Africa
during boreal winter when the Intertropical Convergence Zone (ITCZ), a low pressure
atmospheric boundary, is displaced well south of the lake (Russell et al. 2003). The
extent of migration of the ITCZ determines the interannual climatic variability and the
intensity of convection within the ITCZ (Nicholson, 1980; Mamoudou et al., 1995).
The climate around Lake Bosumtwi is mainly characterized by a dry season in
December – February and a wet season in May – July. Annual rainfall in the region
averages 1380 mm (January to December), with a monthly maximum in June and a
secondary peak in October (Russell et al., 2003). However, (Puchniak, et al., 2009)
recorded annual precipitation peaks in May during 2005 and 2006 but lower annual
rainfall than the basin average of 1380 mm from 1969 to1992 reported by Turner et al
(1996b). Because of the restricted size of the watershed, about 80 % of the annual
water input to the lake is from rainfall directly on the surface, which makes the lake
water budget extremely sensitive to changes in the precipitation/evapotranspiration
balance (Turner et al., 1996 a, b). Water loss is primarily through evaporation from the
water surface (Turner et al., 1996 a). The lake lies within a moist semi-deciduous
forest vegetation zone of West Africa characterized by dominant canopy genera from
the Ulmaceae and Sterculiaceae (Hall and Swaine, 1981; Olson et al., 1983; White,
1983) including an area of natural Imperata cylindrica grassland occurring within the
northeast quadrant of the crater rim (Moon and Mason, 1967).
37
3.3 Limnological Characteristics
The lake waters are highly stratified with a well-mixed surface epilimnion and anoxic
hypolimnion below 15-18 m depth (Turner et al., 1996 a). Deep annual mixing usually
occurs in August (Talbot and Kelts, 1986; Turner, et al., 1996; Puchniak et al., 2009)
due to elevated wind speeds, low maximum daily atmospheric temperature and high
daily average humidity (Puchniak et al., 2009) and the combined effects of high
evaporation and radiative cooling of the surface waters (Talbot and Kelts, 1986).
During the strong mixing period in August, fish kills occur (Turner et al., 1996;
Puchniak et al., 2009) and this is a major harvest for fishermen. “Lake chemistry from
the deep water is more dilute than expected due to past overflow events (Puchniak et
al., 2009).
Average specific conductivity measured was 1150 μScm-1, alkalinity
10320 μmol L-1, salinity 0.32 g L-1, pH 8.9 and the carbon dioxide concentration was
59 μmol L-1”(Puchniak et al., 2009). Mean water temperature recorded was 27.76°C
and Secchi disc transparency ranged from 0.8 to 2.5 m. Highest transparency occurred
during April to July each year during the development of the deep water bacterial
Chlorophyll a maximum by the green sulphur bacteria, c.f. Pelodictyon clathratiforme
(Puchniak et al., 2009). The phytoplankton community is dominated by blue-green
algae (Whyte, 1975; Awortwi, 2009.), whereas information on the zooplankton
community is scanty. Eleven fish species were originally known to inhabit the lake
and its inflowing streams. Cichlids were the dominant group of fishes including the
endemic Tilapia discolor (Whyte, 1975). According to Whyte (1975), no true pelagic
dwelling fish exists but Sarotherodon multifasciatus shows a tendency towards a
pelagic habit although it is not independent of the “estuaries” and the littoral areas.
Recent oral reports from fishermen suggest that, in addition to the decline of fish
38
yields, several species of fish have become rare as the fish resources are subjected to
intense fishing pressure and degrading breeding sites. Additionally, three of the five
permanent inflowing streams which served as habitats and breeding sites for the fish
(Whyte, 1975) have dried up (personal observations) and may contribute to the
dwindling fish resources. Serious deforestation along the crater rim for agricultural
activities could account for the drying up of the previously permanent flowing streams.
Details of distribution, trophic relationships and breeding habits of fish populations are
presented by Whyte (1975).
The littoral region consists exclusively of a belt of emergent Typhus australis (Whyte,
1975) extending from the northern side to the south eastern section of the lake and
occasionally interrupted by short discontinuities. The opposite side of the shoreline is
rather steep and does not adequately support development of littoral vegetation, except
at the estuary of the Abrewa River. The extensive mats of Pistia found to be floating in
the “estuaries” (Whyte, 1975) have disappeared. The littoral serves as a breeding site
for the fishes in the lake (Whyte, 1975), but are subjected to considerable human
disturbance.
3.4 Palaeolimnological and palaeoenvironmental importance
Lake Bosumtwi is one of only 18 confirmed African impact craters (Koeberl, 1994;
Master and Reimold, 2000) and one of only 4 known impact craters associated with
tektite strewn fields (Koeberl et al., 1997 a). It is the longest and continuous
palaeoenvironmental record (Koeberl et al., 1997b) and also “one of the most widely
studied palaeoclimate archives in West Africa” (Russell et al. 2003). Beginning from
the 1970’s Lake Bosumtwi has attracted great international attention owing to its
39
importance to the palaeolimnological and palaeoenvironmental scientific communities
(e.g Talbot and Delibrias, 1977; Talbot and Delibrias, 1980; Talbot and Kelts, 1986;
Talbot and Johannessen, 1992). Talbot et al (1984) have previously demonstrated that
palaeoenvironmental records from Lake Bosumtwi have provided a crucial update in
understanding of tropical climate change and vegetation history and have the potential
to further enhance understanding of climate change over a broad part of the earth. The
2004 Inter-Continental Drilling Programme (ICDP) in Lake Bosumtwi was motivated
by the need to obtain comprehensive information on climatic variability in continental
West Africa over the past 1 million years in order to fill a major gap in the
understanding of global climate dynamics (Koeberl et al, 2007). The fine-scale
resolution of climate history from the Bosumtwi sediment records is expected to
enhance the predictive power of climate change over a broad part of the earth (Koeberl
et al., 2007).
My research involving investigation of the zooplankton community structure and
dynamics is part of a continuous 2 year (2005 and 2006) tripartite limnological
programme that aims to resolve the linkage between meteorology, hydrology,
sedimentology and aquatic ecology and thus to provide valuable information for
enhancing effective interpretation of the 1 million year old Lake Bosumtwi sediment
record recovered by the ICDP drilling.
40
3.5 Field Sampling
3.5.1 Community Structure, Biomass and Production
Bi-weekly zooplankton samples were collected during 2 successive years beginning
from January, 2005 and ending in December, 2006. Four presumptive lake circulation
events were predicted for the entire sampling period based on literature: two strong
mixing periods in August and two weak mixing periods in December/January (Turner
et al., 1996 a; b). Full circulation usually occurs in August whereas weak mixing
usually occurs in January (Whyte, 1975; Beadle, 1981; Talbot and Kelts, 1986).
Sampling took place between 09:30 and 12:00 each day at a fixed central station
throughout the period. Five replicate vertical hauls with a 63 µm mesh zooplankton net
(25 cm diameter) were collected during each sampling event between May, 2005 and
Mid-July 2006. A zooplankton net with 70 µm mesh was used to complete the
sampling programme due to loss of the 63 µm mesh net. The change in nets did not
evidently reduce the qualitative and quantitative coverage of zooplankton as they both
retained microzooplankton such as nauplii and rotifers efficiently. Each haul was
taken from 35m up to the surface in order to obtain quantitative measurements of
Chaoborus which maintained daytime anoxic hypolimnetic residence. The total
volume of water filtered was estimated to be 8.60 m³ according to the formula:
V= πr² x D
Where (V) is volume of water filtered in meter cube, (r) is the radius of filtering net
mouth opening and (D) is the distance hauled (Downing and Rigler, 1984). However,
only 40% (3.45 m3) of water corresponding to the top 15 m productive photic zone
was used for routine calculations of abundance and biomass. Based on the above
41
formula, the total volume of water within the 15 m column is 3.68 m3 representing
43% of the entire 35 m column. But 3 % was taken as sampling inefficiency as a result
of the effect of underwater currents on sampling. So 40 % of the volume of water
filtered was used for enumeration of abundance and biomass. Net efficiency was
estimated to be 90 % average. This was assumed on the basis of the mesotrophic
nature of the lake, shallow oxic and trophogenic zones and the consistent rapid
drainage of the nets when pulled out of the lake (W.D Taylor, personal
communication). No vertical hauls were conducted prior to May 2005. Prior to that
date, relative abundance was estimated from a volume-weighted-depth-integration
procedure using a Van Dorn sampling bottle. This was achieved by sampling 11 fairly
discrete depths between the surface and 35 m. Although this procedure sampled the
whole water column, avoidance of the sampling bottle by larger fast swimming
zooplankters was a concern. Beginning in May 2005, vertical net hauls were done at a
speed of 1m/sec to reduce net avoidance and increase the capture of fast swimming
meso- and macrozooplankton. All zooplankton samples were stored in 500 ml bottles
and subsequently preserved in 4 % sugar buffered formalin to retain eggs and reduce
distortions to cladocerans.
3.5.2 Vertical Distribution
Zooplankton samples were collected at 11 discrete depths; surface (0), 1, 2, 5, 8, 10,
12.5, 15, 20, 25 and 30 m of depth. The various discrete depths were categorized
according to the 3 thermal layers; surface to 10 m constituted the epilimnion, 10 to 15
m constituted the metalimnion (thermocline) and 15 to 30 m constituted the
hypolimnion. The separation of the water column into these discrete strata was based
on temperature profiles taken concurrently with plankton sampling. At least 5 discrete
42
depths were sampled in the epilimnion, whereas only 3 depths were sampled in the
hypolimnion. Due to the relative thinness of the metalimnetic stratum (10 to 15 m),
only 2 depths were sampled. This fairly fine scale sampling was largely intended to
improve coverage of vertical distribution patterns due to gradients in physical and
biotic factors. Sampling took place between January 2005 and December 2006 at
regular bi-weekly intervals at a fixed central station 06° 30' N and 01° 25' W that was
tracked with a Global Positioning Navigation System (GPS) and also marked with a
fishing buoy. On each date, sampling took place between 1130 hrs and 0030hrs noon,
when solar radiation would have been at its greatest intensity and underwater light
penetration would also be at its maximum. A 6 L Van Dorn water bottle was used to
take replicate samples at each depth, beginning from the surface to 30 m. The water
bottle was lowered to the desired depth and closed immediately to obtain a quantitative
sample of the larger and more motile zooplankton (Smyly, 1968). Water samples were
then filtered through a 25 µm mesh to retain zooplankton including small rotifers and
developmental stages of the copepod Mesocyclops bosumtwii. Prior to May, 2005, a
48µm net was used, but no difference in retention efficiency was detected. Filtered
zooplankton were stored in 125 ml glass bottles and preserved with 4 % formalin.
Secchi depth and temperature and oxygen profiles were measured using Hydrolab H 2 O
Multiprobe 6SBP, USA. Samples for analyses of chlorophyll a concentration were
also taken at each discrete depth alongside samples for measurement of zooplankton
densities. 500 ml of water samples were filtered through a 47mm diameter GFF filters.
Filters were then freeze-stored for subsequent analyses. Samples were not obtained
for chlorophyll a analyses between January and March and September to December,
43
2005 due to logistical constraints. During November 2005, due to technical problems
with the Hydrolab, temperature and oxygen profiles were not carried out.
3.5.2.1 Diel Vertical Migration studies
Diel (24 hr) changes in vertical distribution of zooplankton were studied on three
dates, corresponding to different limnological regimes of the lake; 29 and 30th July
2005 during the decent of the thermocline leading to the deep mixing in August; 26
and 27th May, 2006 during stratification, increasing transparency and development of
the seasonal deep algal maximum and 19 and 20th December, 2006, during the
Harmattan windy period which causes weak mixing of the lake. The study in May,
2006 was aimed at tracking the amplitude of the vertical migration of zooplankton to
the deep water Chl a maximum, in order to determine whether zooplankton were
accessing this deep water food resource as a result of the dwindling eplimnetic algal
food base during this period. Triplicate samples were collected at 11 discrete depths,
identical to sampling procedures for vertical distribution studies. On each date,
sampling started at 1200 hrs noon of the first day at a fixed central station, and was
repeated at 6 h intervals to 0600 hrs of the next day. Dissolved oxygen (DO) and
temperature were measured simultaneously with the Hydrolab H 2 O 6SBP multiprobe.
Only two temperature and oxygen profiles could be taken at 12 noon and 7pm on both
26 and 27th May and 19 and 20th December 2006 sampling dates due to the lack of
sufficient battery power to operate the Hydrolab for the entire 24 hours. Filtered
zooplankton were stored in 125 ml glass bottles and preserved with 4% formalin.
44
3.5.3 Chlorophyll a concentration
Water samples for Chl a analyses were taken with a 6 L Van Dorn water bottle over 10
discrete depths, ranging from 0 to 30 m excluding 25 m due to the reduction in Chl a
distribution in deep water. For each water sample, 500 ml of water was filtered
through a 47 mm diameter Whatman GFF filter and stored frozen until subsequent
analysis.
3.5.4 Measurement of temperature and oxygen profiles
To determine variation in depth of mixed layer and water column stability oxygen and
temperature profiles were measured with the Hydrolab recorder. Measurements were
usually made from replicate vertical lowering and raising of the profiler through the
upper 50 m of water at a speed of ~15 cm per second.
3.5.5 Laboratory Procedures
3.5.5.1 Taxonomic analyses
Identification of some of the zooplankton species were carried out with the assistance
of Prof. H.J Dumont, Department of Animal Ecology, Ghent University, Belgium and
Prof. Iskandar Mirabdulayev, Institute of Zoology of the Uzbek Academy of Sciences,
Tashkent, Uzbekistan. Only one species of copepod was observed and it is a novel and
perhaps endemic species (Mirabdulayev et al., 2007). All species of cladocera, and the
sole Chaoborus, C. ceratopogones were identified, but only a single species of rotifer
Brachionus calyciflorus was resolved in routine counts. Further taxonomic work on
the rotifers was carried out at the Biology Department, University of Waterloo,
Ontario, Canada, using the taxonomic aid by Fernando (2002) and fine resolution
computer image-assisted compound light microscopy with enhanced diagnostic output
45
(Northern Eclipse). Consequently, several other rotifer species were discovered and
identified.
3.5.5.2 Measurement of Species Diversity
The Shannon-Weiner Diversity Index (SWDI) was used to quantify the diversity of the
zooplankton species in the community. The SWDI is particularly sensitive to the
number of species and the relative abundances of the species (Krebs, 1985). The
SWDI is given by the following expression;
Where H’ = Index of species diversity
S= number of species
pi = proportion of total abundance belonging to the ith species
Evenness or proportion of individuals among species was also determined by the
expression;
Where H= observed species diversity
In S= maximum species diversity
3.5.5.3 Determination of abundance
3.5.5.3.1 Subsampling and enumeration procedures: abundance
To analyze abundance patterns, zooplankton were counted from 5 ml subsamples
drawn from well mixed samples. Subsampling was carried out with a manual pipette
of mouth diameter > 4 mm (Edmondson and Winberg, 1971) to eliminate potential
selective biases against large size individuals. More than 200 individuals of the most
common species were counted (McCauley, 1984). Where < 200 individuals of a
46
particular species were counted, additional subsamples were enumerated to overcome
the deficit. In contrast, ≥ 50 individuals of the less common species were counted from
several subsamples. Counting was done from a large gridded plastic tray, mounted on
a Zeiss inverted microscope at 100X magnification to record adult non-ovigerous
females, ovigerous females
and nauplii of the copepod Mesocyclops bosumtwii.
Adults of cladocera and B.calyciflorus species were also counted and recorded.
Individuals of the remaining rotifer species and all copepodite stages (CI – CV) of M.
bosumtwii, males of adult M. bosumtwii, ovigerous cladocera and embryos were also
counted and recorded using a compound light microscope. Copepodite stages were
usually sorted out based size and number of body segments > 500 individuals had been
counted. At least 100 individuals of each copepodite stage were enumerated and
measured. Coefficient of variation (CV) of several counts determined for each sorting
and counting session among copepodite stages was usually < 10 %. Prepas (1984) has
suggested that CV of counts < 10 % among subsamples best approximates the sample
mean. Measurements of stage-specific total body lengths of copepodites (Culver et al.,
1985) were accomplished with the Northern Eclipse imaging system. Identification,
measurement and counting were routinely performed at 20 X magnification. The CV
of counts determined for several counts of the other taxa also yielded a value of < 10
%. Major challenges to enumerating Chaoborus species were encountered because the
samples collected from Lake Bosumtwi often had high abundance of Chaoborus from
dense samples. Macrozooplankton such as Chaoborus present major challenges to
subsampling because it is difficult to obtain representative subsamples (Prepas, 1984).
To minimize these difficulties, sample bottles were vigorously shaken to uniformly
mix samples and 20 ml of mixed sample was rapidly poured into a petri dish. All
47
organisms in the petri dish were then allowed to float to the surface because most of
the Chaoborus instars normally floated at the surface in storage bottles and that those
that sank to the bottom were the relatively heavier pupae. Counts of all individuals in
the 20 ml sample were then carried out. Allowing organisms to float to the surface
generally enhanced picking and counting, but all instars and pupae in the petri dish
were counted. Frequently, > 300 individuals were counted and recorded for each
subsample. In this way, coefficient of variation determined for several replicate counts
was < 10 %, and compared favourably to the CV’s of the other taxa.
3.5.5.3.2 Vertical distribution and DVM
Laboratory analyses for both vertical distribution and DVM followed the same
procedure. The subsampling technique described by Schwoerbel (1970) was used to
enumerate zooplankton densities. A manual pipette of mouth diameter > 4 mm was
used to subsample organisms (Edmondson and Winberg, 1971) Counts of adults,
copepodites and nauplii of M. bosumtwii, as well as the rotifer Hexarthra intermedia
were made from successive subsamples drawn from well mixed samples until counts
of >100 individuals of each group were achieved. Subsamples were placed in a 1 ml
Sedgwick-Rafter counting chamber and mounted under a Zeiss inverted microscope at
100X magnification. Counts of the rotifer Brachionus calyciflorus and Moina micrura
were extremely low. Thus, to minimize counting effort on many subsamples from
dilute samples, samples were allowed to stand for at least a week to allow organisms
to settle. Sample volumes were then reduced by siphoning off the top 110 ml and
counting the sample residue in a series of 1 ml subsamples until counts of >50
individuals of the species of organisms involved were achieved. All Chaoborus in the
entire samples were counted and recorded.
48
Chaoborus were large enough to be
identified and sorted with the naked eye, but usually the petri dish containing
Chaoborus were placed on a dark background and illuminated by a bright light source
3.5.5.3.3 Calculation of abundance
Abundances of zooplankton taxa per cubic meter of water were estimated by the
general expression provided by Greeson et al (1977):
Abundance (m3) = Zooplankton/ml of conc. sample x Volume of conc. sample (ml)
Volume of water filtered (m3)
3.5.5.4 Chemical analyses of Chl a concentration
All Chlorophyll a concentration analyses were conducted at the Department of
Biology, University of Waterloo, Canada. Samples for chlorophyll a concentration
were analyzed according to the method described by Stainton (1977). In this method,
Chlorophyll was extracted in 90 % acetone. The Chlorophyll solution was then
fluoresced and the different absorption wavelengths of the Chlorophyll recorded. The
various wavelengths were then used to generate a factor that was substituted into an
equation (Stainton, 1977) to estimate the Chlorophyll concentration in the solution.
3.5.6 Statistical analyses for community structure
To describe fundamental statistical characteristics of variability and distribution of
data for each zooplankton taxon, data (except those from Keratella cochlearis and
Brachionus dimidiatus which had inadequate number of observations) were subject to
calculations to determine means, medians, standard deviations and coefficient of
variation. To resolve the seasonal and temporal correspondence between years in
patterns of zooplankton abundance, counts of each individual species or stage from
samples collected during the same week in the first year (January to December 2005)
were paired with counts from the same week in the second year (January to December,
49
2006). Pairwise Spearman rank correlation coefficients were then computed for each
pair of counts. Additionally, a linear correlation analysis was used to assess the
strength of relationship between Chl a and herbivore abundance. To do this, total
herbivore abundance was lagged 2 weeks behind Chl a to correspond to the time of
energy flow and accumulation at the next trophic level.
3.5.7 Data analyses for vertical distribution
Bi-weekly vertical distribution data were combined into monthly average depth
distribution of each zooplankton species and developmental stages studied. The
monthly mean population depth (MPD) of each zooplankton organism was also
determined to enhance understanding of the temporal changes in the amplitude of
vertical distribution due to alterations in gradients of habitat quality. MPD was
determined by calculating the sum of monthly depth of peak density for each
zooplankton taxon and dividing the sum by the number of sampling dates.
The mathematical expression is given by:
MPD= ∑PDPD
SD
where (MPD) is the mean population depth for each zooplankton taxa, (PDPD) is the
monthly peak depth population density of each zooplankton taxa and (SD) is the sampling
frequency. Additionally, the proportion of the population of each species residing in the
epilimnion was determined to highlight the preferred habitat by each zooplankton.
50
3.6 Measurement of biomass and production
3.6.1 Evaluation of samples
Field samples for the biomass and production analyses were obtained by the same
sample collection scheme for community structure and abundance analyses.
Quantitative samples were used to determine the densities and size-frequency
distributions of Chaoborus instars and pupae as well as adult males, ovigerous females
and non-ovigerous females and developmental stages (nauplii and individual
copepodite stages) of M. bosumtwii. The maximum number of eggs per clutch and the
average number of eggs per adult female copepod were also determined from
qualitative samples. Densities and size frequency of adult M. micrura, B. calyciflorus
and H.intermedia were also measured. Egg ratio – ratio of number of egg or
developing embryo to adult females of M. micrura was also determined and used to
estimate adult production.
3.6.2 Development of length-weight regressions
3.6.2.1 Determination of individual dry weights
Dry weights were determined for all Chaoborus instars and pupae, adults of
M.micrura and adult copepods (males and females) and each of the five copepodite
stages. Prior to the analyses, samples had been preserved for periods ranging from
three months to two years. Determination of dry weights of zooplankton organisms
from preserved samples has been shown to be minimally affected by preservation
effects of formalin (Dumont et al., 1975; Bottrell et al., 1976). They estimate that,
weight losses by organisms to preservation generally do not exceed 10 %. Thus, no
correction for preservation effects on weight loss was made. Samples were thoroughly
washed with distilled water for 30 minutes. Adults of M .bosumtwii and various
51
copepodite stages were identified based on the number of body segments. However,
differentiation of the last caudal somite was used as a discriminant between the fifth
copepodite stage and adults because both possessed equal number of body segments
(Fernando, 2002). Chaoborus instars were distinguished based on the head capsule
length, which was characteristic for each instar (Dumont and Balvay, 1979). Head
capsule length of Chaoborus was taken as the length from the base of the antenna to
the tip of the posterior tentorial pit (Sæther, 1970). Also, the point from the base of the
antenna to the base of the anal gills was measured as the standard body length (Sæther,
1970).
All organisms were sorted out according to taxon and corresponding developmental
stages in the case of the copepods. For each instar or developmental stage, body
lengths were measured on randomly selected individuals. Measurements of individual
lengths of organisms were carried out under a dissecting microscope to obtain a range
of lengths for at least 50 individuals. The range was calculated as the difference
between the maximum and minimum lengths obtained by the random exercise. The
number of size-classes was predetermined by dividing the range obtained by 5. Except
in a few cases where four size classes were used, five size classes which corresponded
to a uniform class width were routinely used for the regressions models. The size
class approach was used because it grouped together similar sized individuals which
then minimized the residual variance in the final regressions (Culver et al., 1985). The
body lengths of individuals were measured to the nearest unit-usually 60 µm at 20X
magnification for large zooplankton and 30 µm at 40X magnification for small
organisms. An ocular micrometer mounted on a Zeiss dissecting microscope was used
for the measurements. Copepod body length was measured as the length from the tip
52
of the cephalothorax to the base of the caudal spines (Persson and Ekbohm, 1980;
Masundire, 1994) whereas for M.micrura, from the tip of the anterior edge of the eye
to the base of the tail spine (Masundire, 1994).Usually for Chaoborus instars, 20
individuals were put into each size class, whilst for copepods ≥ 30 individuals were
assigned to each size class.
Each size class was put in labeled depression slides during sorting and measurement.
Each size class was then transferred into labeled pre-washed, pre-dried and preweighed aluminium boats after computing the mean lengths for each size class. The
aluminium boats containing the size classes were temporarily stored in covered petri
dishes, oven dried at 60 °C for 24 hours and later transferred into a dessicator and
allowed to further cool over dry CaCl 2 for 24 hours . The aluminium boats were reweighed on a Cahn C-31 microbalance using the 25 mg - 0.1µg sensitivity scale to
derive the mean weight for each size class.
The mean length (independent variable) and corresponding mean weight (dependent
variable) for each size class were used to develop species specific size frequency
length-weight regressions that were later used to compute the biomass of the
organisms. The relationship between length and weight was best fitted by the general
form of the allometric power (curvilinear) function;
Where W is the mean weight in micrograms, (a) is the intercept of the regression line,
(L) is the mean length in micrometers and (b) is the slope of the regression line. The
equation was linearized by natural log transformation into the form;
In W = ln a + b ln L
53
The log transformation makes the residuals independent of the independent variable.
Culver et al (1985) cautioned against the extrapolation of a regression equation from
one period to other times of the annual cycle even for the same organism. They
suggest that, comparison among regression equations developed for the same organism
at different times of the annual cycle should be integral in the assessment of the weight
of organisms. Therefore, regression equations for both stratified and mixing periods
were developed for each taxon and developmental stage except for the copepodites
where equations for only the mixing season were derived. This was based on prior
knowledge of minimal effect of seasonality on regressions for both adult copepods and
M.micrura and also to minimize effort. The naupliar regression equation was taken
from Culver et al (1985) for temperate Mesocyclops edax, with adjustment of the
slope. The original equation is given by:
W= 2.5968 L 1.6349
Modified equation with adjusted slope: W = 2.5968 L 1.0039.
The basis for the adjustment was due to the relatively higher dry weights of the
copepods in Lake Bosumtwi compared to the various copepod weights given by
Culver et al (1985) for M. edax. Egg weight of the adult female copepod was assumed
to be a fixed percentage of the weight of the first naupliar stage. The ratio of egg
diameter to the length of the first nauplii was determined and the result was
subsequently multiplied by the weight of the first naupliar stage, to obtain an estimate
of egg weight.
54
3.6.2.2 Determination of rotifer dry weights
Dry weights of the two dominant rotifers, B.calyciflorus and H.intermedia were
determined by first deriving the fresh weight from biovolume calculation by
application of appropriate geometric formulae (Ruttner-Kolisko, 1977) assuming a
specific gravity (density of any material divided by the density of water) of 1 (Pace
and Orcutt, 1981). This means that the density of the rotifers was assumed to equal
1g/cm3. The fresh weight was then converted to dry weight assuming that dry weight
was 10 % of fresh weight (Pace and Orcutt, 1981).
For the cone shape of
H.intermedia, the suggested equation is: V = 0.26 ab2
where (V) is the biovolume, (a) is the body length and (b) the body width. The ratio of
a to b was calculated as 0.694 expressed as: b = 0.694 a. This was then substituted for
b in the first equation. Thus the final biovolume relation was:
V = 0.125a3.
Similarly, the final biovolume equation for B. calyciflorus was determined as:
V = 0.276a³
For H.intermedia, 33% of the initial biovolume was taken as biovolume of appendages
(Ruttner-Kolisko, 1977). This was then added to the initial biovolume to obtain a final
biovolume of the organism. On the basis of a specific gravity of 1 for the rotifers, 1g
was converted to µg to yield 1 x 10-6 µg. This value was then multiplied by the specific
biovolumes calculated for individual H.intermedia and B. calyciflorus species to yield
the fresh weight. Fresh weight was subsequently converted to dry weight assuming dry
weight was 10 % of fresh weight (Pace and Orcutt, 1981).
55
3.6.3 Derivation of population parameters
Development times for zooplankton are normally difficult to determine in nature, so
laboratory experiments are often used instead. These setups are labour-intensive and
difficult to maintain under controlled conditions. Though not completely satisfactory
in reflecting natural conditions, acceptance of laboratory derived population indices is
justifiable due to the difficulty in estimating them in natural populations. Despite these
challenges, some workers have attempted deriving population parameters from natural
populations of copepods (Rigler and Cooley, 1974). Cohort analyses for Chaoborus
and copepods have successfully been derived from field data (Lewis, 1979). The
efficacy of such approaches is based on a sampling regime that is shorter than the
development time of each developmental stage of the population in order to effectively
capture the population changes within the time interval. Because laboratory
measurements were not carried out in this study and the sampling interval was longer
than the development times of the rapidly growing populations, population parameters
were extracted from data on other tropical congeneric species in the literature.
Development times for each of the Chaoborus instars were approximated from the
cohort analysis by Lewis (1979), whilst those for M. bosumtwii were extracted from
the laboratory measurements given by Irvine and Waya (1999) for Mesocyclops
aequatorialis aequatorialis in Lake Malawi, East Africa. Detailed stage-specific
generation times from the two sources are given below.
56
Table 3.0: Development times of individual developmental stages of M. a.
aequatorialis from Lake Malawi. Source: (Irvine and Waya, 1999).
Developmental stage
Egg
Nauplii
Copepodite I
Copepodite II
Copepodite III
Copepodite IV
Copepodite V
Sum ( Egg to Adult)
Mean stage
duration (days)
2.5
7.0
1.6
2.6
2.2
2.6
4.6
23.1
The total development time of 23.1 days agrees well with values reported by Saunders
and Lewis (1988 a), who determined the duration of the life cycle of Mesocyclops
decipiens in tropical Lake Valencia, Venezuela to be 21 days.
Table 3.1: Development times of individual tropical Chaoborus instars from Lake
Lanao, Philippines. Source: (Lewis, 1979). Mean values calculated with permission.
Instar
I
II
III
IV
Sum
Mean Development
time (days)
11.00
8.17
12.80
13.50
45.47
Copepod female production was calculated as the egg production using the equation of
Sabatini and Kiorboe (1994).
EP = N (Eggs) x Egg Wt x D t
N (Females) Female Wt
57
where (EP) is the female egg production, N (Eggs) the egg density, N (Females) the
density of both ovigerous and non-ovigerous females, (Egg Wt) is the dry weight of a
single egg, (Female Wt) is the mean weight of adult female and (Dt) is the
development time of eggs. Similarly, production of M. micrura has been shown to be
equivalent to the production of eggs. “Conceptual and practical difficulties are
associated with calculation of finite birth rate” (Downing and Rigler, 1984). Finite
birth rate describes the average birth rate over an interval of time whereas
instantaneous birth rate specifies the birth rate at a point in time. Because populations
of M. micrura grow exponentially (Murugan, 1975), their birth rates are best
approximated by exponential equations. Several assumptions underline the use of the
exponential equation to estimate birth rate of M. micrura. The finite birth rate of M.
micrura for the sampling interval was estimated using the egg ratio equation of
Edmondson (1968). The life span of M.micrura is estimated to be 13 days between 28°
and 30°C (Murugan, 1975), which is approximately equal to the 14 day sampling
interval of this study. The temperature at which the generation time was determined
falls within the temperature range of the water column of Lake Bosumtwi. The finite
birth rate (β) is given by:
where (β) is the finite birth rate over the sampling interval, (Negg) is egg density in the
population, (N) is the number of females in the population and (D) is the egg
development time. (β) was then converted to instantaneous birth rate under the
assumptions that; birth rate (b) was not equal to death rate (d) during the interval and
that death rate (d) was greater than zero- b ≠ d > 0. These assumptions underlie the
58
equation given by Caswell (1972), which was adopted in this study to convert finite
birth rate to instantaneous birth rate for M. micrura populations. Caswell’s equation is
expressed as:
Where (b) is the instantaneous birth rate, (β) is the finite birth rate, (r) the
instantaneous rate of population increase and (e) the exponential function. The validity
of the conversion of finite birth rate to instantaneous birth rate is subject to the explicit
exponential growth of the population. Furthermore, (r) is given by:
Where (r) is the instantaneous rate of population increase, (t 1) is the initial time of
population size, (t 2 ) the final time of population census, (N 1 ) is the population density
at (t 1 ) and N 2 is the population density at (t 2 ) .
The egg development time of 2.15 days of M.micrura was taken from Lewis (1979) at
mean water temperatures of 25°C at Lake Lanao. Egg development times of
cladocerans are known to be temperature-dependent. Correction was made for the
mean water temperature of 27.76°C of Lake Bosumtwi. Thus, egg development time
for M. micrura in Lake Bosumtwi was estimated at 1.94 days.
3.6.4 Routine body length measurements
Routine body length measurements were carried out on all samples for various instars
and pupae of Chaoborus and adults of B. calyciflorus, H.intermedia and M.micrura,
to derive mean lengths of organisms for estimation of mean weights using the
respective regression models. Similarly, body lengths of nauplii and individual
59
copepodite stages, adult non-ovigerous female and male copepods were routinely
measured. All six naupliar stages were treated as one unit and usually > 150
individuals were measured. Similarly > 40 individuals of adult male and female
copepods and M. micrura were measured and recorded. The number of individuals
each of adult B. calyciflorus and H. intermedia measured was usually >50. Mean
lengths were then computed and subsequently used to estimate corresponding dry
weights of each instar and adults using respective length-weight regressions. The
maximum (Wmax) and minimum (Wmin) weight of each taxon and developmental
stage on each sampling date was simultaneously determined from the routine number
of individuals measured.
3.6.5 Computation of secondary production
The Growth Increment Summation method (Winberg, 1967):
P = N (Δ W)
D
was used to compute the average production of nauplii and copepodites over the
sampling interval. Where (P) is the average production over the sampling interval, (Δ)
(W) is the change in dry weight within each size class or life stage, (N) is the
numerical abundance of a specific size class and (D) is the development time.
Because growth in copepods ceases at adulthood, adult production is given as the
production of eggs. Production lost to mortality is not accounted for by the production
models used, as determination of mortality in continuously growing populations is
difficult (Rigler and Cooley, 1974). Calculation of production of M. micrura followed
60
application of the equations by Elster in Edmondson and Winberg (1971). The
population production is described by:
Where (P) is the production, (P N ) is the recruitment of new individuals and (W) is the
mean adult body dry weight. The P N is also estimated by the following expression:
Where (N F ) is the total number of females and (B) is the instantaneous birth rate.
3.6.6 Calculation of Production-Biomass (P/B) ratios
P/B ratios which provide estimates of the biomass turnover rates were determined for
each taxon and developmental stage except for rotifers. Daily turnover rates were
computed from the biomass on each sampling date over the daily production. Annual
P/B ratios of individual populations as well as for the entire community were also
determined from the mean total monthly biomass over the total annual production.
Daily P/B ratios of individual taxa and developmental stages, and annual P/B ratio for
the entire community were compared with values from other tropical lakes and the
mean of temperate lakes to evaluate the level of productivity of the zooplankton
community in Lake Bosumtwi and to help explain differences and similarities among
the different lake ecosystems.
3.6.7 Statistical analyses
Analysis of variance (ANOVA) was performed on each zooplankton species and life
stage regression model to determine whether the slope of the regression line (value of
b) was significantly different from zero. The slopes and intercepts of all Chaoborus
instars were compared between the stratified and mixing seasons to determine if the
61
pattern of growth was influenced by season. Based on this assessment, if no significant
difference was found between the mean regression slopes of each instar between
seasons, then the regression which had the highest correlation coefficient was used as
representative equation for the routine estimation of mean weights from mean length
for each taxon. A general regression equation for all instars for each season was also
developed. All regression analyses were performed using SIGMA PLOT statistical and
graphing program version 10 whereas SYSTAT version 10 and SPSS were used for
the remaining statistical analyses.
Linear correlation analyses between total herbivore biomass and Chl a were performed
as well as individual herbivore biomass and grazeable phytoplankton biomass to
examine the strength of relationship between the variables. Due lack of data on
photosynthetic microbial abundance, Chl a was taken as the sum of microbial and
algal chlorophyll which is used to indicate total abundance of potential food supply for
herbivorous zooplankton. Grazeable phytoplankton biomass was determined as the
average of the mixed layer phytoplankton biomass (Awortwi, 2009) The Chl a and
phytoplankton biomass data were lagged 2 weeks behind the herbivore biomass to
capture the biomass increment response time by the herbivores to the increase in
phytoplankton biomass. Data for phytoplankton biomass was provided by (Awortwi,
2009) whose phytoplankton community and primary production studies run
concurrently with the zooplankton studies.
62
CHAPTER 4: RESULTS
4.1 Species composition, community diversity and trophic structure
The zooplankton community in Lake Bosumtwi was composed of nine taxa (Fig. 4 (a)
to 4 (d) and Appendix Table A). Mesocyclops bosumtwii and Chaoborus
ceratopogones both of which co-occurred as the sole species representatives of the
copepoda and of the Chaoborus, constituted the invertebrate predator group and were
present in all samples collected throughout the study period. Similarly, Moina micrura
was the sole cladoceran and together with the rotifers constituted the herbivores in the
community. With the exception of Hexarthra intermedia, the remaining rotifer taxa
together with M. micrura showed irregular occurrence in the community. Species
diversity and evenness was higher in 2006 compared to 2005 due in part to reduced
species number in 2006 resulting from the absence of Brachionus dimidiatus in the
community (Table 4.0). The copepod M. bosumtwii was the most common species.
Both M.micrura and B.calyciflorus showed variable abundances between years
compared to the other taxa, which exhibited only slight variation in abundance.
Keratella cochlearis was particularly very rare. H.intermedia was an exception to the
rarity of the rotifers by being the second most common species in the community.
Additionally, the pattern of diversity was repeated in both years and the major feature
of the community diversity is that the numbers of species that are common are
relatively fewer than the number of species that are rare. Most of the rotifers fell into
this rare category except H.intermedia which was the most common among the rotifers
and its proportion to the community abundance remained unchanged in both years.
Abundance of B.calyciflorus was reduced in 2006 and contributed to a decline in
overall community diversity.
63
Mesocyclops bosumtwii Mirabdulayev, Sanful, Frempong 2007 (Adult males)
(Copepod)
Copepod adult female carrying eggs
Moina micrura Kurz 1974
Figure 4 (a): Adult copepod males, adult copepod females of Mesocyclops
bosumtwii and the cladoceran Moina micrura in Lake Bosumtwi at 100 X
Magnification. Reference organisms are indicated by arrows.
64
Copepod Copepodites
Copepod Nauplii
Hexarthra intermedia Wiszniewski 1929
Brachionus calyciflorus Pallas 1766
Figure 4 (b): Photomicrograph representation of copepodites and nauplii of
Mesocyclops bosumtwii and the rotifers Hexarthra intermedia and Brachionus
calyciflorus in Lake Bosumtwi at 100 X magnification. Reference organisms are
indicated by arrows.
65
Filinia pejleri Hutchinson 1964
Brachionus dimidiatus Bryce 1931
Filinia camascela Myers 1938
Figure 4 (c): Photomicrograph representation of the rotifers Filinia pejleri, Filinia
camascela and Brachionus dimidiatus in Lake Bosumtwi at 100X magnification.
Reference organisms are indicated by arrows
66
Keratella cochlearis Gosse 1851
Figure 4(d): The rotifer Keratella cochlearis in Lake Bosumtwi at 100 X
magnification. Organism is amongst the rare zooplankton in the lake.
67
Table 4.0. Interannual species diversity indices of the zooplankton community.
Species
M. bosumtwii
Chaoborus
M.micrura
B.calyciflorus
B.dimidiatus
H.intermedia
F.camascela
F.pejleri
K.cochlearis
2005
2006
pi
pi(lnpi)
pi
pi(lnpi)
0.647
0.435 0.721
0.327
0.046
3.079 0.065
0.178
0.007
4.965 0.010
0.045
0.057
2.865 0.011
0.050
0.063
2.765 ------------0.171
1.766 0.188
0.309
0.003
5.793 0.005
0.027
0.004
5.449 0.006
0.031
0.00007 9.550 0.0006 0.0004
Observed Diversity (H).
Max.Diversity (lnS).
Evenness (E).
1.833
2.200
0.833
-
1.566
2.079
0.755
-
4.2 Patterns in community abundance
The copepods contributed 68 % on average to monthly total zooplankton abundance
throughout the sampling period (Fig.4.1). The proportion of copepods was lowest in
November 2005 - less than 20 %, which coincided with the highest proportion of H.
intermedia to overall abundance. The copepods and H.intermedia formed over 80 % of
total community abundance whilst the remaining 20 % was shared by the other
rotifers, M. micrura and C. ceratopogones. However, B.calyciflorus and B. dimidiatus
increased their contribution to ~ 60 % during the deep circulation in August 2005.
Periods of copepod decline coincided with dominance of H.intermedia and there was a
period in June 2006 where both contributed equally to total zooplankton abundance.
Generally, the proportion of the non copepod members to total abundance was greater
in 2005 compared to 2006.
68
Relative Annual Percent Composition
Figure 4.1: Percent contribution of each zooplankton taxon to monthly total
zooplankton abundance.
50
40
2005
2006
30
20
10
0
A
ri
s
s
ia
ra
ris
lii
es
ela orus
tu
jle
ds
ru floru
ed
dit aup hlea
sc
po mic
b
i
dia rm F.pe
o
a
i
c
e
o
p
N oc
a
m
op
aly . dim . inte
pe
c
M.
ca
Ch
.c
lt c
K.
F.
B
Co
H
u
B
d
Species
Figure 4.2: Annual percent composition of each zooplankton taxon and copepod
developmental stages to total zooplankton abundance.
69
4.2.1 Interannual patterns of community abundance
There were regular seasonal increases as well as short-term increases in community
abundance. The regular seasonal increases were however interrupted by several shortterm fluctuations in total abundance. The copepods showed the greatest similarities in
individual patterns of abundance to patterns of total zooplankton abundance in both
years (Fig. 4.3). This shows that the copepods are driving total abundance trends
relative to the other taxa. Seasonal patterns of increase between the stratified seasons
(February to June) and the circulation (August and December) and post circulation
periods (September to November) were apparent.
Total zooplankton abundances
during the circulation and post circulation periods were generally higher than during
the stratified seasons, but differed between years in the timing and magnitude of
increase. For most individual zooplankton species, correspondence in patterns of
abundance and seasonal maxima were weak and not statistically significant except B.
calyciflorus which possessed a significant and stronger interannual correspondence in
patterns of abundance (Table 4.1). However, correspondence in timing of maxima
during the circulation and post circulation periods were evident for total zooplankton,
adult M. bosumtwii and copepodites, but timing of maxima in nauplii were earlier in
2006 compared to 2005 (Fig 4.3). The seasonal pattern in 2006 was interrupted by
several oscillations in population sizes. Interannual percent composition to total
zooplankton determined for each taxon and all copepod developmental stages showed
little difference except copepodites whose percent contribution was relatively higher in
2006 whilst adults of M. bosumtwii, B. calyciflorus and B. dimidiatus had greater
proportions in 2005 (Fig.4.2). Copepodites dominated in percent abundance in both
70
years whereas F.pejleri, F.camascela and K.cochlearis least contributed to total
zooplankton abundance.
4.3 Trends in abundance of each taxon
4.3.1 Copepod- Mesocyclops bosumtwii
M. bosumtwii adults (males, ovigerous females and non-ovigerous females) and
developmental stages (copepodites and nauplii) constituted the largest numerical
proportion of total zooplankton at most times and in both years (Figs.4.1 & 4.2 &
4.3).The proportion of copepods increased from 65 % in 2005 to 72 % in 2006. Mean
total annual proportion of copepods to total zooplankton abundance was 68 %. Peak
abundances for adults occurred in June and September/October, 2005 following the
deep circulation period. Abundance was variable in both years and there was no
distinct trend between years. Generally, abundance in 2005 was higher than in 2006
though both years recorded lowest abundances in July. Variable patterns in abundance
of copepodites in 2006 were also observed. However, in contrast to adults, a marked
peak occurred in October, corresponding to the highest increase of copepodites over
the year. The highest increase in copepodite abundance in July/ October 2005 matched
similar increases in adults in the same year and both recorded their lowest abundances
in July. Unlike adults, overall abundance of copepodites was higher in 2006 than in
2005 (Fig.4.3). Nauplii showed similar patterns in abundance to copepodites and
adults. Comparison of the abundance trajectories of the copepods to trends in total
zooplankton showed interesting relationships. The timing and magnitude of peaks in
abundance of the copepods in both years and especially in 2005 matched well with
total zooplankton abundance. This clearly shows the contribution and influence of the
71
copepods on patterns of overall community abundance and how they are controlling
community abundance.
There is substantial evidence of temporal and seasonal variation in the observed
patterns of abundance. The temporal changes are marked by short term irregular
oscillations in individual adult, copepodite and nauplii population sizes. The amplitude
of these fluctuations in population sizes was comparatively greater in 2006 (Fig.4.3).
Seasonal differences in abundance can be observed in both individual populations and
total zooplankton. Abundances were lower during the stratified season (between
February and June) and greater during the mixing and post-mixing periods (between
July/August and December) of each year (Fig. 4.3).
4.3.2 Rotifers
All rotifer species had low abundance with periodic peaks in abundance. This could be
seasonal but the pattern was not repeated between the two years. The majority are
absent from the community for most periods of the year and only show unpredictable
increases during brief periods of the year (Figs.4.3 and 4.4). Temporal patterns of
abundance of H. intermedia are much more variable and greater in 2006 compared to
2005 (Fig.4.3). However, the peak in H.intermedia abundances contributed a large
proportion of the total zooplankton abundance in April and November of both years.
This contribution to total zooplankton abundance was shared with copepod nauplii and
the two Filinia species during the same period. Timing of population peaks was
common among H.intermedia, F.pejleri and F.camascela during November 2005. K.
cochlearis also corresponded with F. pejleri in population increases in March 2006.
Population maxima of B.calyciflorus coincided with the deep mixing period of August
72
2005 (which resulted in a fish kill) but were not repeated in 2006. Instead, only low
abundances of B.calyciflorus emerged in September and October of 2006.
500
25
Total Zooplankton Abundance
400
20
300
15
M. micrura
2005
2006
10
200
5
100
No. of Individuals x 103 per m3 of water
0
0
80
Adult M. bosumtwii
70
60
60
B.calyciflorus
50
40
40
30
20
20
10
0
0
200
150
140
H. intermedia
120
Copepodites
100
80
100
60
50
40
20
0
0
120
50
100
Nauplii
Chaoborus
40
80
30
60
Nov
Sept
July
May
Mar
Nov
Sept
July
0
May
0
Mar
10
Jan
20
Jan
20
40
Figure 4.3: Patterns in abundance of total zooplankton, adults and developmental
stages of M. bosumtwii (Copepods), M .micrura (Cladocera), H.intermedia,
B.calyciflorus and Chaoborus in individuals per cubic meter of water in successive
years.
73
4.3.3 Cladocera - Moina micrura
Moina micrura showed a single distinct peak in each year differing in timing and
magnitude (Fig. 4.3). Maximum population numbers were recorded in May 2005, and
September 2006. They were also present in samples from February and April of 2006
but in relatively lower numbers.
4.3.4 Phantom midge larvae - Chaoborus ceratopogones
Comparison of abundance of C. ceratopogones between years show a fairly stable
population in 2005 as opposed to the large temporal variability in population size in
2006 (Fig. 4.3). Highest population densities of ~ 40,000 individuals m-3 was recorded
in January 2006 followed by three lower but significant peaks occurring at
approximately bimonthly intervals between April and November 2006. This almost
predictable cycle of Chaoborus abundance was apparent in 2005 but with a lower
amplitude. Additionally, the timing of these peaks was different between years,
occurring earlier in 2005 than in 2006. However, the timing of the third peak in 2006
was earlier than the related peak of 2005. Similarly, lowest numbers were recorded at
intervals of four to five months beginning from February and ending in December.
Table 4.1: Pairwise Spearman rank correlation coefficients for patterns of interannual
seasonal correspondence (2005 versus 2006) in individual zooplankton abundance and
total zooplankton abundance at 5% significance level.
Species/stage
r
Adult M. bosumtwii
Copepodites
Nauplii
Chaoborus
Moina micrura
B. calyciflorus
H.intermedia
Total zooplankton abundance
74
0.245
-0.095
0.366
0.190
0.244
-0.530
0.309
0.239
P-value
N
0.239
0.651
0.072
0.362
0.240
0.006
0.142
0.297
25
25
25
25
25
25
25
25
140
120
100
B.dimidiatus
2005
2006
80
60
40
20
0
J F M A M J
J A S O N D
2.5
K. cochlearis
2.0
Number of individuals x 103 m -3
1.5
1.0
0.5
0.0
14
12
F.pejleri
10
8
6
4
2
0
8
F. camascela
4
0
J F M A M J J A
S O N D
Figure 4.4: Abundance of B.dimidiatus, Keratella cochlearis, Filinia pejleri and
Filinia camascela in thousands of individuals per cubic meter of water.
75
4.4 Relationship among herbivorous zooplankton, Depth of Mixed Layer (DML)
and Chlorophyll a concentration.
Depth of Mixed Layer (DML) was on average 8 m during the sampling period (20052006).Variation in Chl a and patterns of herbivorous zooplankton abundance appear to
be related to the timing of seasonal changes in the DML which exhibited distinct
seasonal pattern and recurrence in successive years. There was similarity in the timing
of the seasonal increase between Chl a and total herbivore abundance, but there were
Depth of Mixed Layer (m)
140
30
120
25
100
20
80
15
60
40
10
20
5
0
0
-5
Mean Depth Chl a conc.(ug/L)
Tot. Herb. Abund (no. x103per m3)
observable differences in the scale of the seasonal increases (Fig. 4.5).
Average DML
-10
-15
-20
-25
-30
-35
Jan
May
Sep
Jan
May
Sep
Jan
2006
2005
Figure 4.5: Effect of variation in Depth of Mixed Layer on Chl a and total herbivore
abundance. DML (half-filled circles). Chl a (green) and total herbivore abundance
(solid circles).
76
DML increased in January and August in both years during the weak and strong
vertical circulation events respectively. However, seasonal mixing depth during both
periods was greater in 2005 than 2006. There were also sporadic changes in the DML
and these occurred between the regular seasonal maxima of mixing events. These
sporadic changes in DML were more pronounced in 2006 than in 2005 and usually,
the intensity of mixing occurred above the average 8 m of DML.
Thus, there were two different annual scenarios in the character of the DML. In 2005,
there was characteristic deep seasonal mixing in January and August 2005. The
amplitude of seasonal mixing was lower in August 2006, but through the year, there
were greater fluctuations in DML in 2006 compared to 2005 Chl a and total herbivore
abundance were correlated to the patterns of variation in DML. Chl a. showed peaks in
May and September 2005. Whilst the timing of Chl a peaks was repeated in 2006, the
magnitude of the increase in 2006 alternated between both months in the two years.
The seasonal peaks in Chl a seem to follow a time lag with the timing of the deepening
of DML (Fig. 4.5). Gaps in Chl a data in 2005 limited the effectiveness of the seasonal
resolution of the relationship between Chl a and total herbivore abundance. Lowest
Chl a. in 2005 occurred in June whilst March recorded the lowest period for Chl a in
2006. Peaks in herbivore abundance occurred before and sometimes after peaks in Chl
a indicating an initial synchrony of increasing Chl a and increasing herbivore
abundance. This was necessary for the herbivores to respond rapidly to increasing Chl
a and exploit the improvement in food resources to increase their population sizes
before Chl a declined. This could be possible because development of Chl a and
herbivore abundance may likely not occur in phase with each other. From Fig. 4.6 it is
observed that, the timing and magnitude of peaks in Chl a and herbivore abundances
77
show that herbivores probably increase in proportion to the available amount of Chl a.
The pattern of Chl a and herbivore abundance exhibited a regular seasonal pattern and
recurrence between years (Fig.4.6). Maximum Chl a occurred during the stronger
seasonal mixing in August and highest monthly precipitation in May in both years.
The timing of the seasonal increases was repeated in both years but there were
differences in the magnitude of the peaks. The combined seasonal effect of mixing and
atmospheric precipitation on Chl a between the two years is remarkable. Maximum
seasonal rain in May (Puchniak et al., 2009) coupled with stronger seasonal mixing in
August and less temporal variation in thickness of mixed depth resulted in high Chl a
in 2005 during May and September (immediate post-mixing month). However in
2006, there was relatively lower maximum monthly precipitation in May, weaker
seasonal mixing but greater variability of mixing depth compared to 2005. This
matched the overall Chl a produced in both years (Fig.4.6). Thus the amount of
overall Chl a produced by a given set of regulating factors in one year may be matched
by a different interacting strength of the same regulating factors in another year.
4.5 Relationship between individual herbivore species and Chlorophyll a
concentration
Examination of the common rotifer and cladoceran herbivore species demonstrate
relationships with increasing Chl a in the water column. B. calyciflorus, M. micrura
and H. intermedia show strong seasonal responses to increasing Chl a but differ
significantly in the timing of their response (Fig.4.6). Peaks in both Chl a and
abundance of B. calyciflorus coincided in May and September of 2005 but
78
30
120
25
100
Chl a conc. and abundance of B. calyciflorus
20
80
15
60
40
10
20
5
0
0
Jan
May
Sep
Jan
May
Sep
Jan
70
60
50
40
30
20
10
0
Jan
May
Sep
250
200
150
100
50
0
Jan
2005
May Sep
Chl a conc.( ug/L)
22
20
18
16
14
12
10
8
6
4
2
Chl a conc.( ug/L)
Tot. Herb. and Pred. Abund.
300
May Sep
May
Sep
Jan
Chl a ( ug/L)
B.calicyflorus
Chl a (ug/L)
H.intermedia
Jan
Jan
30
25
25
20
20
15
15
5
5
0
0
Jan
Jan
10
10
May
Sep
Jan
May
Tot.Herb.Abund.
Tot.Pred. Abund.
Chl a (ug/L)
Jan
2006
2005
2006
Sep
Chl a (ug/L)
M.micrura
Figure 4.6: Responses of individual herbivore species and total herbivore abundance
to Chl a, seasonal variation in Chl a and relationship between total herbivore and
predator abundance.
showed no response in May 2006 during peak Chl a. However, the response to Chl a
was greater in 2005 than in 2006. M. micrura and H.intermedia exhibited similar
response patterns to B. calyciflorus however, H. intermedia showed greatest responses
to Chl a in all seasons in successive years and, apart from the regular seasonal
interactions with Chl a, they showed dynamic interactions with Chl a during short
79
Abundance of M. micrura
Abundance of H. intermedia
140
term inter-season increases in Chl a. There is evidence to support a differential
exploitation of different stages of Chl a development by the rotifers. H.intermedia
seems to respond to low concentrations, M.micrura - moderate and B. calyciflorus
responds to higher concentrations. Patterns of responses to Chl a in H. intermedia
corresponded with patterns of total herbivore abundance to Chl a indicating greater
influence of H.intermedia on overall herbivore interactions with Chl a. Linear
correlation between Chl a and total herbivore abundance showed no significant
Tot.Herb. Abund. (No. Ind.x 10 3m-3)
relationship (r² = 0.103, P > 0.05) (Fig.4.7).
140
R2= 0.103, P>0.05
n= 24
120
100
80
60
40
20
0
0
5
10
15
20
25
30
Chl a conc.(ug/L)
Figure 4.7: Linear correlation between Chl a and total herbivore abundance.
80
4.6 Interactions between predators and herbivores
Abundances of predators were consistently higher than abundances of herbivores in
successive years. Seasonal increases in Chl a in May and September/October in each
year, increased herbivore abundances appreciably, but they were unable to either
match or exceed predator abundance (Fig.4.6). H. intermedia was an exception by
matching and sometimes exceeding predator densities during the seasonal surges in
Chl a. Predator density was lowest during the major August 2005 circulation and
March 2006. B. calyciflorus increased considerably during this period of low predator
abundance and contributed substantially to the rise in herbivore abundance. There
were also periods of low Chl a low herbivore abundance and high predator densities
(e.g June 2005 and August/November 2006). During March 2006, there was low Chl
a, low predator densities and low herbivore abundance, indicating the possible effect
of low food supply on herbivore abundance. Highest predator abundance occurred in
October of each year following the mixing period and the associated rise in Chl a and
herbivore abundance.
81
4.7 Regression model, statistics and general zooplankton biometrics
Length-weight regressions with the exception of rotifers and copepod nauplii, were
developed for individual zooplankton taxa and developmental stages of copepods and
Chaoborus for both stratified and circulation periods. Regression statistics for the
various length-weight relationships are presented in Tables 4.2 - 4.5.
100
R2 = 0.98, P < 0.05, n = 4
Mean weight (ug)
80
W=aLb
60
40
20
0
3.5
4.0
4.5
5.0
5.5
6.0
Mean length (mm)
Figure 4.8: Generalized curvilinear (power function) regression model describing the
growth pattern of zooplankton and the relationship between zooplankton mean body
length (independent or predictor variable) and the mean body weight (dependent or
response variable). Where a and b are fitted constants of the regression.
The growth model above shows that the weight of the zooplankton organisms
increased according to a certain power (slope) of the body length (Fig.4.8). The slope
(value of b) of all regression equations for all taxa and developmental stages were
significantly different (P < 0.05) from zero, indicating that a relationship existed
between the length and weights of all taxa and developmental stages. The mean slopes
and intercepts of all Chaoborus instars during one season were compared using
unpaired t-test.
82
Table 4.2: Regression statistics for various length-weight relationships of individual
Chaoborus instars during the stratified period. Regressions equations used for routine
calculations are indicated in bold.
Instar
a
b
RMS
F-ratio
R2
W= ln a + b ln L
I
5.0949 0.8841
1.789
15.335
0.78
W=ln5.094+0.884ln L
II
0.0523 3.9096
22.189
16.912
0.92
W=ln0.052+3.909lnL
III
0.0505 3.6621
6.120
123.911
0.97
W=ln0.051+3.662lnL
IV
0.4876 2.5533
32.592
16.068
0.79
W=ln0.488+2.553lnL
Pooled 0.3492 2.6917
45.039
67.415
0.79
W=ln0.349+2.692lnL
Table 4.3: Regression statistics for various length-weight relationships of nonovigerous adult female copepod and adult non-ovigerous M.micrura during the
stratified season.
Species
a
b
RMS
F-ratio
R2
W=In a + b In L
Females
8.792
1.558
0.021
58.647
0.95
W=ln8.792+1.55lnL
Pooled
10.489 2.349
0.127
56.154
0.88
W=ln10.49+2.349lnL
Males
59.517 4.329
0.077
74.654
0.96
W=ln59.517+4.32lnL
Pooled
2.373
2.927
0.717
10.781
0.57
W=ln2.373+2.927lnL
M.micrura 9.331 1.108
Pooled
10.998 1.264
0.172
0.568
23.461
20.111
0.88
0.72
W=ln9.331+1.108lnL
W=ln10.99+1.264lnL
Table 4.4: Regression statistics for various length-weight relationships of individual
Chaoborus instars and pupae except instar I during the mixing period.
Instar
a
b
RMS
F-Ratio
R2
W= ln a + b ln L
II
3.4605 1.4548
25.145
7.0181
0.60
W=ln3.4605+1.4548lnL
III
3.3056 1.4558
5.276
33.335
0.88
W=ln3.3056+1.4558lnL
IV
1.5562 1.8295
3.541
50.548
0.93
W=ln1.5562+1.8295lnL
Pupae
1.7590 8.546
105.93
37.795
0.93
W=ln1.7590+8.546lnL
Pooled
3.1695 1.4737
13.612
37.147
0.72
W=ln3.1695+1.4737lnL
83
Table 4.5: Regression statistics for various length-weight relationships of individual
developmental stages of M. bosumtwii during the stratified period.
RMS
F-ratio
R2
W= ln a + b ln L
11.237 2.784
0.405
13.929
0.82
W=ln11.237+2.784lnL
Males
22.849 3.118
0.253
16.907
0.85
W=ln22.849+3.118lnL
C1
13.450 1.823
0.025
18.742
0.82
W=ln13.450+1.823lnL
CII
11.085 2.446
0.005
34.659
0.89
W=ln11.085+2.446lnL
CIII
457.83 10.072
0.151
31.995
0.91
W=ln457.83+10.07lnL
CIV
73.186 7.509
0.567
98.572
0.96
W=ln73.186+7.51lnL
CV
12.086 3.134
0.061
26.040
0.86
W=ln12.086+3.134lnL
Pooled
7.640
1.727
0.832
26.146
0.58
W=ln7.640+1.727lnL
M.micrura 17.760 1.852
0.504
19.553
0.86
W=ln17.760+1.825lnL
Species/
stage
Females
a
b
There was no significant difference between slopes and intercepts for all Chaoborus
instars between seasons (slope: P = 0.78, n = 4; intercepts: P = 0.36, n = 4). Similarly
comparisons between the slopes and intercepts of Chaoborus instars and all other taxa
and their developmental stages between seasons showed no significant difference
between slopes but differences in intercepts during the mixing season were significant
(slopes: P = 0.43, n = 4, intercepts: P < 0.05, n =11). Seasonal differences between
slopes and intercepts of adult males and female copepods were also not significant (P
= 0.996, n = 2). Generally, regressions equations with higher R-square values were
used for the routine estimation of weights from respective lengths. Regressions were
not developed for various copepodite stages during the stratified season. For copepod
adult males, the equation with the lower R-square value was used for the routine
calculations due to the overestimation of the weights by the equation with the higher
R-square value. Similarly, regressions could not be developed for Chaoborus instar I
84
because of its absence in the samples used for the development of mixing season
regressions. Generally, the length-weight relationships for the stratified season were
stronger for most taxa than for the mixing season although the lack of regressions for
copepodites during the stratified season was assumed on the basis of prior knowledge
of lack of statistical difference between slopes and intercepts of adult organisms. A
general length-weight equation for each taxon showed lower R square values than the
individual regression equations
4.7.1 Zooplankton Biometrics
4.7.1.1 Copepod body length and weight distribution
Total body length but not weight increased progressively across all taxa and
developmental stages (Table 4.6). Body lengths of copepodites and adult copepods are
characteristic of each stage and show no overlap between life stages. Adult male
copepods showed marked overlap in body length with copepodite stages II and IV.
Generally, mean body length changes 1.15 times across successive stages but the
greatest change in body length occurred between the first and second copepodite
stages whilst adult females are approximately twice the length of the first copepodite
stage (Table 4.6). The weight distributions among the adult copepods and development
stages show contrasting patterns relative to their body length. For example, weight
overlap occurred between adjacent stages except the fifth copepodite stage and adult.
The fifth copepodite stage also show greatest variability in weight, as opposed to the
first copepodite stage which exhibited the least. Changes in mean weight of 1.22 µg
across all stages correspond well to the changes in body length.
85
Table 4.6: Monthly mean values of selected biometrics of all zooplankton taxa and
developmental stages. SEM is the standard error of the monthly means. Range
represents the range of monthly means recorded, Wt (weight) and L is the total body
length of organisms.
Species/
stage
Copepods
Females
Males
Nauplii
Copepodite I
Copepodite II
Copepodite III
Copepodite IV
Copepodite V
Copepod Eggs
Chaoborus
Instar I
Instar II
Instar III
Instar IV
Pupae
M.micrura
B.calyciflorus
H.intermedia
Mean
L.(mm)
Range
(mm)
SEM
±
Mean
Wt.
(µg)
Range
(µg)
SEM
±
N
0.79
0.57
0.19
0.40
0.49
0.59
0.65
0.77
0.08
0.72 - 0.87
0.52 - 0.64
0.13 - 0.31
0.36 - 0.44
0.47 - 0.52
0.55 - 0.62
0.65 - 0.67
0.73 - 0.78
-----
0.017
0.003
0.948
0.006
0.002
0.002
0.019
0.047
-----
6.03
4.60
0.22
2.57
1.99
2.30
2.97
5.27
0.68
4.77 - 8.20
2.66 - 6.21
0.18 - 0.30
1.99 - 2.89
1.79 - 2.29
0.89 - 4.09
2.30 - 3.95
4.49 - 6.00
-----
0.137
0.080
0.022
0.019
0.031
0.146
0.291
0.137
-----
24
24
24
24
24
24
24
24
-
4.58
5.60
5.98
6.02
3.91
0.51
0.20
0.13
3.63 - 5.58
4.63 - 6.63
4.73 - 6.90
4.93 - 6.80
3.05 - 4.90
0.38 - 0.68
0.11 - 0.29
0.09 - 0.23
0.123
0.225
0.039
0.344
0.048
0.011
0.005
0.005
15.20
42.11
45.13
43.79
56.31
6.24
0.24
0.13
16.60-22.50
17.57-58.90
22.80-88.80
24.39-68.90
47.72-68.90
2.09 - 8.56
0.04 - 0.68
0.08 - 0.14
3.704
2.172
0.766
2.452
0.588
0.104
0.013
0.002
23
24
24
24
20
14
15
24
The greatest weight change occurs between the fourth and fifth copepodite stages.
Whereas mean adult weight is 2.35 times the mean weight of the first copepodite
stage, the ratio is much greater, 2.8 times between the maximum weights in the
respective stages.
4.7.1.2 Chaoborus body length and weight distribution
Unlike the copepods, Chaoborus instars showed much overlap in body length. The
mean change in unit body length of 1.09 mm among all instars is comparable to the
copepods, but has a lower ratio of adult to first instar body length of 1.31. Chaoborus
86
instars show no weight change between the fourth larval and the pupal stages, but
show greatest change between the first and second instar stages. The ratio of the mean
weight between the first instar and the fourth larval is 2.88. This is comparable to the
ratio of the maximum weights between the two instars, which is 3.06. The mean ratio
across all stages is approximately 1.5. This shows that the instars change more in
weight per unit change in length than the copepods, evidenced by the general
curvilinear relationship between lengths and weight. The third Chaoborus instar
showed the greatest maximum weight of 88.90 µg, which is 20µg more than both the
fourth and final larval and pupal stages, which had equal maximum weights of 68.90
µg.
Table 4.7: Characteristic Head Capsule Lengths (HCL) of individual Chaoborus
instars used as the primary discriminant among instars.
Instar
I
II
III
IV
Head Capsule
Length (µm)
480
660
780
840
The head capsule length was characteristic for each instar. The ratio of change in head
capsule length between successive instars decreases by an order of magnitude (60 µm)
from the first to the fourth instar (Table 4.7).
87
4.8 Biomass and production of dominant taxa
Proportion to annual biomass and production
4.8.1 Patterns of community biomass and production
120
Biomass
Production
100
Copepodites
Adult Copepods
Nauplii
Chaoborus
M.micrura
B.calyciflorus
80
60
40
20
0
2005
2005
2006
2006
YEAR
Figure 4.9: Percent contribution of each zooplankton group to total annual
zooplankton biomass and production in 2005 and 2006. B. calyciflorus and
H. intermedia are not represented in the production estimates given.
The predatory copepods and Chaoborus constituted the largest proportion of (> 98.5
%) of the total community biomass and production in successive years (Fig.4.9). M.
micrura, the largest herbivore contributed < 2 % to total community biomass and
production. Shifts in dominance of biomass occurred between the two study years for
the copepods and Chaoborus, such that, whilst copepods exceeded Chaoborus
biomass by 7 % in 2005, Chaoborus surpassed copepods by 10 % in 2006. Total
zooplankton biomass recorded in 2005 was 9761 mg dw m-3.This figure increased by
30 % to 12614 mg dw m-3 in 2006. In contrast, no such shifts in dominance of
production occurred between the copepods and Chaoborus. Total annual production
88
increased from 1230 mgdwm-3 y-1 in 2005 to 1890 mg dw m-3 y-1 and the copepods
made up 64 % and 72 % of this annual production respectively. Mean monthly
community biomass for 2005 was 813 mg dw m-3, which was lower than the 1051
mgdwm-3 recorded for 2006. Likewise, mean monthly production for 2005 was 102
mg dw m-3 y -1, which was also lower than the 157 mg dw m-3 y-1 recorded for 2006.
Overall community patterns in biomass and production were comparable in both years.
Community biomass
and
production
were
maximum
during
August
and
September/October of both years in fairly equal measure but patterns were more
variable in 2006 (Fig.4.11).
4.8.2 Trends in copepod biomass and production
Patterns of total copepodite and total copepod biomass and production compare very
well with the patterns of the overall zooplankton community biomass and production
(Fig.4.11). This is a further illustration of the influential role of the copepods in overall
community dynamics and energy flux. Copepodite stage III dominated total copepod
production contributing about 384 mg dw m-3day-1 and 599 mg dw m-3day-1,
representing 49 % and 44 % of the daily total copepod production in 2005 and 2006
respectively (Fig. 4.10). Adult males however dominated copepod biomass in 2005 but
co-dominated with copepodite I in 2006. Total copepod biomass and production were
lower in 2005 compared to 2006. Total copepod biomass in 2005 was 5119 mgdwm-3
and 5547 mg dw m-3 in 2006. Similarly, total copepod production in 2005 was 783 mg
dw m-3y-1 and 1360 mg dw m-3y-1 in 2006 representing an increase of 74 % over
production in 2005.
89
Percent.contribution to total biomass and prodn.
120
Biomass
Production
100
Cop.I
Cop.II
Cop.III
Cop.IV
Cop.V
Nauplii
Females
Males
80
60
40
20
0
2005
2005
2006
2006
YEAR
Figure 4.10: Percent contribution of adult copepods and developmental stages to total
copepod biomass and production in successive years.
Total copepod production excludes males because males are short lived and cease
growing at adulthood. Seasonality among the various copepodite stages was muted,
exhibiting dynamic patterns in biomass and production in successive years with
relatively greater amplitude in 2006 (Fig.4.12). Despite the high degree of variability
in magnitude and timing of peaks in biomass and production, common patterns among
all groups are apparent. The timing of maximum peaks during October 2005 was
synchronized in all stages and was commonly repeated in October 2006 but to a lesser
extent in the fourth copepodite stage and completely muted in adult females. Mean
biomass of all stages during the synchronized peak in October 2005 was 85 mg dwm-3.
The 56 % contribution of adult males and females to the average biomass was the
largest. The proportion of nauplii biomass of 12 mg dw m-3 to the overall average was
the least. In both years, biomass and production during the mixing and immediate post
90
mixing period in August and October/November were on average higher than during
the stratified season. Daily egg production by adult copepod females was very low in
500
Adult males
200
150
100
50
400
Total Copepodite
120
100
300
80
200
60
40
100
20
0
0
Jan
160
140
Biomass (mg dry wt.m-3)
Biomass (mg dry wt.m-3)
250
0
May
Sep
Jan
May
Sep
Jan
Jan
May
Sep
Jan
May
Sep
Production (mg dry wt.m-3day-1)
2005. Production recorded was 8.86 mg dw m-3day-1.
Jan
Biomass
Production
1200
140
120
500
100
400
80
300
60
200
40
100
20
0
0
1000
200
800
150
600
100
400
50
200
0
Biomass (mg drywt. m-3)
600
250
total zooplankton
Production (mgdrywt m-3day-1)
160
Total Copepod
Production (mgdrywt m-3day-1)
Biomass (mg drywt. m-3)
700
0
May
Sep
Jan
May
2005
Sep
Jan
Jan
2006
May
Biomass (mgdry wt. m 3)
May
Sep
Jan
2006
7
6
Biomass
Production
Nauplii
5
15
4
3
10
2
5
1
0
Jan
Jan
2005
25
20
Sep
Production (mg dry wt.m-3day-1)
Jan
0
May
Sep
Jan
May
Sep
Jan
2006
2005
Figure 4.11: Interannual patterns of variation in biomass and production of nauplii and
adult males of M. bosumtwii, as well as total copepodite, total copepod and total
zooplankton biomass and production in successive years.
91
Annual egg production of adult copepods increased by about 6 fold in 2006
representing an increase of 82 % over production in 2005 (Fig.4.9).
200
15
150
9
1
100
7
50
4
0
25
Jan
May
Sep
Jan
May
Sep
20
120
18
100
Copepodite II
16
14
80
12
60
10
8
40
6
20
4
2
0
0
Jan
Jan
May
Sep
Jan
May
Sep
Production (mg drywt.m-3day-1)
14
Biomass (mg dry wt. m-3)
Biomass (mg dry wt. m-3)
Copepodite I
Production (mg dry wt. m-3 day-1)
9
250
Jan
Biomass
Production
40
40
20
20
0
0
May
Sep
Jan
May
Sep
Jan
80
7
Biomass (mgdrywt.m-3)
Copepodite V
6
60
5
4
40
3
20
2
1
0
0
16
50
14
40
12
10
30
8
20
6
10
4
2
0
Jan
0
May
Sep
Jan
May
Sep
Jan
200
35
180
Biomass (mgdry wt.m-3)
Jan
18
Copepodite IV
60
Adult Females
30
160
140
25
120
20
100
15
80
60
10
40
5
20
0
0
Jan
May
2005
Sep
Jan
May
Sep
Jan
Jan
May
Sep
2005
2006
Production (mg drywt.m-3day-1)
60
20
70
Jan
May
Sep
2006
Production (mg dry wt.m-3day-1)
60
Production (mgdry wt.m-3day-1)
Biomass (mg dry wt.m-3)
Copepodite III
80
Production (mg dry wt. m-3 day-1)
Biomass (mg dry wt. m-3)
80
100
Jan
Figure 4.12: Interannual patterns of variation in biomass and production of individual
copepodite stages and adult females of the copepod M. bosumtwii in successive years.
92
Biomass and production during the mixing and immediate post-mixing period in
August and October/November were on average higher than during the stratified
periods. For copepodite IV and adult females, the amplitude of variation in biomass
and production declined markedly in 2006 whereas biomass and production of
copepodite IV remained very low from January to May 2006. The opposite occurred
for both copepodites I and II, which showed lower biomass and production values for
most part of 2005 except the temporal punctuation by peaks in August of the same
year.
Relative Annual Biomass (mg dry wt. m-3)
4.8.3 Trends in Chaoborus biomass and production
8000
2005
2006
6000
4000
2000
0
Instar 1
Instar II Instar III Instar IV
PupaeTotal Biomass
Developmental stage
Figure 4.13: Biomass of each Chaoborus instar in successive years
Instar III contributed the highest mean percentage of 36 % and 59 %, to total
Chaoborus biomass and production in 2005 and 2006 respectively (Figs.4.13 and
4.14). Percentage biomass of instar III only did not change. Biomass was 36 % in both
years but production between the two years was 18 % greater in 2005. Total
percentage production of instar III in 2005 was 68 %, whereas that of 2006 was 50 %.
93
This shows that instar III contributed more than half of the total Chaoborus production
in successive years. Differences in biomass and production between years for instar I
was very small (< 3 biomass and production units), but unlike instar III, production in
2006 was higher than in 2005.Overall, instar II contributed the least (17 %) to biomass
in 2005, whilst instar IV contributed the lowest proportion of biomass in 2006
(Fig.4.13). Biomass and production of instar IV in 2006 was lower than in 2005. Apart
from the larval instars, the pupal stage of Chaoborus contributed the least to total
biomass in both years. Production was however not estimated for pupae since growth
Relative annual Instar production (mg dry wt.m-3day-1)
ceases during that stage of development (Cressa and Lewis, 1986).
600
Instar 1
Instar 2
Instar 3
Instar 4
500
400
300
200
100
0
2006
2005
YEAR
Figure 4.14: Interannual differences in proportion of total production contributed by
each larval instar of Chaoborus.
Mean total annual biomass among Chaoborus instars increased from 891 mg dw m-3 in
2005 to 1370 mg dw m-3 in 2006. Similarly, mean total annual production also
increased from 108 mg dw m-3y-1 in 2005 to 126 mg dw m-3 y-1 in 2006. Highest daily
Chaoborus production in 2005 occurred in August at 176 mg dw m-3day-1 whereas
94
production was at a low of 0.27 mg dw m-3day-1 during May. Lowest daily production
however was 78 mg dw m-3day-1 during January of 2006.
10
8
200
6
150
4
100
2
50
0
0
Jan
May
Sep
Jan
May
Sep
30
300
Biomass (mg dry wt.m-3)
Biomass (mg dry wt.m-3)
250
Instar I
Production (mg dry wt. m-3 day-1)
300
Instar II
250
25
200
20
150
15
100
10
50
5
0
0
Jan
Jan
May
Sep
Jan
May
Sep
Production (mg dry wt. m-3 day-1)
4.8.3.1 Patterns in Chaoborus biomass and production
Jan
Biomass
Production
200
150
100
50
0
May
Sep
Jan
May
Sep
Biomass (mg dry wt.m-3)
Pupae
25
20
15
10
5
0
Jan
May
2005
Sep
Jan
May
Sep
Jan
8
6
100
4
50
2
0
Jan
35
30
10
150
Jan
Monthly Total Biomass (mg dry wt.m-3)
Jan
0
May
Sep
Jan
May
Sep
Jan
200
1000
180
800
160
140
600
120
100
400
80
60
200
40
20
0
0
Jan
May
2005
2006
12
200
Sep
Jan
May
Sep
Production (mg dry wt.m-3day-1)
250
Instar IV
Jan
Mon. Tot. Production (mg dry wt.m-3 day-1)
300
14
250
Biomass (mg dry wt. m-3)
Biomass (mg dry wt. m-3)
Instar III
Production (mg dry wt. m-3 day-1)
200
180
160
140
120
100
80
60
40
20
0
350
2006
Figure 4.15: Interannual patterns of variation in biomass and production of individual
Chaoborus instar stages and biomass of pupae in successive years.
95
Biomass and daily production of all Chaoborus instars were highly correlated as
evidenced by the stable population growth in numbers especially during 2005 (Fig.
4.3). Daily biomass changed in proportion to the changes in production for all instars
(Fig. 4.15). Seasonal patterns were apparent for instars I, II and IV, but were less
distinct and variable for the third instar. Highest production for both first and second
instars occurred in February 2006. Biomass of both instars reached their highest values
(250 mgdwm-3) during the same month but production differed between the two
instars by more than 3 fold; 27 mgdwm-3day-1 for instar II, whereas 8 mgdrywt.m-3day1
was recorded for instar I. Lowest biomass and production for instar I occurred during
the onset of the Harmattan season in November 2005 at < 2 mgdwm-3 and 0.019 mg
dw m-3day-1 of biomass and production respectively. In contrast, greatest decline in
biomass and production for instar II occurred in August 2005 during the deep mixing
event at < 3 mg dw m-3 for biomass and 0.023 mg dw m-3day-1 for production
respectively. Similar patterns were observed for both the third and fourth instars.
Peaks in production occurred repeatedly in August of both years during the major
circulation (Fig. 4.15). Production reached 172 mg dw m-3day-1 during the same month
in 2005, but decreased in magnitude the following year to 31 mg dw m-3day-1,
reducing by ~ 6 fold. However, whereas the timing of peaks in biomass and production
differed in 2005 - November and August respectively, the situation was different in
2006 where peaks in biomass and production were synchronized. Production of instar
3 was highest among all instars increasing by > 11 fold above average production of
the other 3 instars. Overall, biomass was lowest in instar 1 but all instars showed
similar magnitude of biomass peaks. Biomass and production were also relatively
96
higher in 2006 for all instars compared with 2005. Variable patterns in total
Chaoborus biomass and production were observed in both years. Highest Chaoborus
production was recorded in August 2005 whereas highest biomass was recorded in
February 2006. However, patterns were not repeated in successive years. Total annual
Chaoborus biomass was 54 % higher in 2006 compared to 2005. In contrast,
production was only 17 % higher in 2006 compared to 2005.
4.8.4 Relationship between herbivore biomass and Chl a concentration
Biomass and Chl a showed varied relationships among herbivores. This response was
well marked in the herbivore B. calyciflorus during May and August of 2005
corresponding to peaks in Chl a B.calyciflorus was absent for most of 2006, and
during periods when it was present in the community, its biomass was very low
(Fig.4.16). H. intermedia comparatively showed a much more dynamic and variable
biomass pattern interannually, compared to both B.calyciflorus and M.micrura.
Maximum biomass of H.intermedia occurred during December 2005. M.micrura
dominated the biomass distribution among the herbivores. Highest biomass and
production of M. micrura occurred in October 2006, with a biomass maximum of (>
92 mg dw m-3). Total annual biomass of M. micrura exceeded biomass of B.
calyciflorus by 2 fold and H. intermedia by ~5 fold. Differences in biomass were more
pronounced during 2006 when biomass of M.micrura exceeded that of B.calyciflorus
and H.intermedia by as much as 21 and 7 fold respectively. Total annual herbivore
biomass was 189 mg dw m-3 in 2005. This was 27 times lower than that of the
copepods and ~ 24 times lower than biomass of Chaoborus. In 2006, total annual
herbivore biomass recorded was 217 mg dw m-3, a fair increase over that of the
previous year. However, the biomass ratio of herbivores and the copepods only
97
reduced by an order of magnitude whereas the biomass ratio of copepods and
Chaoborus increased to ~32 orders of magnitude in 2006. Mean monthly biomass of
the herbivores was also very low in both years compared to the other taxa. Similarly,
copepod production exceeded that of M.micrura by 56 times in 2005 and 65 times in
2006. However the production ratio between Chaoborus and M. micrura in 2006 was
Chlorophyll a conc.(ug/L)
30
20
18
16
14
12
10
8
6
4
2
0
25
20
15
10
5
Biomass (mg dry wt. m-3)
unaltered between years (Table 4.10)
0
May
Sep
Jan
May
Sep
Jan
30
6
25
5
4
20
3
15
2
10
1
5
Biomass (mg dry wt. m-3)
Chlorophyll a conc. (ug/L)
Jan
0
0
Sep
Jan
May
Sep
Jan
30
100
12
25
80
10
20
60
15
40
10
20
5
0
0
Jan
May
Sep
Jan
May
2005
Sep
2006
Jan
8
6
4
2
0
Production (mg dry wt. m-3day-1)
May
Biomass (mg dry wt.m-3)
Chl a conc. (ug/L)
Jan
Chl a conc.
Biomass
Production
Figure 4.16: Interannual patterns of variation in biomass and Chl a of the major
herbivores; B.calyciflorus, H.intermedia including production of M.micrura
respectively in successive years.
98
Biomass (mg drywt m-3)
Variation in Thickness of Mixed Layer (m)
Biomass
Production
1000
200
800
150
600
100
400
50
200
0
Production (mgdrywt m-3day-1)
250
1200
0
-5
-10
-15
-20
-25
Mixed Depth
-30
-35
Jan
May
Sep
Jan
May
Sep
Jan
2006
2005
Figure 4.17: Temporal patterns of the effect of variation in thickness of mixed depth
on seasonal total biomass and production of zooplankton.
The pattern of variation in total zooplankton biomass and production is comparable to
patterns of numerical abundance (Fig.4.3). The depth of mixed layer appears to have
had considerable influence on the biomass and production patterns of zooplankton
especially during the deep seasonal overturn in August 2005 (Fig. 4.17). The periods
of mixing (overturn) coincided with periods of maximal biomass and production of
zooplankton. However, there was greater variability in the pattern of biomass and
production resulting from the corresponding variable thickness of the mixed depth
layer in 2006 compared to 2005. This greater variability corresponded with higher
average biomass and production of zooplankton in 2006 compared to 2005 (Fig. 4.17).
99
4.8.5 Correlation Analyses: herbivore biomass versus grazeable phytoplankton
biomass (GPB)
20
6
H. intermedia
Biomass (mgdw-3)
Biomass (mgdwm-3)
16
R2 =0.24
4
B.calyciflorus
18
5
n= 45 P>0.05
3
2
1
14
R2 = -0.02
n = 25, P>0.05
12
10
8
6
4
2
0
0
0
500
1000
1500
2000
2500
0
500
1000
1500
2000
2500
Average Grazeable Phytoplankton Biomass (mgdwm-3)Average Grazeable Phytoplankton Biomass (mgdwm-3)
100
500
Copepodite
R2 = 0.23
n = 24 P>0.05
80
Biomass (mgdwm-3)
Biomass (mgdwm-3)
M.micrura
60
40
20
0
0
200
400
600
800 1000 1200 1400 1600
R2 = 0.20
400
n= 46 P>0.05
300
200
100
0
0
500
1000
1500
2000
2500
Average Grazeable Phytoplankton Biomass (mgdwm-3) Average Grazeable Phytoplankton Biomass (mgdwm-3)
Biomass (mgdwm-3)
25
R2 = 0.01
n = 46 P>0.05
Nauplii
20
15
10
5
0
0
500
1000
1500
2000
2500
Average Grazeable Phytoplankton Biomass (mgdwm-3)
Figure 4.18: Linear correlation between grazeable phytoplankton biomass and 2-week
lagged individual herbivore biomass. Copepodites and nauplii are included for
speculative analyses.
100
Patterns in individual herbivore biomass were weakly correlated with patterns of
grazeable phytoplankton biomass (GPB) and the relationship was statistically not
significant for all organisms (Fig.4.18). B. calyciflorus showed a negative relationship
with GPB. Copepod nauplii and copepodites were included in this analysis to
determine if the juvenile copepods were accessing algal food sources but the
relationship with GPB did not yield any statistical significance. Almost no relationship
existed between copepod nauplii and GPB due to a very weak correlation coefficient
(Fig. 4.18). Overall, herbivore biomass and GPB were not linearly correlated.
Maximum biomass of most of the zooplankton categories tend to occur at lower to
moderate algal abundances. At low to moderate phytoplankton biomass there is quite a
range in zooplankton biomass from low to high.
GPB exhibited repeated timing of single peaks in November of both years during the
seasonal deep circulation and restratification period in August/September (Fig.4.19).
Lowest GPB in 2005 occurred in September whereas lowest GPB in 2006 was
recorded in May during the development of the deep water microbial maximum.
However, the magnitude of the annual peaks of GPB was similar between years. Only
H. intermedia and M.micrura showed biomass peaks that corresponded to the seasonal
peaks in GPB, but this occurred in different years for both organisms. Periods of high
GPB coincided with low herbivore biomass particularly for both B. calyciflorus and
M. micrura (Fig. 4.19). H. intermedia showed two annual biomass peaks differing in
the timing of the second peak but patterns of its biomass were variable and lacked any
clear seasonality much like the other herbivores.
101
2500
B.calyciflorus
16
2000
14
12
1500
10
8
1000
6
4
500
2
0
0
Jan
May
Sep
Jan
May
Sep
Jan
6
2500
H.intermedia
5
2000
4
1500
3
2
1000
1
500
0
0
Jan
May
Sep
Jan
May
Sep
Jan
100
2500
M.micrura
80
Biomass (mgdwm-3)
Grazeable phytoplankton biomass (mgdwm-3)
Biomass (mgdwm-3 )
18
Biomass (mgdwm-3)
Grazeable phytoplankton biomass (mgdwm-3) Grazeable phytoplankton biomass (mgdwm-3)
20
2000
60
1500
40
1000
20
500
0
0
Jan
May
Sep
Jan
2005
May
Sep
Jan
2006
Figure 4.19: Patterns of herbivore biomass and grazeable phytoplankton biomass.
Grazeable phytoplankton biomass (dotted-lines) and individual herbivore biomass
(solid lines).
102
Mean GPB was estimated as 917 mg dw m-3 (n = 45) and mean cyanobacteria biomass
was 1226 mg dw m-3 (n = 45) (Awortwi, 2009). Similarly, mean herbivore biomass
was 4 mgdwm-3 (n = 99). Thus, the ratio of herbivore biomass to GPB was 229
compared to a ratio of 307 between mean herbivore biomass and cyanobacteria
biomass. The differences in the biomass ratios illustrate the concentration of
phytoplankton community biomass in the cyanobacteria.
4.8.6 Production-Biomass (P/B) Ratios
Daily turnover rates of copepodites were highest but slightly different in successive
years (Tables 4.8 and 4.9). Similarly, P/B ratios least varied for both copepod adults
and Chaoborus, but were constant (0.126 d-1) for M. micrura in both years. The
greatest change in turnover rates occurred in copepod nauplii, for which P/B ratios
increased from 0.28 in 2005 to 0.32 in 2006 representing an increase of 12 %.
Generally, the P/B ratios for copepod nauplii, copepodites and adults where relatively
higher in 2006 compared to 2005. The reverse however, occurred for Chaoborus. No
distinct pattern in the timing of P/B ratio maxima and minima among taxa was evident
among taxa. Generally, high turnover rates of the copepods occurred in March and
during the deep mixing period in August of 2005. The timing of the increase shifted to
November/December in the succeeding year. Chaoborus exhibited a pattern
comparable to that of copepods, as P/B ratio reached its maximum of 0.94 during the
deep mixing in August 2005 and declined to its lowest of 0.001 in November of the
same year. In contrast, the timing of P/B ratio increases in M. moina recurred during
June of both years but differed slightly in the timing of its decline. Highest community
rates of zooplankton in 2006 occurred in May. Turnover rates were as low as 0.09 in
February 2005. Annual turnover rate of the community was higher in 2006 compared
103
to 2005. Mean annual P/B ratio for both years was 23.4. The copepods formed the
largest proportion of annual community turnover rates contributing as high as 78 %.
The 9 % contribution of Chaoborus to community turnover rates was the least (Table
4.10).
104
Table 4.8: Summary statistics of biomass, production, P/B ratios of dominant zooplankton taxa and developmental stages
in 2005. Mass is given as mgdwm-3 and production, mgdwm-3day-1
Species/
Stage
Mean
daily
mass
Mean
daily
Prodn.
Daily
P/B
ratio
min.
mass
max.
mass
mean
mass
Std. ±
dev.
min.
prodn.
max. mean Std.± n
prodn. prodn. dev.
Nauplii
Copepodites
Adult
Copepods
Chaoborus
M.micrura
B.calyciflorus
H.intermedia
135.80
2173.76
39.79
733.86
0.28
0.33
0.71
10.81
23.17
281.55
5.43
86.95
5.02
76.57
0.18
3.63
5.95
105.62
1.59
29.35
1.46
28.61
25
25
2809.18
4453.87
112.32
51.77
24.66
8.86
433.20
13.89
-
0.02
0.09
0.13
-
1.29
5.08
0.43
0.02
0.03
383.67
441.1
59.86
17.15
5.123
112.37
178.15
10.15
3.21
1.12
102.86
128.05
17.88
4.81
1.61
0.07
0.27
0.05
-
1.48
176.08
7.28
-
0.35
17.33
1.24
-
0.42
34.96
2.20
-
25
25
12
20
25
Overall
Mean
9761.37
1394
1229.69
255.0
0.17
105
Table 4.9 : Summary statistics of biomass, production, P/B ratios of dominant zooplankton taxa and developmental stages in 2006.
Mass is given as mgdwm-3 and production, mgdwm-3day-1
Species/
Stage
Mean
daily
mass
Mean
daily
Prodn.
Daily
P/B
ratio
min.
mass
max.
mass
mean
mass
Nauplii
Copepodites
Adult
Copepods
Chaoborus
M.micrura
B.calyciflorus
H.intermedia
172.34
3250.02
54.28
1256.03
0.32
0.37
0.50
30.42
17.95
391.72
7.18
135.42
2125.0
6850.23
180.40
8.57
27.64
50.15
507.29
21.81
-
0.04
0.07
0.13
-
10.60
34.17
1.05
0.09
0.01
218.61
841.10
94.90
3.70
4.25
88.54
285.43
18.89
1.42
1.23
Overall
Mean
12614.21
1802.03
1889.56
377.90
0.19
106
Std.±
Dev.
min.
prodn.
max.
prodn.
mean
prodn.
Std.±
Dev.
n
4.73
91.43
0.15
9.88
4.85
133.70
2.26
52.33
1.46
52.33
24
24
54.03
185.49
25.96
1.38
1.27
0.26
2.29
0.14
-
32.07
78.34
11.08
-
0.26
21.14
2.25
-
2.09
16.85
3.04
-
24
24
13
5
24
Table 4.10: Mean annual biomass, total production, annual P/B ratios and mean P/B ratios of dominant zooplankton
in Lake Bosumtwi in 2005 and 2006.
Species/
stage
Nauplii
Copepodites
Adult
Copepods
Chaoborus
M. micrura
Total
Zooplankton
2005
Production
Biomass
(mgdwt.m-3y-1) (mgdwtm-3)
39.78
5.22
733.86
83.60
8.86
433.20
13.99
1229.69
48.73
171.30
4.32
P/B
ratio y-1
7.62
8.88
2006
Production
Biomass
(mgdwt.m-3y-1) (mgdwtm-3)
54.28
6.89
1256.03
130.00
0.18
2.53
3.22
50.15
507.29
21.89
22.44
1889.64
107
25.58
274.0
7.22
P/B
ratio y-1
7.88
9.66
Mean
P/B ratio y-1
7.41
8.86
1.96
1.85
3.02
0.33
2.11
1.27
24.37
23.40
4.9 Indicators of Habitat Quality: vertical gradients in light, temperature, dissolved
oxygen, food availability and predation risk
Environmental factors relevant to habitat quality; i.e. vertical gradients in light
distribution (Secchi depth), temperature, dissolved oxygen and Chl a conc. are shown in
Figures 4.20 and 4.21.
Stratified Period
Mixing Period (August 2005)
Temp.(oC)
27.0
27.5
28.0
28.5
Temp.(oC)
29.0
29.5
30.0
30.5
27.0
0
0
5
5
10
10
Depth (m)
Depth (m)
26.5
15
20
27.3
27.4
27.5
2005
15
20
30
30
35
35
0
2
4
6
8
0
10 12 14 16 18 20
1
2
3
4
5
6
7
Dissolved Oxygen (mg/L)
Dissolved Oxygen (mg/L)
Mixing Period (August 2006)
Temp. (oC)
26.8 27.0 27.2 27.4 27.6 27.8 28.0 28.2
0
0.6
0.8
27.2
25
Temp.
D.Oxygen
25
27.1
Secchi Depth
5
2006
1.0
10
Depth (m)
Depth (m)
1.2
1.4
1.6
1.8
15
20
25
2.0
2005
2006
2.2
30
2.4
35
Dec
Oct
Nov
Sep
Jul
Aug
Jun
Apr
M ay
M ar
Jan
Feb
2.6
0
1
2
3
4
5
6
7
Dissolved Oxygen (mg/L)
Month
Figure 4.20: Variation of Secchi depth and vertical distribution of temperature and
dissolved Oxygen (DO) during the stratified and interannual mixing seasons of 2005 and
2006. Temperature and Oxygen profiles for the stratified season were taken in May 2006
whereas those of the mixing periods were taken in August of each year.
Persistent stratification of the water column occurred throughout most of the year. The
epilimnion ranged from the surface to 10 m depth, whilst the strata of water between 10
108
and 15 m constituted the metalimnion. The hypolimnion extended from 15 m to 35 m.
The hypolimnion was hypoxic and H 2 S rich as evidenced by its strong sulfurous odour.
Seasonal circulation in August 2005 resulted in near isothermal conditions, deepening of
the thermocline and a decline in oxygen concentrations in the upper water column. DO
was comparatively higher in the epilimnion during the mixing period in 2006 than 2005.
Transparency, measured as the Secchi depth was highest in June of both years (Fig. 4.20).
However, seasonal and interannual differences in transparency were observed; Secchi
depth increased from 1.8 m in 2005 to 2.5 m in 2006 during the same month of both
years. Transparency was usually lowest during the second half of the annual cycle, from
July to December of both years. Highest transparency in June of both years coincided
with the development of the deep (15 m) microbial maximum (Fig.4.21).
Chlorophyll a was usually distributed from the surface to 15 m of depth. This chlorophyll
distribution included the deep water photosynthetic microbial plate at 15 m Peak Chl a.
occurred in the epilimnion throughout most of the year except periods between April and
July of each year when the development of the deep water microbial maxima shifted
higher levels of Chl a to the 15 m depth. During the period of the development of the
deep water microbial maximum, epilimnetic concentrations of chlorophyll a were much
reduced, thereby increasing transparency. The deep water microbial maximum was
disrupted during seasonal circulation and high epilimnetic Chl a conc. were restored
during post mixing periods.
109
Chl a conc. (ug/L)
0
15
0
50
0
15
0
0
6
0
18
5
Depth (m)
10
15
20
25
30
35
Apr
May
Jun
Jul
Aug
2005
0
22 0
12
0
0
14
0
50
0
50 0
30
0
25 0
25
0
15 0
10 0
10
0
10
5
Depth (m)
10
15
20
25
30
35
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sept.
Oct
Nov
Dec
2006
Figure 4.21: Depth distribution of monthly mean Chl a (µg/L) in 2005 and 2006 showing
deep water microbial maximum from April to July each year. Lower frequency of
measurements in 2005 is attributed to logistical constraints.
Predation risk to herbivores was assessed on the basis of the depth distribution of the
main invertebrate predator Chaoborus (Fig. 4.22). The daytime depth distribution of
Chaoborus was usually limited to the water column below 10 m. Peak densities occurred
between 20 and 25 m, well into anoxic waters (Fig. 4.22). During October and November
2006, Chaoborus was dispersed between the surface and 30 m, however, the highest
densities in the hypolimnion were maintained. Additionally, during June 2006,
Chaoborus was found only in the epilimnion between 2 and 12 m during the day.
110
Density (No.of Individuals/m-3)
0
0
20000.0 1.2e+5
15000
1600 0 7000 0 4000 0 8000 0
5000 0
350 0 7000 0
9000 0
4000
Nov
Dec
4000
0
5
Depth (m)
10
15
20
25
30
35
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
2005
Strong seasonal circulation
0 15000 0 4000 0
1800 0
2000 0 2700 0 20000 0 4000 0 10000 0
3500 0
20000
3000 0
Sep
Oct
Nov
0
Depth (m)
5
10
15
20
25
30
35
Jan
2006
Feb
Mar
Apr
May
Jun
Jul
Aug
Dec
Weak seasonal circulation
Figure 4.22: Patterns of vertical distribution of Chaoborus during 2005 and 2006
4.9.1 Patterns of vertical distribution of zooplankton and responses to seasonal
alterations in habitat quality parameters
4.9.1.1 Copepods
Nauplii, copepodites and adult copepods exhibited similar vertical distribution patterns
and persistent residence in the epilimnion throughout most of the year. Nauplii were
usually concentrated at the top 10 m of the water column throughout most of the year
when there was thermal stratification (Fig. 4.23). The depth range of their distribution
was increased to 15 m in 2006. Maximum densities occurred in the upper 5 m. During
111
March, 2005, nauplii unusually extended their depth distribution to well below 10 m, but
this was not repeated in 2006.
Density (No. of Individuals/m-3)
0
7000
3000
0
300000 15003000 0
0
20000
35000100016000
25000 0
1e+5
0 1e+5
6000
1.5e+5
Oct
Nov
Dec
0
Depth (m)
5
10
15
20
25
30
35
Jan
Feb
Mar
Apr
May
Jun
Aug
Jul
Sep
2005
Strong seasonal circulation
0 30000 0
15000
5000
0
10000 0
5000 0 10000
0
0
150000
0
30000
10000 0
100000
1e+5
0
25000
5
Depth (m)
10
15
20
25
30
35
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Dec
Nov
2006
Weak seasonal Circulation
Density (No. of Individuals/m-3)
0
6e+4 0
50000 0
30000 0
45000
0
80000
20000
0
8e+4 0
7e+4 0.0
1.2e+5
0.0
0
1.4e+5
20000 0
20000
0
5
Depth (m)
10
15
20
25
30
35
Jan
Feb
Mar
Apr
May
Jul
Jun
Aug
Sep
Oct
Nov
Dec
2005
Strong seasonal circulation
0
400000
60000
0
1e+50
0
7e+40.0 1.2e+50
7e+4 0
1e+5 0 50000 0
22000 0
Jun
Jul
Sep
1e+5 0
600000
8e+4
5
Depth (m)
10
15
20
25
30
35
Jan
Feb
Mar
2006
Apr
May
Aug
Oct
Nov
Dec
Weak seasonal Circulation
Figure 4.23: Patterns of vertical distribution of copepod nauplii (top panel) and
copepodite (lower panel) developmental stages respectively. Seasonal circulation periods
are illustrated with the arrows.
112
There was a broader distribution of nauplii to 30 m between June and September during
the strong seasonal mixing in 2005, but this was not repeated in 2006 due to weaker
mixing. The mean population depth (MPD) of nauplii was approximately 15 m. Nauplii
responded to the deep water microbial maximum only in June of each year by
aggregating close to the 15 m depth (depth of deep water microbial maximum). The
period immediately following the collapse of the deep water microbial maximum marked
the period of dramatic epilimnetic distribution and peak densities for nauplii. This was
more evident in 2006 than in 2005.
Copepodites were distributed within the upper 12 m, corresponding to the entire
epilimnion and a portion of the metalimnion. Copepodites showed a similar pattern of
vertical distribution with nauplii during 2005. Copepodites however showed a more
uniform distribution compared to nauplii during 2006. Peak densities of copepodites were
measured in the upper 5 m of the epilimnion during 2006. For both copepodites and
nauplii, seasonal mixing events resulted in a broader distribution in the water column to
depths of 30 m. MPD of copepodites was 14.3 m which is similar to the MPD of nauplii.
Copepodites showed no response to the deep microbial maxima in both years.
Adult copepods also showed no response to the deep seasonal algal maxima. Seasonal
mixing also ensured a broader, but somewhat staggered distribution during August 2005
(Fig.4.24). MPD of 12.8 m of adults was shallower than both nauplii and copepodites.
Adult copepods were mostly concentrated in the top 10 m with maximum densities in the
upper 5 m.
113
4.9.1.2 Cladocera
M.micrura was absent from the water column for most of the year. Additionally, its
absence during the mixing season made it impossible to capture effect of mixing on the
character of its vertical distribution. The vertical distribution of M. micrura was restricted
to the upper 5 m in 2006 in contrast to the variable depth distribution in 2005 (Fig. 4.24).
The MPD of M. micrura was approximately 6 m whilst peak densities occurred in the
upper 5 m of the water column. Together with copepod nauplii, M. micrura showed some
response to the deep water algal maximum by peaking close to it in June 2005.
Density (No.of Individuals/m-3)
0 20000 0
80000
8000
15000 0
0
30000 0
30000 0
1000 0
12000 0
30000 0
200000
4000 0 4000
0
Depth (m)
5
10
15
20
25
30
35
Jan
Feb
Mar
Apr
May
Jun
Jul
2005
0
100000
150000
Aug
Oct
Sep
Nov
Dec
Strong seasonal circulation
5000 0
20000
20000
0
6000
10000 0
700
0
1100
0
2000 0
3000 0 4000
0
5
Depth (m)
10
15
20
25
30
35
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
2006
Weak seasonal circulation
0
2000 0
Density (No. individuals/m-3)
500 0
1000 0
2500 0
14000 0
2000
0
5
Depth (m)
10
15
20
25
30
35
Mar
0
4000 0
Apr
700 0
Jun
May
2005
1500 0
4000
Jul
0
Dec
12
0
5
Depth (m)
10
15
20
25
30
35
Mar
Apr
May
Jun
Jul
2006
Figure 4.24: Patterns of vertical distribution of adult copepods (top panel) and M. micrura
(lower panel) respectively.
114
4.9.1.3 Rotifers
Seasonal mixing in August 2005 resulted in a broader distribution for B. calyciflorus (Fig.
4.25). There was also a broader distribution in June and December 2005, but densities
were lopsided and tapered off in the hypolimnion. During August 2006, B.calyciflorus
were found in 15 m deep water but in a relatively lower density compared to epilimnetic
densities. The MPD of B. calyciflorus was restricted to the epilimnion at ~ 6 m. Maximal
population densities were found within the upper 2 m.
Like most zooplankton, H. intermedia showed a fairly stable vertical distribution pattern
in successive years (Fig. 4.26). The stability in vertical distribution was disrupted by the
seasonal mixing in August 2005 but not in August 2006. During this period, H.
intermedia population was dispersed down to 25 m depth. Peak population densities were
recorded in the upper 5 m of the epilimnion. The MPD and habitat range were both 10 m
for H. intermedia.
Density (No.of Individuals/m-3)
0
5000 0
2000 0
2000 0
27000
38000
30000 0 15000
0
8e+4
0
4000 0
5000
0
Depth (m)
5
10
15
20
25
30
35
Jan
Apr
Mar
Feb
May
Jun
Jul
Aug
Nov
Dec
2005
0
3000 0
11000 0
10000 0
2000 4000
0
Depth (m)
5
10
15
20
25
30
35
Aug
Sep
Oct
2006
Nov
Figure 4.25: Patterns of vertical distribution of B.calyciflorus in 2005 and 2006.
115
Density (No.of Individuals/m-3)
0
1000
0
2000 0
1600
0
8000
0
3000
0
0
2000 0
2000
0
7000
0
6000
0
6000
0
6000
0
16000
5
Depth (m)
10
15
20
25
30
35
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
2005
Strong seasonal circulation
0
40
0
4000
0
4000
0
10000 0
3000 0
9000
0
7000
0
1000
0
9000
0
8000
0
10000 0
10000
0
5
Depth (m)
10
15
20
25
30
35
Jan
Feb
Mar
Apr
May
Jun
2006
Jul
Aug
Sep
Oct
Nov
Dec
Weak seasonal circulation
Figure 4.26: Patterns of vertical distribution of H.intermedia in 2005 and 2006.
4.9.2 Epilimnetic proportion of each zooplankton species
Over 90 % of each zooplankton species and copepod developmental stages resided in the
epilimnion at all times throughout the year (Fig. 4.27). The exception was copepod
nauplii which had > 80 % of its population regularly living in the epilimnion. During
2006, copepodites, copepod adults and M. micrura exhibited a complete epilimnetic
residence. Interannual differences in the epilimnetic proportion of each species were
apparent but not substantial.
116
Percent Abundance in Epilmnion
100
90
80
70
60
50
40
30
20
10
0
up
Na
lii
Co
od
p ep
i t es
ul
Ad
ts
a
ra
rus
ed i
cru
flo
i
m
i
r
c
m
e
nt
M.
aly
H.i
B.c
2005
2006
Species
Figure 4.27: Annual mean percent of each species and copepod developmental stages
occurring in the epilimnion of Lake Bosumtwi.
117
4.9.3 Diel Vertical Migration (DVM) pattern of zooplankton
One diel vertical migration (DVM) study was conducted during the onset of the strong
circulation event on 29th July, 2005, evident from the temperature and dissolved oxygen
profiles below (Fig. 4.20 & 4.29).
29-09-05
0
30000 0 8e+4
Density (No.individuals/m-3)
0 22000 0
2000 0 30000
0
300027.0 27.3 27.6
0 40 80 120
Depth (m)
0
3
6
9
12
15
18
21
24
27
30
33
Nauplii
Copepodites Adults
M.micrura
B.caly.
H.int.
Chaoborus
(Temp.
O
C) DO(%sat)
12 noon
Depth (m)
0
30000 0
3e+5 0
8000 0
4000
50000
0
0
3000
0
6e+4
27.0
27.3
27.6 0
40
80
0
3
6
9
12
15
18
21
24
27
30
33
Nauplii
Copepodites Adults
M.micrura
B.caly.
Chaoborus
H.int.
(Temp.
O
C)
DO(%sat)
7-8pm
Depth (m)
0
25000 0.0
2.5e+5
0
7000
0
4000
0
40000
3000
0
6e+4
27.0
27.3
27.6 0
40
80
0
3
6
9
12
15
18
21
24
27
30
33
Nauplii
Copepodites Adults
M.micrura
H.int.
B.caly. Chaoborus
(Temp.
O
C)
DO(%sat)
30-7-05: 12-1 am
Depth (m)
0 12000 0
6e+4 0 10000
0
1200
0 12000 0
500
0
2500027.32 27.36 27.40
0
80 160
0
3
6
9
12
15
18
21
24
27
30
33
Nauplii
Copepodites Adults
M.micrura
B.caly. Chaoborus
H.int.
(Temp.
O
C)
DO(%sat)
6-7am
Figure 4.28: Patterns of DVM of zooplankton including temperature and dissolved
oxygen profiles during the onset of the strong seasonal mixing on 29th July 2005.
Chaoborus was not detected in the water column (surface-30 m) at 12 noon.
118
Analyses of the temporal changes in densities of individual species and copepod
developmental stages with depth reveal vertical migration in nauplii, copepodites,
M.micrura, B.calyciflorus and H.intermedia, but these species did not migrate in the same
way (Fig. 4.28). Naupliar densities of 30,000 individuals m-3 were maintained from 12
noon to 7 - 8 pm but decreased through the night to 12,000 individuals m-3 in the surface
waters at dawn. Copepodites exhibited a similar pattern to nauplii, whilst M.micrura
increased in surface waters at night, but a portion of the population was found at 10 and
15 m during the night and at dawn respectively. Copepod adults showed little migration
whilst Chaoborus ascended into surface waters at sunset and descended at dawn. Night
depth range of species distribution was about surface to 25 m for all organisms. Water
column turbulence and upwelling currents during the period may have opposed the active
movements of zooplankton and resulted in a passive distribution of zooplankton
throughout the water column.
A contrasting pattern of vertical migration pattern was observed during the 26th May,
2006 study in which the water column was more strongly stratified (Fig.4.29). Copepod
nauplii and copepodites did not migrate at all throughout the 24 hr period and remained
between 3 and 15 m for nauplii and between the surface and 15 m for copepodites.
Copepod adults increased from 0 to 20,000 individuals m-3 at surface waters between
noon and sunset, but disappeared from the surface at dawn to depths between 3 and 15 m.
The pattern of vertical migration of M. micrura was similar to copepod adults. Only
Chaoborus showed distinct vertical migration by moving en masse from below 20 m
during the day to the surface at night and descending again to the hypolimnion at dawn
(Fig. 4.29). H. intermedia increased continuously at every 6 h interval at the surface
119
waters until reaching 150,000 individuals m-3 at dawn from 0 at noon the previous day.
All zooplankton were concentrated in the epilimnion and metalimnion throughout the 24
hr period.
26-5-06
Depth (m)
0
Density (No.of Individuals/m-3)
7e+4
0.0 1.6e+5
0
Nauplii
12-1pm
Cop.
0.0 1.2e+5 0.0 1.2e+5 0
Depth (m)
30000 0
12000
0
2500 0
50000 24
28
32
0
3
6
9
12
15
18
21
24
27
30
33
Adults
M.micrura Chaoborus
20000 0
25000 0
2500
Temp. (0C)
H.int.
0
4000
M.micrura Chaoborus
H.int.
24
28
32
0
3
6
9
12
15
18
21
24
27
30
33
Nauplii
Cop.
Adults
Temp. (oC)
7-8pm
Depth (m)
0
7e+4
0
1e+5 0
10000
0
13000
0
3000
0
15000
0
3
6
9
12
15
18
21
24
27
30
33
Nauplii
Cop.
Adults
M.micrura Chaoborus
H.int.
27-6-06: 12-1 am
Depth (m)
0
8e+4 0.0 1.6e+5 0
15000 0
900
0
2000 0.0
1.5e+5
0
3
6
9
12
15
18
21
24
27
30
33
Nauplii
Cop.
Adults
M.micrura Chaoborus
H.int.
6-7am
Figure 4.29: Patterns of DVM of copepod nauplii, copepodites, adult copepods, M.
micrura, Chaoborus and H.intermedia including temperature profiles on 26th May, 2006.
B. calyciflorus was absent from the water column. Night temperature profiles were not
taken due to lack of battery power for CTD. Dissolved Oxygen profiles were not included
due to their similarity with temperature profiles.
120
Adults copepods, M.micrura and B. calyciflorus were not detected in the water column
during the night in the course of the December 19, 2006 study. Nauplii maintained the
same depth range much like the May 26th study and though copepodites followed the
same pattern, their density decreased at surface waters at sunset and during the night,
from a high 140,000 individuals/m-3 at noon (Fig.4.3). Peak densities for copepodites
were recorded at 5 m at dawn. All zooplankton with the exception of Chaoborus
maintained epilimnetic and metalimnetic residence throughout the 24 hr period.
Chaoborus showed distinct mass migration by ascending to surface waters at sunset and
descended into the hypolimnion at dawn.
19-12-06
0
15000
Density (No.of Individuals/m-3)
0.0 1.4e+5
0
4000
0
9000
0
1200
0
8000
27
29
31
Depth (m)
0
3
6
9
12
15
18
21
24
27
30
33
Naupliii
Cop.
Adults
B.caly.
Chaoborus
Temp. (oC)
H.int.
12-1pm
Depth (m)
0
14000
0
6e+4
0
1000 0
2500
0
1200
0
8000
27
29
31
0
3
6
9
12
15
18
21
24
27
30
33
Naupliii
Cop.
Adults
B.caly.
Chaoborus
H.int.
Temp. (oC)
7-8pm
Depth (m)
0
17000
0.0
Nauplii
Depth (m)
6.5e+4 0
1500
0
1500
0
3
6
9
12
15
18
21
24
27
30
33
20-12-06: 12-1 am
0
30000 0
0
3
6
9
12
15
18
21
24
27
30
33
Nauplii
Cop.
6e+4
Cop.
Chaoborus
0
2000
Chaoborus
H.int.
0
4000
H.int.
6-7am
Figure 4.30: Patterns of DVM of copepod nauplii, copepodites, adults, Chaoborus, B.
calyciflorus, H. intermedia recorded on 19th December, 2006.
121
CHAPTER 5: DISCUSSION
5.1 Species richness and community diversity
Characteristic of many tropical lakes, the zooplankton community of Lake Bosumtwi is
less diverse and consists of only nine species, namely; the endemic single species’ of the
cyclopoid copepod- M. bosumtwii (Mirabdulayev et al., 2007) the cladoceran M. micrura,
the invertebrate predator, C.ceratopogones and six rotifer species; B.calyciflorus,
B.dimidiatus, H.intermedia, F. pejleri, F.camascela and K. cochlearis. Interplay of biotic
and physical factors may be responsible for the low species diversity of zooplankton in
Lake Bosumtwi. The lake is uniquely isolated from the drainage network of Ghana and
its hydrologically closed basin and nature of the surrounding topography may restrict
species import through hydrological transfers. For species which may be introduced via
other processes apart from hydrological routes, successful establishment in the pelagic
zooplankton community may be constrained by fewer niches of the less complex physical
environment which may not support additional species (Dumont, 1994 a). Immigrant
species, therefore, may have little chance of survival from inevitable competitive
exclusion by native species with comparable trophic specialization (Dumont personal
communication.). Resistance by local species interactions (Shurin et al., 2000) may also
reduce success of foreign species. In some cases, however, available niches may allow
successful establishment of non-native species (Shurin et al., 2000). Similarly, the
unstructured and resource limited pelagic physical environment of ancient lakes such as
Victoria, Malawi, Albert and Edward may be driving the zooplankton diversity to
competitive equilibrium (Dumont, 1994 a). Dumont (1994a) identified Lakes Tanganyika
122
and Baikal as being closest to achieving low community diversity equilibrium compared
to the aforementioned East African Lakes.
5.1.1 Effect of biotic processes on diversity
Biotic processes of predation and competition may be principally responsible for the low
diversity of the Lake Bosumtwi pelagic zooplankton. Paine (1966) argues that predation
should reverse the impact of competition by reducing prey populations to allow the
coexistence of more species and consequently increase species diversity. This argument
has been supported by reports of the role of invertebrate predators and planktivorous
fishes in regulating pelagic zooplankton species diversity in the tropics (Kerfoot and
Lynch, 1987; Kerfoot and Sih, 1987; Fernando et al., 1990). Nonetheless, unlike
temperate communities where predation effect is seasonal and allows prey populations to
alternately flourish at certain periods, continuous predation by invertebrate and fish
predators on tropical pelagic zooplankton communities have eliminated certain species
and confined others to fishless lakes (Kerfoot and Lynch, 1987). Low limnetic
recruitment of zooplankton from the littoral regions, which harbour more species, has
also been attributed to high invertebrate predation (Fernando et al., 1990). But extreme
eutrophy reduces littoral zone complexity by eliminating benthic plant production.
Examples of the excessive impact of predation are reported in the literature; elimination
of Cladocera from Lake Tanganyika (Hecky, 1991) and the disappearance of Daphnia
curvirostris Eylmann from Lake Kivu following the introduction of the Lake Tanganyika
sardine, Limnothrissa miodon (Isumbisho et al., 2006). According to Whyte (1975) and
more recently (Poste et al., 2008) based on stable isotope analysis of the food web
structure, true pelagic zooplanktivorous fishes are absent in Lake Bosumtwi. Despite the
123
rarity of pelagic fish in the lake, zooplankton diversity is still low and further
demonstrates the efficiency of control by invertebrate predators (Fernando and Holicik,
1982) and perhaps food competition. The cichlids Tilapia busumana, T.discolor and
Hemichromis fasciatus are facultative planktonic feeders and are unable to breed
successfully in the limnetic zone as a result of the low resource availability (Whyte,
1975). Consequently, low predation pressure coupled with possible predator avoidance
strategies, has allowed the invertebrate predators, Chaoborus and the copepods, to
dominate and shape the diversity of the pelagic zooplankton community in Lake
Bosumtwi. Fernando et al (1990) suggest that, although invertebrate predators generally
preceded fish predators in exerting major control on zooplankton, the diversity of present
day zooplankton in most tropical lakes can largely be attributed to fish predation. Based
on this assertion, Lake Bosumtwi zooplankton may be assumed to yet undergo this
ecological displacement, despite the ancient age of Bosumtwi (Koeberl et al., 2007). The
low diversity of zooplankton and the lack of zooplanktivorous fish given the evidence for
rapid evolution among haplochromines in Lake Victoria (Johnson et al., 1996) suggest a
very harsh habitat inimical to most zooplankton species in Lake Bosumtwi. Similarly, the
high abundance of cyanobacteria phytoplankton compared to herbivores zooplankton in
Bosumtwi may indicate a trophic “bottleneck” restricting energy flow from the
phytoplankton to the herbivorous zooplankton. The phytoplankton is dominated by
filamentous and colonial cyanobacteria and dinoflagellates (Awortwi, 2009) which may
constitute a difficult to graze or even toxic ration for many zooplankton. But changes in
the diversity of pelagic zooplankton occur over ecological and geological time and the
lack of long term historical records on possible alterations in the food web structure limits
124
accurate interpretation of the present day status of the pelagic zooplankton community
diversity of Lake Bosumtwi.
5.2 Patterns of zooplankton abundance
Patterns and scale of total zooplankton abundance were characterized by interannual
differences in short-term changes and seasonal mixing cycles of the water column and the
seasonal amount of precipitation as implied by the association of peak zooplankton
abundances with the onset of the rains in April of each year. Rainfall has been implicated
as major source of nutrient input into tropical systems either through run off or direct
precipitation onto the lake surface (Burgis, 1974; Mavuti, 1990; Kizito, 1998;
Tamatamah et al., 2005). Together with nutrient input from lake mixing, they
characterize the seasonality in many tropical lakes (Mengestou and Fernando, 1991;
Mavuti, 1990; Kizito; 1998). M. bosumtwii, H. intermedia, B. calyciflorus and M.micrura
showed clear seasonal maxima in response to chlorophyll a and improvement in the
quality and quantity of food supply. The remaining species; C. ceratopogones, F. pejleri,
F. camascela, K. cochlearis and B. dimidiatus lacked any seasonality. The improvement
in food quality and quantity was associated with mixing of the water column that result in
entrainment of nutrient rich water into the epilimnion further stimulating phytoplankton
growth for utilization at higher trophic levels. This increase in food resource leads to
increases in total zooplankton abundance. The seasonality of tropical zooplankton species
is now well established and represents a major paradigm that is supported by evidence
from many tropical lakes of varying depth, notably, Lake Malawi (Degnbol and Mapila,
1982; Twombly, 1983; Irvine, 1995), Lake Kivu (Isumbisho et al., 2006), Lake Chilwa
(Kalk, 1979), Lake Lanao (Lewis, 1979), Lake Tanganyika (Mulimbwa, 1988, 1991;
125
Kurki et al., 1999) and Lake Awasa (Mengestou and Fernando, 1991). The evidence
provided by the array of investigations suggests that the short term fluctuations in
zooplankton abundance are superimposed on the seasonal cycle and these elicit varying
responses from individual species populations. Twombly (1983) emphasized the
demographic advantages that such short-term episodes of water column instability confer
on opportunistic species, particularly rotifers, to offset continuous predation. Such
populations possess rapid development rates and short generation times and can readily
respond to the brief favourable conditions. Not all species in the Lake Bosumtwi
zooplankton exhibit such characteristic tropical behaviour. Patterns of seasonal and
intraseasonal variation in abundance are demonstrated by adults, copepodites and nauplii
of M. bosumtwii and to a lesser extent H.intermedia. The remaining species may show
response to favourable seasonal conditions but the timing and magnitude of the response
may vary from year to year. Generally, short-term oscillations in the population sizes of
individual taxon and corresponding seasonal increments in individual population size
contributed to overall interannual variations in total zooplankton abundance in Lake
Bosumtwi. Twombly (1983) has documented similar patterns for Lake Malawi.
Additionally, correlation coefficients determined for paired counts of each species
population during the same week in successive years showed weak correspondence in the
timing and periodicity of seasonal patterns of abundance; which further indicates that the
species differ regarding the timing of their population increase and decline (Twombly,
1983; Kurki, 1996). Seasonal trends in many African lakes support this observation.
Moreover, total zooplankton abundance varies between the stratified periods and the
circulation and post circulation periods. Total zooplankton abundance was higher during
126
and after the circulation period between (or “in”) August and December each year. The
pattern of this difference was repeated in successive years but was greater in 2006.
Reasons for this difference are clearly due to the intensity of rejuvenation of the
trophogenic zone by climate driven nutrient input (Lewis, 1987; Talling and Lemoalle,
1998; Puchniak et al., 2009) which sets the limit for the potential biotic utilization of the
plankton community (Carpenter et al., 1985).
5.2.1 Patterns of overall community abundance
Analysis of the community showed that whereas the copepods dominated the
zooplankton community in Lake Bosumtwi majority of the rotifers was rare. Dominance
of the cyclopoid copepod
in the zooplankton community of Lake Bosumtwi is
comparable to observations in many tropical zooplankton communities; Lake George
Uganda (Burgis, 1971, 1973, 1974), Lake Awasa Ethiopia, (Mengestou and Fernando,
1991) Lake Malawi (Irvine and Waya, 1999; Twombly, 1983) (but a calanoid herbivore
is the most common copepod in Malawi), Lakes Nkuruba and Nyahira, Uganda, ( Kizito,
1998), Lake Tanganyika (Kurki, 1996) also a herbivorous calanoid dominates), Opi Lake,
Nigeria (Hare and Carter, 1987), Lake Kivu (Isumbisho et al., 2006), Lake Victoria
(Mwebaza-Ndawula, 1994), and five Kenyan lakes (Uku and Mavuti, 1994). Average
monthly contribution to total zooplankton abundance by the Copepods was 68 %. During
certain periods in September/October 2005, copepods made up 90 % of the total
zooplankton abundance in Lake Bosumtwi.
Proximal factors to explain this dominance by cyclopoids are ascribed to low pelagic fish
abundance and consequently low predation pressure. But more importantly, copepods are
K-strategist having low reproductive rates but high survival rates of juveniles and
127
consequently high recruitment rates allowing for the maintenance of large population
sizes (Allan, 1976). Furthermore, evasive strategies such as vertical migration and rapid
burst swimming to avoid visual feeding predators (Maier, 1993; Ghan et al., 1998;
Perticarrari et al., 2004), investment in less conspicuous earlier developmental stages to
balance adult losses to predation (Saunders and Lewis, 1988 b) and versatile mode of
feeding by cyclopoid copepods (Kumar and Rao, 1998; Rao and Kumar, 2004) may
contribute to their dominance in most tropical lakes. The high monthly average
abundance of the copepods seems unlikely to be adequately supported by an unreliable
seasonal rotifer food supply. It suggests that, the copepods may probably be utilizing
algal and microbial production in the lake to sustain their dominance in the community.
Also cannibalistic predation by M. bosumtwii on nauplii and copepodites may allow the
dominance of the copepod but at low and constrained population densities. Lake
Bosumtwi zooplankton lacks an efficient grazer, such as a calanoid copepod and large
Cladocera, to effectively utilize the dominant blue greens (Whyte, 1975, Awortwi, 2009).
The copepods could possibly be supplementing their animal diet with both algal and
microbial production but the pathways are unclear. Omnivory has been implicated in the
cyclopoid copepods and found to enhance reproductive success (Adrian and Frost, 1993;
Makino and Ban, 2000). Nauplii and early copepodites are however more herbivorous
(McQueen, 1969; Maly and Maly, 1974; Gophen, 1977; Jamieson, 1980) than adults
(Zankai and Ponyi, 1986; Adrian, 1991a, 1991 b), and this favours greater survival and
recruitment to offset losses to adult predation (Saunders and Lewis, 1988 b). Similarly,
evidence for algal nutrition has been reported for cyclopoid copepods in tropical lakes
(Hare and Carter, 1987). The transition from herbivory of nauplii and copepodites to
128
predation in adults (McQueen, 1969; Maly and Maly, 1974; Gophen, 1977; Jamieson,
1980) may reflect an evolutionary adaptation of cyclopoid copepods to increase
survivorship and recruitment among nauplii and copepodites by reducing competition
with rotifer and cladoceran herbivores, due in part, to the susceptibility of the early
ontogenetic stages of copepods to starvation (Soto and Hulbert, 1991; Santer and Van den
Bosch, 1994).
5.2.2 Rarity of rotifers in Lake Bosumtwi
Whilst dominance by the copepods has been evaluated on the basis of various strategies
employed, assessment of the rarity of rotifers in Lake Bosumtwi is equally worthy. Rare
species are very important in bioassessment techniques and community analyses (Gannon
and Stemberger, 1978; Cao et al., 1996) because they enhance species richness (Krebs,
1985; Putman, 1994; Gotelli and Graves, 1996) and are valuable indicators of ecosystem
health (Schelske, 1984; Ford, 1989; Minns et al., 1988; Lyons et al., 1995). Therefore,
their exclusion from community analyses may result in loss of useful information
(Goodall, 1969; Fore et al., 1996; Courtemanch, 1996; Karr and Chu, 1997). The low
diversity of rotifers in some African lakes is related to invertebrate predation by
Chaoborus and Mesocyclops like in Lake Malawi and Mesocyclops in Lake Tanganyika
(Lehman, 1988) but control of invertebrate predators by planktivorous fish in Lake Kivu
minimizes their effect on rotifers (Isumbisho et al., 2006). Zooplanktivory is also strong
in Lake Tanganyika and Lake Malawi but rotifers are not common (R. E Hecky, personal
communication.).
129
5.2.3 Community dominance of predators
The invertebrate predators, M. bosumtwii (Copepoda) and C. ceratopogones dominated
the zooplankton community in Lake Bosumtwi due to the absence of true pelagic
zooplanktivores. Microzooplankton is the preferred food for invertebrate predators (Hall
et al., 1976; Fernando et al., 1990). Except H.intermedia, the invertebrate predators
suppressed all the other members of the herbivore community throughout the year.
Typically, among the invertebrates, with the exception of few temperate species of
rotifers, cladocera and other groups, most members of the cyclopoid Copepoda and
Chaoborus are the major predators among zooplankton (Fernando et al., 1990).
Chaoborus however appears to have limited distribution, but spectacular abundance in
the tropics compared to temperate regions and its absence in Lakes Tanganyika, Kivu and
Ethiopian Rift Valley lakes has been noted (Fernando et al., 1990). Compensatory
dominance by other invertebrate zooplanktivores and competition among herbivores has
been shown to influence community structure in the absence of planktivorous fishes
(Brooks and Dodson, 1965; Hall et al., 1976). Such trophic interactions are visible and
pronounced in Lake Bosumtwi during periods of nutrient enrichment in the peak rainy
season in May/June and during the major mixing events in August. Nutrient input from
these sources stimulates phytoplankton growth leading to marked response by the
herbivore populations. Such a response suggest a food constraint on herbivorous
zooplankton and more importantly such nutrient driven events may allow growth of
“grazeable” phytoplankton. Corresponding increases in predator abundances enhance
total zooplankton abundance during the period. The seasonal growth activity of both
herbivore species- M. micrura and B. calyciflorus could be described as opportunistic,
130
even though they are noted as the main grazers in tropical freshwater ecosystems due to
the absence of the more efficient Cladocera and calanoid copepods (Aka et al., 2000;
Fernando, 2002). The other rotifer species, B.dimidiatus, K.cochlearis, F.pejleri, and
F.camascela, are rarely present in the zooplankton community. Incisive environmental
phenomena such as deep mixing and relatively high amounts of rainfall in 2005
rejuvenated both B.dimidiatus and K.cochlearis. This led to an appreciable contribution
of ~ 50 % by B.dimidiatus to monthly total zooplankton abundance. According to
Nogrady et al (1993), rotifers display opportunistic tendency towards changing food
levels and their abundance usually increases linearly with rising food concentrations
(Dumont et al., 1995). H. intermedia increases its populations at low concentrations of
phytoplankton, M. micrura responds at moderate to high concentrations whilst B.
calyciflorus requires high concentrations. Available information on partitioning of food
resources are based on experimental observations as has been reported for tropical
species of M.micrura, Diaphanosoma excisum, B.calyciflorus (Pagano, 2008).
Nonetheless, fine-scale measurements may provide adequate prospects for enhancing
resolution of nutrient and food web interactions in tropical natural systems. Differences
in threshold food concentration among the herbivore populations may govern the timing
of the response to the seasonal peak in phytoplankton growth. M. micrura has been
shown to require very high food concentrations (Romanovsky, 1984) as well as B.
calyciflorus (Lehman, 1988; Sarma et al., 1999) but relatively low concentrations may
apply to H.intermedia in maintaining positive population growth. The major food sources
for Hexarthra sp however is detritus and associated bacteria (e.g Sanders et al., 1989;
Bouvy et al., 1994) which could be abundant in Lake Bosumtwi and could account for
131
the persistence of H. intermedia in the plankton in the face of limiting algal food of the
required quality and quantity. Sensitivity of H.intermedia to phytoplankton growth was
higher in 2006 than 2005, due to variable thickness of mixed depth and corresponding
nutrient supply. Peaks in phytoplankton chlorophyll were brief and declined sharply
preventing both M.micrura and B. calyciflorus to sustain positive population growth in
order to match or offset losses to invertebrate predators. Conversely, the earlier response
of H.intermedia to the quantity and quality of food ensured a prolonged food supply that
supported high population growth leading to its dominance of the herbivore community.
5.3 Relationship between Chl a concentration and total herbivore abundance
Linear correlation between mean Chl a and 2-week lag total herbivore abundance yielded
no statistically significant correlation, (R²= 0.103, P>0.05). This indicates a low
statistical correspondence between Chl a and total herbivore abundance thus Chl a was an
unreliable predictor of total herbivore abundance.
5.4 Length-weight relationships
Length-weight regressions of a species examined for different seasons represent the best
approach to accurately resolve the length-weight relationships of zooplankton taxa,
because length-weight relationships indeed vary seasonally in temperate lakes (Culver et
al., 1985), owing to changes in quantity and quality of food supply (Dumont et al., 1975).
In contrast, changes in fecundity may be the best indicator of varying food conditions in
tropical zooplankton populations due to the limited information on length-weight
relationships (Amarasinghe et al., 1997). Though seasonality in numerical abundance has
been implicated in tropical zooplankton populations, the concept has not been extended to
patterns of length-weight relationships. Few length-weight equations exist for common
132
tropical zooplankton species (Lewis, 1979), particularly in African lake systems. In the
context of European and American work, length-weight relationships have been
developed for their common species (Dumont et al., 1975; Bottrell et al., 1976; Culver et
al., 1985). In some instances, within the tropical situation, only fresh weight have been
used (Lewis, 1979), whereas in other cases, combinations of both fresh and dry weights
have been employed (Mengestou and Fernando, 1991). The choice of fresh weight is
basically to minimize the analytical effort and also to circumvent technological
inadequacies associated with dry weight estimates in the tropics, which are regarded as
the best means of assessing the weight of organisms (Dumont et al., 1975; Bottrell et al.,
1976; Persson and Ekbohm, 1980). Equations have been obtained for some crustacean
zooplankton taxa and their developmental stages in Lake Kariba (Zambia/Zimbabwe)
(Masundire, 1994), Lake Malawi (Irvine and Waya, 1999), Lake Awasa-Ethiopia
(Mengestou and Fernando, 1991) and Lake Nkuruba-Uganda (Kizito, 1998). Whilst the
slopes of the different equations for Mesocyclops a. aequatorialis female adults of Lakes
Malawi, Nkuruba and Awasa compare (difference of > 2 orders of magnitude) with those
of Mesocyclops bosumtwii in Lake Bosumtwi, the intercepts widely differ with the latter
having higher values (Table 5). Table 5 below shows the comparison of the mean lengths
and weights of adult Mesocyclops spp and their developmental stages in a temperate and
some African lakes for which data is available. The mean lengths and corresponding dry
weights of various stages of Mesocyclops sp in volcanic crater Lake Nkuruba and Lake
Bosumtwii compare very well. The same genus in temperate Lake Ontario (CanadaUSA) has comparable lengths for all stages but differs from Mesocyclops in Lake
Bosumtwi and Nkuruba by lower weights for similar lengths. Irvine and Waya (1999)
133
have also demonstrated a similar relationship for Mesocyclops in Lake Malawi. They
report lower weights for corresponding lengths. However, for comparable body lengths,
adults of Mesocyclops in Lakes Bosumtwi and Nkuruba have higher body weights.
Table 5.0: Comparison of mean lengths (µm) and weights (µg) of Mesocyclops sp in
Lake Bosumtwi (Bos.) Ghana, Lake Nkuruba (Nku.) Uganda, Lake Awasa Ethiopia and
Lake Ontario (Ont.) Canada-USA. Fourth and fifth copepodite stages in Lake Awasa are
integrated in to 3 general size classes. Values in bold are estimated from data given by
Culver et al (1985) for Lake Ontario. Source of Lake Nkuruba data is Kizito (1998).
Species
stage
Nauplii
Cop. I
Cop. II
Cop. III
Cop. IV
Cop. V
Adult Male
Adult Fem.
Egg
Bos
(µm)
190
400
490
590
650
770
570
790
79
Bos.
(µg)
0.22
2.57
1.99
2.30
2.97
5.27
4.60
6.03
0.68
Nku.
(µm)
230
400
400
560
682
818
702
908
230
Nku.
(µg)
1.12
1.95
1.95
3.14
4.16
5.36
4.32
6.22
0.14
Awa.
(µm)
187
355
475
535
745
882
67
Awa. Ont.
(µg)
(µm)
0.17
221
0.22
507
0.42
579
0.55
634
670
742
1.16
1.70
914
0.005
-
Ont.
(µg)
0.24
0.97
1.20
1.77
2.73
2.40
6.70
0.18
This indicates a greater weight per unit length in these two lakes compared to comparable
species in both Lake Awasa and Malawi. The differences and similarities in lengthweight relationships exhibited among tropical species and between tropical and temperate
relatives reflect underlying differences in season (Green, 1956) climate, geographical,
altitudinal, local habitat conditions and productivity (Dumont et al., 1975) and body
shape. Variations in trophic interactions within the same habitat can cause considerable
variations in length-weight relationships (Dumont et al., 1975).
5.4.1 Length-weight relationships of Chaoborus instars
Length-weight relationships were determined for individual larval instars and pupae of
Chaoborus ceratopogones in Lake Bosumtwi. Chaoborus exerts a major force on
134
zooplankton community structure through its predation pressure (Fernando et al., 1990).
When present in significant numbers, it can exert major influences on the energy flux of
zooplankton communities. Estimates of biomass from length-weight regressions have
been carried out on populations of Chaoborus brasilienis in tropical Lake Valencia
(Cressa and Lewis, 1986). McGowan (1974) also provided dry season estimates of the
biomass of C. ceratopogones in tropical Lake George Uganda using carbon
measurements, but she was inconclusive about the patterns of weight variations over the
annual cycle. Additionally, Hare and Carter (1986) provided measurements of head
capsule lengths of C.ceratopogones. Clearly, allometric studies on Chaoborus
populations in African lake systems are rare and evaluation of energy budgets of
zooplankton communities have excluded Chaoborus; a feature which limits
understanding of the functioning of tropical freshwater systems. Chaoborus instars were
identified by differences in head capsule length (HCL) because there was no overlap in
this measure among instars. Dumont and Balvay (1979) have documented this specificity
of HCL among larval instars of temperate Chaoborus flavicans, but Balvay (1977)
suggests that deviations from this pattern may occur under fewer instances. HCL is also a
useful indicator of the maximum size of prey eaten (Fedorenko, 1975). Head capsule
length for the largest instar (instar IV) was 840 µm and differed between consecutive
instars by about 60 µm unit of the order of magnitude, starting from the first instar.
With respect to African work, Hare and Carter (1986) have measured both antennal and
HCL’s as well as dry weights of individual larval instars of C. ceratopogones in Opi
Lake, Nigeria. McGowan (1974) also measured antennal length and carbon-content dry
weights for the same species in Lake George, Uganda, but neither measured total body
135
length which consequently limits effective comparison of total body length with the Lake
Bosumtwi population. HCL of instars II and III of the Bosumtwi population are higher
(60 % in instar II; 52 % instar III) than those in Opi Lake, but those of the fourth instars
are comparable and only differ by 30 µm. Similarly, only mean dry weights of the fourth
instars are comparable between organisms in Opi and Bosumtwi lakes, with almost equal
variability. Hare and Carter (1986), however, did not record mean dry weights for instars
I and II. The pattern of the length-weight relationship between the populations in the two
lakes indicates that both populations have comparable maximum lengths and weights, but
food limitation of early instars in Opi Lake may be constraining this potential. That is,
mean dry weight of the fourth Chaoborus instar among the three lakes is comparable, but
the first three instars in the Bosumtwi populations have higher mean dry weights than the
populations in both Lake Opi and Lake George.
Generally, the average sizes of the early instars (I and II) of Chaoborus in Lake
Bosumtwi are comparable with those in related temperate species (Dumont and Balvay,
1979), but they differ widely among late instars. Populations of Chaoborus in Lake
Valencia exhibit similar characteristics in that mean total body lengths and weights are
lower than the Lake Bosumtwi populations by at least 2 orders of magnitude in all instars,
while the fourth instar possesses a greater minimum and maximum dry weight.
5.5 Patterns of biomass and production
5.5.1 Overall community patterns
There was no distinct seasonality in patterns of variation of zooplankton community
biomass and production, and the patterns of community biomass and production were
comparable between the two years of this study. Mean daily community biomass for the
136
two years was 457 mg dw m-3 whereas mean daily production was 64 mg dw m-3day-1.
Copepods contributed the largest proportion (48 %) to daily mean community biomass.
The contribution to daily mean community production was even higher at 69 %. The
contribution of the herbivores to community biomass and production was the least < 2%).
It is possible that our inability to determine production for the rotifers limited resolution
of their contribution to the total energy flux, because they can form a significant portion,
and sometimes exceed crustacean portion of total community production, due to their
high growth rates and short development time (Makarewicz and Likens, 1979). Neglect
of rotifer production in this study is not uncommon because the task is difficult to
undertake due to the high labour intensity of this particular analyses. High community
production occurred during the deepest mixing period in August 2005. But highest daily
community production over the two years occurred in October 2006 reaching 162 mg dw
m-3day-1 representing an increase of 46 % over the value in 2005. This difference in the
timing of the annual peaks in community production is striking and outstanding between
years. The reason is that the water column was more stable throughout the early part of
2005 compared to all of 2006 and experienced particularly deep mixing in August 2005.
This resulted in entrainment of nutrient-rich hypolimnetic water into the mixed layer
(Puchniak et al., 2009) stimulating phytoplankton growth (Awortwi, 2009.). The result
was elevated herbivore biomass response and ultimately high overall community
production. In contrast, the amplitude of seasonal mixing in August 2006 was lower than
in 2005. This deficit was compensated for by frequent short-term mixing of the water
column that produced aseasonal increases in nutrient supply and a generally deeper
mixed depth during 2006. This stimulated phytoplankton growth resulting in higher
137
overall total annual community production in 2006 than in 2005. Therefore, in Lake
Bosumtwi, water column variability could potentially create interannual differences in
patterns of community biomass and production.
An unusual aspect of Lake Bosumtwi is its high biomass of invertebrate predators relative
to herbivore biomass, which at face value seems inadequate to support the high biomass
and production of predators. Predators are probably supplementing their diets with a
potentially rich microbial biomass (Puchniak et al., 2009) and reducing energetic costs of
predation as in the case of Chaoborus (Cressa and Lewis, 1986), compared to fish
planktivores. This is because evidence of food limitation of copepods and Chaoborus was
not apparent due to the imbalance between predator and herbivore biomass.
5.5.2 Biomass and production patterns of copepods
Most data on tropical crustacean zooplankton communities suggest that biomass and
production are concentrated in the copepods (e.g Mengestou and Fernando, 1991; Irvine
and Waya, 1999). These attributes enhance the productivity of tropical systems because
copepods have evolved several mechanisms to ensure their relative success. Notable
among these mechanisms are; their elaborate life history which enables them to benefit
from investment in less vulnerable developmental stages when adult predation is high
(Saunders and Lewis, 1988 a), greater energy acquisition through a dominant nauplii and
copepodite herbivory (McQueen, 1969; Maly and Maly, 1974; Gophen, 1977; Jamieson,
1980) which favours survival and recruitment (Saunders and Lewis, 1988 b) and evasive
mechanisms from predators such as relatively high burst swimming and diel vertical
migration (Maier, 1993; Ghan et al., 1998; Perticarrari et al., 2004). It has also been
shown that Chaoborus which is a dominant invertebrate predator rarely consumes
138
copepod nauplii (Hare and Carter, 1987) further increasing naupliar survival and overall
copepod recruitment. Furthermore, calanoid copepods compared to rotifers and
cladocerans are more efficient at utilizing cyanobacteria abundant in tropical systems
(Haney, 1987; Amarasinghe et al., 1997) including Lake Bosumtwi (Awortwi, 2009). It
has also been shown that some predatory cyclopoids can even survive on a diet of
cyanobacteria (Burgis, 1973; Lewis, 1973). A stable tropical climate, high rates of
primary productivity and constant temperature (Fernando et al., 1990; Talling and
Lemoalle, 1998) support this vivacious growth.
In common with many African lakes, copepods dominated the biomass and production of
planktonic metazoa in Lake Bosumtwi. Values recorded were high compared to most
tropical lakes and particularly African lake systems. Mean total annual biomass and
production were 5.3 g dw m-3 and 1.07 g dw m-3y-1 respectively. Copepodites contributed
the largest proportion, about 93 % to total annual copepod production and exactly half of
total mean annual biomass. Except in Lake Awasa, Ethiopia, where the cladoderan,
Diaphanosoma contributes 69.5% of annual zooplankton production (Mengestou and
Fernando, 1991), several African lakes have >50 % of community biomass and
production allocated to the copepods. Lake George, Uganda (Burgis, 1974), Lake Malawi
(Irvine and Waya, 1999), Lake Kivu (Isumbisho et al., 2006) and Lake Tanganyika
(Kurki, 1996) are several such examples, but many of these have herbivorous calanoid
copepods. Furthermore, in these systems, the copepodites contribute the majority of the
copepod biomass and production. Thus the copepods in Lake Bosumtwi drove the
patterns of total zooplankton biomass. No strong seasonal patterns existed in the annual
biomass and production dynamics of the copepods in Lake Bosumtwi, although maximal
139
biomass and production were recorded during the deep mixing and immediate postmixing periods.
5.5.3 Biomass and production patterns of Chaoborus
Total annual Chaoborus standing stock increased from 4.5 g dw m-3 in 2005 to 6.9 g dw
m-3 in 2006. Similarly, total production was higher in 2006, reaching 0.51 gdwm-3 y-1 as
compared to 0.41 g dw m-3 y-1 in 2005. Mean daily production in each year was highest
during the strong and weak mixing seasons in August 2005 and January 2006
respectively. The strongest mixing period over the 2 year sampling occurred in August
2005 when mean daily production was highest at 176 mgdwm-3 y-1.Comparison of the
mean daily biomass and production of Chaoborus with similar tropical populations in
Lake Valencia, Venezuela (Saunders and Lewis, 1988 b), and Lake Lanao Philippines
(Lewis, 1979) demonstrates exceptionally high total annual and mean daily biomass and
production of Chaoborus in Lake Bosumtwi. In these lakes, Chaoborus is the chief
invertebrate predator and has been proven to exert major control on the zooplankton
community, particularly the herbivores. The differences in biomass and production can be
explained on the basis of the food web structure, resource availability and effect of
limnological regime shifts of stratification and mixing events. Lake Bosumtwi, in contrast
to Lake Valencia and Lanao possesses a simplified food web structure common to ancient
lakes like Baikal and Tanganyika (Dumont, 1994 a). Persistent resource limitation and
mortality associated with lake mixing in Lake Valencia is responsible for the low biomass
and production of Chaoborus (Saunders and Lewis, 1988 b). They report that Chaoborus
incurs heavy losses due to mortality during the annual mixing event and takes about 2
months to recover after restratification. Two factors interact to drive Chaoborus mortality
140
during mixing events at Lake Valencia; chemical changes in the water column during
mixing and increased fish predation due to loss of the hypolimnetic refuge for
Chaoborus.
The situation in Lake Bosumtwi sharply contrasts with that in Lake
Valencia. There is loss of hypolimnetic refuge volume during deep mixing in Lake
Bosumtwi as well, but catastrophic fish kills occur also, relieving possible predation on
Chaoborus by fish (Puchniak et al., 2009). Thus, Chaoborus seem unaffected by mixing
in Lake Bosumtwi and because standing stocks of zooplanktivorous fish are consistently
low due to heavy removal by fishermen (according to reports by local fishermen), the
Chaoborus population remained fairly stable especially during 2005 (Chapter 2).
Consequently, highest mean daily biomass and production of Chaoborus occurred during
the deep mixing in August 2005 and weak mixing in January 2006. These striking
system differences from other tropical lakes and unique adaptations may account for the
unique ways by which Chaoborus ensure their widespread distribution among freshwater
ecosystems. The ecological success of Chaoborus has been shown to be a consequence of
its omnivorous tendencies (Hare and Carter, 1986; Moore et al., 1994), low energetic cost
as an ambush predator and efficient use of food especially under limiting conditions
(Cressa and Lewis, 1986). The lack of adequate data on Chaoborus dynamics limits our
understanding of the array of strategies that populations employ in different tropical
systems.
5.5.4 Biomass and production patterns of Cladocera and Rotifera
One cladoceran species-Moina micrura and two rotifers Brachionus calyciflorus and
Hexarthra intermedia were the major herbivores in the zooplankton community of Lake
Bosumtwi. Herbivorous biomass and production were low. Persistent Chaoborus
141
predation and food limitation may be proximal factors contributing to this low herbivore
biomass and production. M.micrura and B.calyciflorus are among the major diet
components of Chaoborus (Hare and Carter, 1987). Also the dominant cyanobacteria in
Lake Bosumtwi (Whyte, 1975; Awortwi, 2009.) contribute to the low production,
because cyanobacteria are generally poorly utilized by herbivores (e.g Haney, 1987).
Cyanobacteria affect herbivore nutrition in 3 principal ways. Firstly, energy deficits
incurred from low ingestion rates because of filamentous, colonial and gelatinous cell
morphologies (Schindler, 1971; Arnold, 1971; Porter and McDonough, 1984). Secondly,
low nutritional value of some species lower growth and reproduction (Schindler, 1971;
Arnold 1971; Porter and McDonough, 1984) and thirdly, toxins released by cyanobacteria
affect herbivore filtering efficiency (Lampert, 1981; Ostrofsky et al, 1983). The response
of the herbivores to the seasonal increases in Chl a is likely attributable to improvement
in food conditions due to the increased fractions of edible phytoplankton during the peak
rainfall in May (Awortwi,2009) and following nutrient enrichment of the water due to
water-column mixing in August of each year. Herbivores are also limited by threshold
food concentrations (Lehman, 1988) and M.micrura and B.calyciflorus are known to have
high requirements for food (Gliwicz, 1980; Romanovsky, 1984; Lehmann, 1988;
Gliwicz, 1990; Sarma et al., 1999). Correlation analysis between biomass of each
herbivore species and Chl a showed no statistical relationship (r2 < 0.3, P > 0.05 for all
species) in addition correlation between individual herbivore species biomass and
grazeable phytoplankton biomass. The maximum zooplankton biomass occurring at low
to moderate phytoplankton biomass suggests a likely negative relationship between
zooplankton biomass and GBP especially when phytoplankton is abundant. At low to
142
moderate phytoplankton abundance there is quite a range in zooplankton abundance from
low to high. This suggests that predation on zooplankton by Chaoborus and M.
bosumtwii is likely holding their populations below the carrying capacity possible based
on phytoplankton biomass. Additionally, rapid removal of herbivore biomass by
predators, a long sampling interval relative to the generation times of most herbivore
species and other unexplained factors may account for the lack of statistical correlation
between herbivore biomass and GPB. This lack of relationship between herbivore and
phytoplankton has been documented in Lake Valencia (Saunders and Lewis, 1988 b)
where algal biomass also could not explain the variation in herbivore biomass.
5.6 P/B ratios (biomass turnover rates)
Despite the high total annual biomass and production, the P/B ratio - a commonly used
index of the biomass turnover rates of a species population and community was lower at
Lake Bosumtwi than in several African and temperate lakes ((Winberg et al., 1972)
partly due to the relatively high standing stock of the zooplankton (Table 5.1)
Table 5.1: Annual P/B ratio of the zooplankton community (including Chaoborus) of
Lake Bosumtwi compared with several tropical lakes and the mean of temperate lakes
(IBP). Mean daily biomass is given as mgdwm-3 whereas total annual production is given
as gdwm-3y-1. Data for the first 7 lakes were extracted from Mengestou and Fernando
(1991) and Winberg et al., (1972). Lakes containing Chaoborus in bold.
Lake
Chad
George
Sibaya
Lanao
Nakuru
1BP mean
Awasa
Malawi
Bosumtwi
Biomass
Production
333.0
368.0
50.8
222.0
6438.0
3000
44.9
1600
75.62
132.5
17.3
2.00
9.10
510.0
79.0
2.50
49.0
1.56
P/B
(yr-1)
69.7
28.7
40.6
41.0
119.9
28.5
55.8
36.0
23.4
143
Reference
Carmouze et al (1983)
Burgis (1974)
Hart and Allanson (1975)
Lewis (1979)
Vareschi and Jacobs (1984)
Winberg et al (1972)
Mengestou and Fernando (1991)
Irvine and Waya (1999)
This report
It has been suggested that assessments of ecosystem productivity of tropical lakes, based
on differences in P/B ratios, should not be carried out without reservations because of
variations in P/B under different environmental and ecological conditions (Mengestou
and Fernando, 1991). Nonetheless, it is emerging that tropical lakes may have higher
production per unit biomasses than their temperate counterparts (Saunders and Lewis,
1988 b; Mengestou and Fernando, 1991).
Annual P/B ratio of the zooplankton in Lake Bosumtwi is lower than the mean of those
reported for the tropical and temperate lakes. Mean daily turnover rates for the
community were estimated at 0.17. Temperature largely determines the rate of production
of biomass (Shuter and Ing, 1997). However, temperature cannot explain the differences
in community turnover rates among tropical lakes because they vary little in temperature
(Talling and Lemoalle, 1998). Low food quality available to herbivores may be the
immediate factor determining the relatively lower turnover rates of Lake Bosumtwi
zooplankton community. There is evidence that dominant cyclopoids can often utilize the
abundant cyanobacteria in the tropics, yet copepods in Lake Bosumtwi may be poorly
assimilating cyanobacteria because of the low nutritional quality of the latter
(Haney,1987) as levels of herbivore biomass are determined by both quantity and quality
of food resources (Wetzel, 2001). Additionally, Chaoborus, which is the second
dominant species, contributes half of community biomass but with a correspondingly
lower proportion of production of 31 %. The relatively longer life cycle of Chaoborus of
about 45 days is partly contributing to reduction in community turnover rates. Some of
the P/B ratios quoted for the tropical lakes above (Malawi, George, Chad) do not
incorporate Chaoborus biomass and production in their estimates of community turnover
144
rates although Chaoborus is recognized as an important component of their zooplankton
communities. Thus, inclusion of Chaoborus biomass and production may permit a more
useful comparison of P/B ratios among these lakes, since biomass allocation to
Chaoborus in tropical zooplankton communities may be quite high.
5.7 Variation in habitat quality and vertical distribution of zooplankton
Stability of habitat in terms of habitat quality supported the dominant epilimnetic life of
all zooplankton species throughout the annual cycle. Secchi depth and vertical gradients
of temperature, oxygen and food availability (chlorophyll a concentration) were
maintained within a limited range but with marginal seasonal oscillations. Disruption of
water column stability as indicated by the vertical gradient of temperature and DO during
the major circulation event in August 2005 resulted in a passive transport of zooplankton
throughout the water column. The resulting dispersive action altered the pattern of
vertical distribution by opposing the active habitat selection of zooplankton. Changes in
the strength of physical forces during mixing have been suggested to oppose the active
habitat choice of zooplankton (Mavuti, 1992; Kramer et al., 1997). Seasonal changes in
the distribution of Chl a affected the vertical distribution of only nauplii and M. micrura
in June 2006 and June of both years for nauplii. The deep water photosynthetic microbial
maximum in Lake Bosumtwi develops between April and July of each year (Puchniak et
al., 2009) and is disrupted by the circulation event in August. During the period of deep
water microbial development, nauplii and M. micrura aggregated in hypoxic water at 15
m, at the depth of the Chl a maximum possibly due to low epilimnetic food supply, whilst
H. intermedia gradually aggregated at 5 m in May and 8 m in June. The interesting
feature about the response of nauplii and M. micrura to the hypoxic deep water Chl a
145
maximum concerns their delayed response. M. micrura, H.intermedia and nauplii are
herbivores and likely compete for algal food among themselves and with B.calyciflorus.
The
progressive
reduction
of
epilimnetic
food
supply
during
stratification,
contemporaneous with the development of a deep water chlorophyll maximum, likely
resulted in an altered depth distribution of M. micrura, H. intermedia and nauplii. The
aggregation of copepod nauplii to hypoxic deep water microbial food sources has been
documented in Lake Nyahira, Uganda by (Kizito, 1998). Similar movements of
cladocerans in Lake Kinneret, Israel, have also been described (Gophen, 1972). This
behaviour was explained by the tolerance of nauplii to hypoxic conditions. Such spatial
segregation among competing species during periods of limiting food resources may
allow their coexistence within the same water column (Demott, 1989; Leibold, 1991;
Kizito, 1998).
The effect of light on the vertical distribution of zooplankton is seen in the pattern of
distribution of nauplii and Chaoborus. Average Secchi depth over the two years was
approximately 2 m and only copepod nauplii avoided the euphotic zone from January to
May of each year. However, they were present in this zone during the second half of both
years (June to December). Light and temperature have been shown to have limited
control of the vertical distribution of tropical zooplankton (Kizito, 1998). Reasons for the
absence of nauplii in the euphotic depth during the first half of both years are unclear
since the main predator, Chaoborus, maintains a day time hypolimnetic residence
throughout the year and food conditions in the euphotic zone are relatively stable during
this period.
146
5.7.1 Percent zooplankton in epilimnion
With the exception of copepod nauplii, more than 90 % of all zooplankton species and
developmental stages resided in the food rich, low predation risk (because of daytime
hypolimnetic residence of Chaoborus and low fish abundance) warmer and oxygenated
epilimnion throughout the year. This was only interrupted by episodic circulation events
which promoted broader zooplankton dispersion along the water column, yet, high
epilimnetic concentrations of zooplankton were maintained during this period. The
broader distribution of zooplankton during the strong seasonal circulation could also be a
result of deeper oxygen penetration allowing the expanding suitable habitat and
broadening of the distribution of food supply for most species. This is in accordance with
the stability of the epilimnetic habitat which guarantees a faster pace of development in
the warmer temperatures and increased reproductive potential from reliable food supply.
Gophen (1979) found that in Lake Kinneret, Israel, 90 % of zooplankton was
concentrated in the epilimnion and metalimnion during the stratification period, but this
was reduced to 72 % during the turnover period. A similar reduction in epilimnetic
zooplankton concentration was observed in Lake Bosumtwi during the turnover period in
2005 where percent densities ranged from 66 to 82% among zooplankton organism with
copepod nauplii at the top of the range. The maintenance of high epilimnetic
concentration of zooplankton during turnover periods has been documented in some
fairly deep temperate lakes (e.g Burns and Mitchell, 1980). Reasons for this may be
attributed to the passive transport of zooplankton by unconstrained water movements
during circulation, but this may also imply that zooplankton can distribute themselves
over a broader depth range and still have access to food. This assertion is supported by
147
the higher aggregation of zooplankton species in the epilimnion in the face of deep
mixing. In contrast, an even distribution of zooplankton along the water column during
circulation was observed in shallow Lake Naivasha (mean depth 6 m), Kenya, (Mavuti,
1992). These differences in patterns of vertical distribution of zooplankton likely pertain
to differences in lake depth and mixing dynamics.
5.8 Patterns of Diel Vertical Migration (DVM) of zooplankton
5.8.1 DVM during stratified periods
The contrasting patterns of DVM during the circulation and stratified periods demonstrate
the role of physical forces such as water currents in influencing the shape of vertical
distribution of zooplankton. There is a trend to increasing amplitude of vertical migration
among the zooplankton during the stratified periods. Diurnal variation in zooplankton
densities occur predominantly in the upper layers. Amplitude of DVM decreases from
mass population migration in Chaoborus to partial population migration in copepod
adults, M. micrura, B. calyciflorus and H. intermedia to no migration in copepod nauplii
and copepodites. DVM in Chaoborus can be a fixed trait that is principally controlled by
light, but can also be induced by fish predation (Gliwicz, 1986; Dawidowicz et al., 1990).
Light principally plays a role in synchronizing circadian rhythms of organisms which
may subsequently regulate migratory behaviour of zooplankton (e.g Hart and Allanson,
1976). The adaptation of Chaoborus to withstand anoxic and hypoxic conditions enables
it to dwell in darker, deeper waters during the day time (e.g Dawidowicz, 1993). Despite
the low fish predation in the pelagic of Lake Bosumtwi (inferred from low pelagic fish
abundance), Chaoborus still performed DVM indicating endogenous trait.
148
Differential migration patterns among the developmental stages of copepods have been
documented (Redfield and Goldman, 1980; Nauwerck, 1993). The non-migration of
nauplii and copepodites in the presence of Chaoborus has been observed (Kizito, 1998).
Migration may not confer any advantage to nauplii and copepodites as they seem to
occupy an optimum habitat between 3 and 15 m which is characterized by a reliable food
supply as indicated by the (high Chl a low predation risk (sub euphotic zone) and warmer
temperatures (decreased development time). Gliwicz (1986) has noted that DVM may not
occur in situations with low predation risk. Kizito (1998) further showed that copepod
adults descended the water column to avoid predation and habitat overlap with
Chaoborus larvae at night. In Lake Bosumtwi, copepod adults rather ascended
simultaneously with Chaoborus to the surface waters at dusk, but their stay in surface
waters was short-lived and they began to descend gradually at night (12 am) through to
dawn to maintain their daily residence in sub-euphotic zones. A proportion of the
populations of M. micrura and H. intermedia spent the night in surface waters. However,
densities of H. intermedia increased up to dawn whereas densities of M.micrura declined
considerably till dawn. This inverse migration relationship between M. micrura and H.
intermedia may have an adaptive value in reducing interspecific competition in surface
waters during the night. M. micrura co-existed with its tactile predator Chaoborus in
surface waters at night, perhaps because predation-driven migration only occurs above a
certain threshold of predation risk (Loose and Dawidowicz, 1994).
5.8.2 DVM during circulation period
The water column during the circulation period was virtually homogenous through the
upper 30 m. Although a broader vertical distribution of all zooplankton species occurred
149
during this period, high aggregations of each zooplankton species in surficial waters
during the night and corresponding minima at dawn characterized the DVM pattern.
Copepod adults, however, maintained similar densities at each 6 hr interval in surface
waters and disappeared at dawn. The broader water column dispersion of zooplankton is
promoted by physical forces of water movement that partially oppose the active depth
habitat selection of zooplankton (Primicerio, 2000). Additionally, during the circulation
period, the habitable volume of water column is expanded rapidly so redistribution may
also partially reflect the increased habitat volume.
150
CHAPTER 6: CONCLUSIONS & RECOMMENDATIONS
6.1 Research Overview and General Comments
This research enabled fundamental assessment of the biological structure and function of
the zooplankton community in Lake Bosumtwi (impact crater). Zooplankton are
undoubtedly critical to ecosystem functioning and their exclusion from any lake
ecosystem assessment scheme results in limited understanding of ecological processes
and productivity of lake ecosystems. The zooplankton community in Lake Bosumtwi is
very simple with predominance of predators possibly reducing the trophic efficiency
between algal production and consumer trophic levels due to low numerical abundance as
well as low biomass and production of herbivores. The low zooplankton diversity
parallels that of most tropical lake zooplankton communities and many evolutionary
causes have been assigned to this pattern of diversity in tropical systems. The biotic
processes are strongly driven by variation of climatic and physical processes of lake
mixing and stratification which ultimately determine seasonal and annual variability of
community structure and the attainable levels of community biomass and production.
The strong interannual differences in biomass and production and physical processes
suggest that the dynamics of ecological processes in the lake may differ from year to year
in magnitude, timing and recurrence. The different annual patterns of the biomass
relationship between individual herbivores and GPB demonstrate considerable
interannual differences in response of herbivores to the timing of the availability of food
resources which consequently affect herbivore growth and abundance. The low fish
production in the lake as noted from oral reports from local fishermen indicate a “sink” of
151
most organic production to the deep waters of the lake suggesting a low trophic
utilization by higher consumers such as fish.
6.2 Major findings and Conclusions
6.2.1 Community structure and dynamics
1. The zooplankton community in Lake Bosumtwi is species poor with predominance of
predators (M. bosumtwii and Chaoborus) and less abundant herbivores (M. micrura,
B.calyciflorus, B. dimidiatus, K.cochlearis). This low herbivore to predator ratio indicates
a trophic bottleneck restricting energy flow to predators.
2. The zooplankton community dynamics in Lake Bosumtwi are influenced by climatic
factors affecting column stratification and mixing and seasonal precipitation.
Considerable interannual differences in the pattern of mixing and rainfall are linked with
changes in zooplankton abundance.
3. Intensity of seasonal hydrodynamic events and precipitation elicit corresponding
changes in community diversity and abundance.
4. The zooplankton community also exhibited short term fluctuations in abundance.
These intraseasonal oscillations in abundance may be associated with sporadic changes in
water column properties affecting thermal stability, nutrient availability and biological
productivity.
5. The herbivore niche structure may permit a large grazer, notably a calanoid copepod to
efficiently utilize the abundant Cyanobacteria and enable less competitive but more
palatable phytoplankton taxa to increase their fractions to support a vibrant herbivore
growth. The corresponding enrichment of the limnetic environment will likely attract
152
facultative zooplanktivores in the lake to establish a dominant pelagic mode of life and
thereby increase overall fish productivity.
6.2.2 Biomass and Production
1. Biomass and production of the zooplankton in Lake Bosumtwi falls within the range
of values recorded for most tropical lakes. Turnover rates are low compared to most
tropical lakes and the mean of temperate lakes. However standing stock is very high.
2. Low fish planktivory on Chaoborus and copepods, sometimes constrained by
catastrophic fish kills resulting from seasonal mixing events contributed to the sustained
high biomass and production of Chaoborus and copepods seasonally and interannually.
3. Herbivore zooplankton food in the lake may be limiting in quality and quantity. This is
evidenced by the dominance of cyanobacteria which are largely ungrazeable by herbivore
zooplankton. Herbivore growth and production is consequently very low and invertebrate
predation virtually offsets any gains in growth and biomass.
4. There is monopolization of limiting food resources by predators and a high
maintenance cost of herbivores in the face of limiting food resources constrains their
growth. Herbivore biomass is not correlated significantly with phytoplankton biomass.
The lack of correlation between herbivores and the grazeable phytoplankton fractions
may be largely attributed to predation effect. Therefore, maximum herbivore biomass is
potentially limited by invertebrate predation.
3. Chaoborus in Lake Bosumtwi and in other tropical systems may be an important
energy link to higher trophic levels such as fish. This conduit of energy transfer can
compensate for the energy deficits incurred by herbivores resulting from their low
153
productivity which is partly caused by their inability to effectively utilize the abundant
cyanobacteria present in tropical systems.
4. A large proportion of zooplankton production in Lake Bosumtwi may be unutilized at
higher consumer levels and much of this production may enter detrital pathways, which
will consequently feed a potentially rich microbial food web. But potentially, production
is mostly lost to the deeper anoxic waters of Bosumtwi which may be resuspended during
deep seasonal lake mixing as nutrients for phytoplankton production. Zooplankton
production is lost also because of the intense fishing pressure on the lake. Reduction in
fishing pressure can lead to significant planktivory on zooplankton predators, which will
consequently reduce predation on herbivores allowing an increase in herbivore
populations in the lake.
5. Phytoplankton in the lake is not under strong grazing control by the zooplankton
herbivores.
6.2.3 Vertical distribution and migration
1. Zooplankton in Lake Bosumtwi have distinct vertical distributions and that few
undergo diel changes in vertical distribution.
2. Seasonal circulation resulted in minimal but brief habitat disturbances and
consequently had a minimal influence on the character of daytime vertical distribution
and diel migration. Interannual differences in seasonal mixing dynamics resulted in
marked effects on vertical distribution pattern of zooplankton. During annual cycles
characterized by weak seasonal mixing as in August 2006, vertical distribution patterns
could be fairly stable throughout the year.
154
3. Changes in seasonal depth distribution of food resources promote appreciable spatial
segregation among herbivores that may reduce the intensity of interspecific competition
in the shared epilimnetic habitat.
4. Predation risk imposed by both invertebrate (Chaoborus) and vertebrate (mostly fish)
predators on other zooplankton are consistently low in daytime surficial waters and light
intensity affects the vertical distribution of copepod nauplii, Chaoborus and M.micrura.
5. Certain prey species migrate to coexist with predators in surface waters during the
night because predation risk may be below threshold levels. The stable patterns in
zooplankton vertical distribution may enhance effective predictions about community
structure and dynamics of zooplankton.
6.3 Implications of Research for Ecosystem Resource Management
It is becoming increasingly imperative that contemporary policy development and
management schemes for conservation of natural resources should be developed at the
ecosystem and watershed scale. This represents a new paradigm in management goals
and resource protection programmes necessitated by the increased demand for more
effective tools to be developed from a system perspective due to increasing human
pressure on natural resources. Such schemes are opposed to the traditional methods of
reliance on administrative permits to regulate resource use. Aquatic organisms serve as
continuous monitors of ecosystem health and provide an integrated assessment of the
physical, chemical and biological status of freshwater ecosystems (Davis and Simon,
1995) which are sometimes more useful techniques than the chemical methods of
measuring water quality (Davis and Simon, 1995). Freshwater ecosystems especially
155
lakes are very sensitive to environmental and human impacts and manifest system-level
pathological symptoms in various structural and functional responses (Xu et al, 1999).
Influence of the information on the structure and functional role of zooplankton in
ecosystem-driven management schemes has the potential to affect policy development at
two levels; effective risk assessment based on sound scientific theory and solid data for
local problems (Davis and Simon, 1995) and to inform decision making at broader
regional and national levels which is key for effective policy implementation.
Additionally, a comprehensive monitoring programme of an aquatic ecosystem is
essential for tracking long-term changes and for construction of reliable historical records
for reference towards solution of current problems and accurate forecasting. Knowledge
of the structure and function of zooplankton from a model system will also enhance
development of rigorous biological criteria for lakes and reservoirs and promote the
establishment of reliable bioassessment schemes. Such biocriteria and bioassessment
profiles of freshwater ecosystems will equip water resources and environmental
regulatory agencies like the Ghanaian Environmental Protection Agency (EPA) and
Water Resources Commission (WRC) with the requisite scientific tools and framework
for developing appropriate benchmarks for decision-making and effective evaluation of
various uses and protection of water resources in Ghana. Currently, the operational
capacity of these regulatory agencies in the management of water resources is weakened
by limited understanding of the ecological integrity and dynamics of freshwater systems
in the country.
156
6.3.1 Implications of Research for Fisheries Management
Zooplankton are an important source of energy and nutrients for fish growth and
development and forms part of the production base of many marine and freshwater
fisheries. Assessment of fish stocks and fish productivity as part of fishery management
programmes in lakes can benefit from knowledge of zooplankton productivity through a
deepening of the understanding of the link between fish and zooplankton dynamics for
effective management. Knowledge of zooplankton production dynamics is vital for
effective evaluation of the carrying capacity of a fishery based on zooplankton production
and to enable understanding of the extent to which changes in zooplankton production
caused by changes in environmental factors can impact the performance of a fishery.
With the benefit of long term records of the relationship between fish productivity and
zooplankton production and the understanding of the dynamics imposed by
environmental and climate changes, reliable models can be constructed to regulate
exploitation of fish resources as part of sustainable management initiatives. Additionally,
knowledge of zooplankton production can be used in aquaculture through
biomanipulation techniques to increase fish yield and help optimize operational and
production cost of the industry.
6.4 Recommendations and future research directions
The potential for secondary production to be lost to the deep anoxic waters of Lake
Bosumtwi indicates that trophic or ecological efficiency (the transfer efficiency from one
trophic level to the next) from zooplankton to fish could be very low. To clarify this
assertion, a detailed study of the ecological efficiency of the food web is a compelling
prospect to enhance complete understanding of the trophic dynamics and energy flux
157
through the biotic components of the ecosystem. Stable isotope techniques will be an
invaluable tool in this regard.
Because the sustainability of the fish resources are the most immediate concern of the
natives, elucidation of the trophic efficiency between zooplankton and fish should be
amongst the primary goals of future research programmes on the lake. Additionally,
suspicions about a dynamic and energetically important microbial food web of the lake
ecosystem is very high and future research programmes should incorporate investigation
of the microbial ecology of the lake. Research on the feeding habits of the dominant
copepod zooplankton, especially, trophic diversification through ontogenetic shifts in
feeding habit is essential to identifying the principal sources of their energy supply since
it was apparent by this research that, the level of herbivore biomass could not support the
biomass of the copepods. Furthermore, it will be very interesting to know whether the
adult copepods are feeding on their young as an alternative food supply.
Because it was discovered in this research that predation was a limiting factor to
achieving maximum herbivore biomass, enclosure experiments will be useful to elucidate
the interactions between zooplankton herbivores and grazeable phytoplankton biomass
(GPB) to see if herbivore zooplankton biomass will increase proportionally to GPB in the
absence of predators. Feeding inhibition by algal toxins on zooplankton herbivores may
have also accounted for the low herbivore biomass. This assertion can be tested by
experiments examining the effect of algal abundance on growth rates of zooplankton
herbivores.
158
Finally, nutrient cycling and whole-sale ecosystem mass balance analyses of the Lake
Bosumtwi ecosystem will provide a useful tool for effective ecosystem management.
159
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APPENDIX
187
Table A: Taxonomy of zooplankton taxa in Lake Bosumtwi
Taxon
Mesocyclops bosumtwii
Chaoborus ceratopogones
Moina micrura
Brachionus calyciflorus
Brachionus dimidiatus
Hexarthra intermedia
Keratella cochlearis
Filinia pejleri
Filinia camascela
Authority
Date
Mirabdulayev, Sanful, Frempong
Theobald
Kurz
Pallas
Bryce
Wiszniewski
Gosse
Hutchinson
Myers
2007
1903
1974
1766
1931
1929
1851
1964
1938
Table A2: Mean abundance and proportion of individual species to total
zooplankton
Mean Abundance
(in1000’s m-3) ± SD
Proportion to total
zooplankton (%)
Cladocera
1011 ± 2739
0.70
Chaoborus
8363 ± 7879
6.00
Rotifera
33029 ± 38525
25.60
Copepoda
95947 ± 65082
67.70
Total zooplankton
138350 ± 81713
100
Taxonomic group
188
Table B. Basic descriptive statistics of abundance of zooplankton taxa
Table B1. Mean (x), median (MD) Standard Deviation (SD), Coefficient of Variation
CV (%), minimum (min) and maximum (max) values computed for each taxa and
copepod developmental stages for 2005, n= 26 n (2006)= 25. For both F.camascela in
2005 and F.pejleri in 2006; n = 2
Developmental
Stage
Adult copepods
Copepodites
Nauplii
C.ceratopogones
M. micrura
B.calyciflorus
H.intermedia
F.camascela
F.pejleri
Monthly total
zooplankton
abundance
2005
x
18656
32227
24836
6028
1906
9304
27816
4947
4681
MD
9109
24560
15132
6286
449
1111
10298
4947
4656
128252 132822
min
2037
4074
3288
349
87
80
582
4656
3218
max
64491
116458
102374
12222
11640
62584
121871
5328
6169
SD
20312
31992
23619
3631
3537
16726
38555
412
1475
CV
1.09
0.86
0.95
0.60
1.86
1.80
1.39
0.08
0.32
34820
336620 73157
0.57
Table B 2: Mean (x), median (MD) Standard Deviation (SD), Coefficient of Variation
CV (%), minimum (min) and maximum (max) values computed for each taxa and
copepod developmental stages 2006, n = 25
Developmental
Stage
Adult copepods
Copepodites
Nauplii
C.ceratopogones
M. micrura
B.calyciflorus
H.intermedia
F.camascela
F.pejleri
Monthly total
zooplankton
abundance
2006
x
16884
59385
33103
10754
2948
5748
32455
2235
11931
MD
18463
49721
28780
8381
897
4685
19348
1745
11931
164964 157452
min
2008
14375
2444
873
254
407
2037
1454
11640
max
35561
184727
79972
39576
19963
16703
126003
4418
12222
SD
10740
45604
21366
8403
5401
5662
33346
1237
412
CV
0.64
0.77
0.65
0.78
1.83
0.99
1.03
0.55
0.03
44155
301112 73695
0.45
189
Table C: Correlation analyses between individual herbivore biomass and 2-week
lagged and unlagged grazeable phytoplankton biomass (GPB) at P-value of 5%.
Variables
(Herb. Biomass vrs GPB)
H.intermedia
B.calyciflorus
M.micrura
Copepodites
Nauplii vrs
Herb. Biomass vrs Log GPB
H.intermedia
B.calyciflorus
M.micrura
Copepodites
Nauplii
R
0.24
-0.02
0.23
0.19
0.01
Lagged
P-value
0.11
0.93
0.28
0.19
0.94
0.16
0.12
0.09
0.16
0.06
0.29
0.57
0.65
0.29
0.70
2
190
R
-0.02
-0.15
0.26
0.16
-0.08
Unlagged
P-value
0.90
0.48
0.22
0.28
0.59
N
46
25
24
46
46
0.04
-0.07
0.25
0.16
-0.07
0.79
0.75
0.24
0.29
0.64
46
25
24
46
46
2
TABLE D
Community Vertical Distribution Data (Volumetric)
(Measured in number of individuals per cubic meter of lake water)
AF: Adult female copepods, MM: M. micrura, BC: B. calyciflorus, CP: Copepodites
NP: nauplii, CH: C. ceratopogones, HX: H. intermedia, DO: Dissolved Oxygen
TEMP: water temperature, Chl a conc.: Chlorophyll a conc. DEPTH (meters)
Tables D1: 14-01-05
Depth
AF
Surface 68,664
1
0
2
0
5
6000
8
3167
10
5666
12.5
3750
15
3500
20
0
25
0
30
0
MM
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
HX
DO
TEMP
8500 141,828
0
0
0
19,666
0
0
0
0
0
0
0
84,663
0
0
0
13,999 6500
0
0
14832 3500
0
0
4000
3833
0
0
3833
0
0
0
0
0
417
0
0
0
2083
0
0
0
417
237
2234
645
1352
0
0
0
0
0
0
0
79.4
80.6
83.6
94.4
105.1
110.7
118.7
121.5
118.4
95.2
94.3
27.59
27.49
27.40
27.34
27.33
27.33
27.33
27.33
27.30
27.01
27.01
Chl a
conc.
No Data
Tables D2: 14-01-06
Depth
AF
Surface 4666
1
19,499
2
12166
5
4167
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
74997
6250
0
76,997
0
417
58,831
3917
0
41,498 38,165 333
16,666 24,832
0
6250
36000
0
6166 104,829 167
0
0
500
0
0
2917
0
0
7166
0
0
1417
191
HX
DO
TEMP
0
0
0
0
0
0
0
0
0
0
0
128.7
127.2
129.2
108.7
71.7
49.5
27.5
17.0
7.7
6.2
10.6
29.29
29.07
28.92
28.57
28.32
28.20
27.59
27.97
26.92
26.83
26.82
Chl a
conc.
9.95
18.71
19.50
11.33
9.55
6.05
4.21
2.51
1.31
-
Tables D3: 28-01-05
Depth
AF
MM
BC
CP
Surface
1
2
5
8
10
12.5
15
20
25
30
2083
4250
4083
4333
0
2083
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
2250
0
0
2000
0
0
0
0
0
0
0
NP
CH
HX
DO
TEMP
0
0
0
0
0
0
0
0
0
0
0
127.2
128.7
128.4
74.8
45.6
50.01
46.0
37.3
11.70
5.8
4.5
29.06
28.58
28.16
27.29
27.23
27.22
27.19
27.19
27.16
27.06
27.00
14,832
0
0
72,747
0
0
68747
0
0
76,747
0
0
17,166 8250 333
8666 4083 167
0
1917 83
0
0
83
0
0
83
0
0
1333
0
0
4167
Chl a
conc.
No Data
Tables D4: 28-01-06
Depth
Surface
1
2
5
8
10
12.5
15
20
25
30
AF MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
18,749
0
0
14,832 2083
0
12,666 2250
0
18,832
0
0
3917 12333
0
2333
0
0
0
0
0
0
0
0
0
0
34,000
0
0
7333
0
0
167
192
HX
DO
0
83
0
0
0
0
0
0
0
0
0
112.4
105.4
110.7
117.7
112.9
72.6
59.6
55.7
57.8
55.3
59.0
TEMP Chl a
conc.
28.18
29.27
6.85
29.03
9.02
28.95
5.21
28.56 10.74
28.21
3.82
27.54
5.92
27.18 10.86
26.96
1.33
26.84
26.83
1.09
TABLES D5: 11-02-05
Depth
AF
Surface
0
1
6500
2
0
5
0
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
HX
DO
TEMP
0
2083
0
0
2250 96,329 2083
0
0
18,833
0
0
0
16,749 1833
0
0
8583 8666 417
0
0
0
1167
0
0
0
1333
0
0
0
1000
0
0
0
1333
0
0
0
417
0
0
0
0
1126
0
0
2083
0
0
0
0
0
0
0
116.0
118.1
119.6
105.4
53.0
22.4
14.7
14.4
13.5
5.9
0
30.70
29.63
29.25
28.54
27.69
27.36
27.36
27.23
27.18
26.99
0
Chl a
conc.
No Data
TABLES D6: 10-02-06
Depth
AF
Surface 22083
1
2167
2
18,666
5
0
8
2417
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
8333
0
0
0
108.2
37,499
0
0
2083 101.7
87,497
0
0
1126 103.7
16666 2083
0
83 109.6
20,666 27,082
0
0
81.8
17,166 18,666
0
0
42.1
0
0
0
0
37.8
0
0
4
0
33.6
0
0
0
0
24.9
0
0
9416
0
18.6
0
0
1083
0
7.9
193
TEMP Chl a
conc.
29.47
0.97
29.70
7.25
29.60
3.13
29.48
9.16
28.63
27.84
3.06
27.43
4.56
27.28
3.47
26.99
2.50
26.90
26.83
0.84
TABLES D7: 26-02-05
Depth
AF
MM BC
Surface
0
1
10,416
2
6166
5
6333
8
1917
10
0
12.5
0
15
0
20
0
25
0
30
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
12,500
0
0
20,915
0
0
61,830
0
0
56,248 4250
0
23,249
0
0
0
0
333
0
0
1750
0
0
252083
0
0
1250
0
0
1750
0
0
500
HX
DO
TEMP
0
83
2083
1126
0
0
0
0
0
0
0
122.6
124.0
125.0
124.0
105.6
37.1
8.7
4.6
3.1
2.5
2.2
31.88
31.71
30.58
29.88
28.88
28.04
27.41
27.24
27.17
27.01
26.97
Chl a
conc.
No Data
TABLES D8: 24-02-06
Depth
AF
Surface
0
1
0
2
4167
5
0
8
2333
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
0
0
0
6250
0
0
38748
0
0
5750
0
0
4666 8416 0
4083
0
0
0
0
83
0
0
250
0
0
167
0
0
417
0
0
167
0
6250
1126
0
0
0
0
0
0
0
0
118.6
112.2
115.4
129.6
139.8
136.0
111.9
102.2
101.9
101.6
102.8
194
TEMP Chl a
conc.
29.23
6.33
29.85 11.53
29.71
9.09
29.58
9.81
29.53 12.85
28.29
27.61
8.96
27.61 13.96
27.03
1.99
26.87
26.83
1.26
TABLES D9: 11-03-05
Depth
AF
MM
Surface
0
1
0
2
12,333
5
2000
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
HX
DO
50
2083
0
0
0
91.1
0
21,082
0
0
642 93.6
6333 14,833 2000
0
1126 91.8
0
20,833 2250
0
2083 86.8
0
0
0
0
0
35.4
0
4416
0
0
0
5.7
0
0
0
250
0
2.8
0
0
0
417
0
2.0
0
0
0
1417
0
1.6
0
0
0
1917
0
1.5
0
0
0
667
0
1.3
TEMP
30.76
30.30
30.10
29.98
28.22
27.82
27.45
27.23
27.15
27.03
26.98
Chl a
conc.
No Data
TABLES D10: 10-03-06
Depth
AF
Surface
0
1
8500
2
0
5
0
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
0
2083
0
39,248
0
0
72,664
0
0
16,916 2083
0
0
0
0
0
0
0
0
0
0
0
0
167
0
0
500
0
0
2083
0
0
667
83
0
1126
6250
0
0
0
0
0
0
0
100.7
102.9
105.9
114.6
118.8
101.4
85.7
89.8
93.3
99.2
100.4
195
TEMP Chl a
conc.
30.44 10.93
30.25 11.00
30.07
9.16
29.77 10.61
29.42 10.27
28.24 20.09
28.04 12.38
27.70
8.69
27.39
7.25
26.99
26.83
0.91
TABLES D11: 25-03-05
Depth
AF
MM
BC
CP
NP
Surface
0
0
2333 6250
0
1
2000
0
0
25,166
0
2
8333 2583
0
31,499
0
5
0
0
0
45,998
0
8
0
0
0
8500
0
10
0
0
0
5916
8166
12.5
0
0
0
10,166 16,499
15
0
0
0
0
24,666
20
0
0
0
0
33,332
25
0
0
0
0
75,080
30
0
0
0
0
56,164
CH
HX
DO
TEMP
0
0
0
0
0
0
0
0
0
0
0
83
0
0
1126
0
0
0
0
0
0
0
91.1
93.6
91.8
86.8
35.4
5.7
2.8
2.0
1.6
1.5
1.3
30.76
30.30
30.10
29.98
28.22
27.82
27.45
27.23
27.15
27.03
26.98
Chl a
conc.
No Data
TABLES D12: 24-03-06
Depth
AF
MM
Surface 2000
0
1
0
1917
2
10,416 4500
5
4250
0
8
0
0
10
0
0
12.5
0
0
15
0
0
20
0
0
25
0
0
30
0
0
BC
0
0
0
0
0
0
0
0
0
0
0
CP
NP
4250
0
76,914
0
143,661
0
71,997
4250
54,331 16,666
25000
6250
0
4167
0
0
0
0
0
0
0
0
196
CH
HX
DO
0
0
167
500
1250
2917
1000
0
1083
5250
5833
0
1126
0
2083
0
0
0
0
0
0
0
88.8
91.7
94.4
89.8
62.7
37.1
19.9
13.1
11.3
10.9
8.1
TEMP Chl a
conc.
31.21
8.16
30.62
7.44
30.28
7.44
29.78 11.72
29.48 10.86
28.90
6.58
28.12
6.38
27.94 21.93
27.32
7.04
26.91
26.85
1.24
TABLES D13: 8-04-05
Depth
AF
MM
BC
CP
Surface
0
0
1750 8166
1
6500
0
6000 14,249
2
12666 1583
0
18,416
5
4250
0
0
14,583
8
8583
0
0
6250
10
2083
0
0
1917
12.5
0
0
0
0
15
0
0
0
0
20
0
0
0
0
25
0
0
0
0
30
0
0
0
0
NP
0
0
0
0
0
0
0
0
0
0
0
CH
HX
DO
0
2083 112.7
0
14684 113.5
0
12624 111.6
0
8333 102.3
0
1126 41.7
1667
0
4.9
3333
0
2.2
833
0
1.8
250
0
1.4
667
0
1.3
333
0
1.2
TEMP Chl a
conc.
31.26
9.17
31.25 10.38
31.14
7.44
30.93 12.38
28.79 10.54
27.85
6.58
27.46
5.0
27.26 13.10
27.15
0.87
27.59
26.99
-
TABLES D14: 7-04-06
Depth
AF
MM
Surface
0
0
1
0
0
2
0
1333
5
4416 1500
8
0
0
10
0
0
12.5
0
0
15
0
0
20
0
0
25
0
0
30
0
0
BC
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
0
0
0
6250 84.9
55,914
0
0 22464 87.2
106,246
0
0 12842 89.9
68,664
4250
0
8333 94.6
52,081 25,082 0
1126 79.0
1250
14,000 0
0
15.1
6166
1500
0
0
9.8
0
0
167
0
8.9
0
0
167
0
9.8
0
0
833
0
9.8
0
0
167
0
6.3
197
TEMP Chl a
conc.
31.76
5.56
31.06
8.04
30.84
8.56
30.51
8.82
29.92
7.11
28.93
8.95
27.81
3.93
27.23
0.73
26.96
1.06
26.96
26.85
1.03
TABLES D15: 22-04-05
Depth
AF
MM
BC
CP
NP
CH
HX
DO
Surface 6250
0
25
4250
0
0
83 116.8
1
1250
0
0
1500
0
0
1126 117.7
2
12,166 4167 2250 15,332
0
0
6250 115.4
5
4250 2417
0
26,666
0
0
2083 116.3
8
30,665
0
0
35,499 3167
0
0
39.5
10
2083
0
0
0
0
0
0
19.9
12.5
7750
0
0
9916
0
0
0
20.8
15
0
0
0
0
0
0
0
21.6
20
0
0
0
0
0
333
0
20.7
25
0
0
0
0
0
500
0
19.4
30
0
0
0
0
0
1333
0
17.9
TEMP Chl a
conc.
31.50
5.59
31.50
5.37
31.45
6.95
31.09
9.68
29.88
11.0
28.62 12.05
27.58 34.90
27.29 17.34
27.18
1.29
27.10
27.00
1.95
TABLES D16: 21-04-06
Depth
AF
Surface
0
1
2083
2
0
5
0
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
1250
0
0
654 51.1
20,666
0
0
1126 53.2
47,498
0
0
2083 56.2
36,332 3250
0
6250 57.7
6166
0
0
0
56.3
2083
0
0
0
49.7
0
0
833
0
16.8
0
0
2500
0
11.1
0
0
3750
0
8.9
0
0
3333
0
7.8
0
0
0
0
7.7
198
TEMP Chl a
conc.
32.11
6.71
31.22
30.87
7.77
30.61
6.94
30.50 17.85
29.88 11.10
28.40 98.87
28.14 35.85
27.20
7.97
26.98
26.86
1.01
TABLES D17: 7-05-05
Depth
AF
MM
BC
CP
NP
CH
HX
Surface
0
0
0
7333
1583
0
1123
1
1583 8333 3917 7750
0
0
2083
2
31,165 9833 1417 40498
0
0
6250
5
25000
0
0
33332 4250
0
0
8
4000 7333
0
4250 42,664
0
0
10
9916
0
0
6500
3750
0
0
12.5
0
0
0
0
0
12500
0
15
0
0
0
0
0
2750
0
20
0
0
0
0
0
12,250
0
25
0
0
0
0
0
4833
0
30
0
0
0
0
0
14,499
0
DO
TEMP
161.6
106.6
110.6
112.9
108.4
95.3
42.6
30.6
22.3
14.2
9.1
31.83
31.03
30.82
30.68
30.23
29.76
28.67
27.71
27.24
27.07
26.99
Chl a
conc.
No Data
TABLES D18: 6-05-06
Depth
AF
MM
Surface
0
0
1
0
0
2
2083 1583
5
0
0
8
0
0
10
0
0
12.5
0
0
15
0
0
20
0
0
25
0
0
30
0
0
BC
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
30,665
0
0
52,081
0
0
156,160
0
0
46,165 4167
0
27,332 2083 833
11500
0
0
0
0
3333
0
0
2083
0
0
2500
0
0
3167
0
0
1167
2083
6250
2083
1128
0
0
0
0
0
0
0
24.6
25.0
25.6
26.1
24.8
22.1
9.4
4.9
2.0
1.8
1.6
199
TEMP Chl a
conc.
31.58
8.30
31.53
8.36
31.15
7.90
30.43 15.09
29.96
29.74 12.71
29.03 89.58
28.15 58.46
27.11 13.77
26.93
26.86
3.82
TABLES D19: 20-05-05
Depth
AF
MM
BC
CP
NP
CH HX
Surface
0
0
0
8500
2083
0
1
2083 11,000 4167 12500
0
0
2
40,998 19,833 1333 59,498
0
0
5
32165
0
0
44,665 9833
0
8
5916
9000
0
10,166 68,664 0
10
20833
0
0
6166
7416
0
12.5
0
0
0
0
0
500
15
0
0
0
0
0
167
20
0
0
0
0
0
500
25
0
0
0
0
0
250
30
0
0
0
0
0
583
0
0
0
0
0
0
0
0
0
0
0
DO
116.2
113.3
117.2
104.5
61.7
4.4
1.8
1.3
1.6
1.2
1.2
TEMP Chl a
conc.
32.61
4.90
31.74 11.86
31.44
6.07
30.96
7.90
30.19
29.59
8.37
28.26 66.31
27.47 47.70
27.18
1.39
27.01
27.00
1.72
TABLES D20: 19-05-06
Depth
AF
Surface
0
1
0
2
2667
5
0
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
7333
0
0
1126 18.8
51,331
0
0
678 18.8
93,996 4167
0
2083 18.9
23,166 6250
0
83 18.9
0
0
0
0
18.1
0
0
500
0
18.1
0
0
833
0
5.5
0
0
2500
0
2.9
0
0
3333
0
1.9
0
0
1750
0
1.7
0
0
250
0
1.4
200
TEMP Chl a
conc.
30.09
6.32
30.09
7.90
30.05
8.50
29.85
6.13
29.65
8.23
29.65 12.90
29.09 16.40
28.00 48.28
27.20
27.03
26.95
2.54
TABLES D21: 2-06-05
Depth
AF
MM
Surface
1
2
5
8
10
12.5
15
20
25
30
0
1583
10666
14499
4250
12833
21082
12500
0
0
0
0
0
25
0
0
0
0
0
0
0
0
BC
CP
NP
CH
HX
DO
33332 7916 8333
0
828 112.0
56248
0
2083
0
234 110.8
53165 16333 2083
0
0 110.4
91663 5416
0
0
0 111.4
34332 11833 6166
0
0
99.0
0
5666 5750
0
0
76.9
0
7166 9333
0
0
33.0
0
4000 3833 2500 0
3.6
0
0
0
1250 0
2.3
0
0
0
1833 0
1.8
0
0
0
1250 0
1.8
TEMP Chl a
conc.
32.17
4.42
31.05
5.45
30.83
9.35
30.49
30.34
7.64
29.98
6.59
29.29 15.13
27.54 16.41
27.20
7.45
27.09
27.01
1.22
TABLES D22: -06-06
Depth
Surface
1
2
5
8
10
12.5
15
20
25
30
AF
MM
0
0
8333 2333
2083
0
4500
0
4666
0
0
0
0
0
0
0
0
0
0
0
0
0
BC
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
0
0
0
0
108.7
56,498
0
0
4167 109.7
92,663
0
0
4167 111.4
74,664 21499
0
8333 115.6
39582 21416
0
0
112.7
0
15416 1083
0
94.6
0
0
8333
0
64.3
0
0
1000
0
53.0
0
0
417
0
53.3
0
0
500
0
58.0
0
0
833
0
56.7
201
TEMP
31.23
31.06
30.43
30.12
29.67
29.46
29.12
28.75
27.29
26.94
26.85
Chl a
conc.
No Data
TABLES D23: 17-06-05
Depth
AF
MM
BC
CP
NP
CH
HX
DO
0
0
0
0
0
0
0
0
0
0
0
97.0
97.2
99.4
96.5
94.3
82.7
48.5
9.5
4.2
3.1
2.3
Surface 18749
0
1750 7250
1250
0
1
10333
0
0
11000 1500
0
2
93,746 1833
0
35,499 2333
0
5
20499
0
1833 18,583 32,999
0
8
12500
0
0
16666 34,632
0
10
2083
0
0
14583 25166
0
12.5
2083
0
0
2083 42665 333
15
0
0
0
0
0
2500
20
0
0
0
0
0
2500
25
0
0
0
0
0
1667
30
0
0
0
0
0
1000
TEMP Chl a
conc.
29.75 0.002
29.75
2.74
29.71
6.20
29.63 0.002
29.61 0.002
29.46
3.32
29.22
7.12
28.07 41.89
27.18
0.81
27.07
27.02 0.002
TABLES D24: 16-06-06
Depth
AF
MM
Surface
0
0
1
4666 5666
2
2083 1500
5
0
0
8
0
0
10
0
0
12.5
0
0
15
0
0
20
0
0
25
0
0
30
0
0
BC
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
6500
0
0
75,997
0
83
56,664
0
583
23,166 2083 500
20,916 19499 1000
6333
9666 500
0
0
500
0
0
1083
0
0
1083
0
0
333
0
0
1667
202
HX
DO
0
0
0
0
0
0
0
0
0
0
0
93.6
97.5
98.7
94.5
96.6
96.7
35.0
32.7
30.5
30.1
26.5
TEMP Chl a
conc.
30.31
7.13
30.16
5.45
29.98
8.24
29.84
4.84
29.80
29.78
6.34
29.03
4.65
28.46
2.74
27.34
4.98
27.02
26.90
0.85
TABLES D25: 30-06-05
Depth
Surface
1
2
5
8
10
12.5
15
20
25
30
AF
MM
BC
4167
0
8333
14583
0
5416
2083
0
0
12500
0
4000
10333
0
9833
10166
0
8333
0
0
1583
0
1250 3167
4500
0
0
1500
0
4167
0
0
0
CP
14583
25000
12500
20833
8333
8333
4167
4167
2083
4000
0
NP
CH
HX
DO
0
0
0
84.5
0
0
2083 84.3
1333
0
0
82.5
1417
0
2083 83.0
1500
0
6250 78.6
6333 833
0
76.1
6500 417
0
77.1
7166 583
0
72.0
6750 2917
0
5.9
6500 8750
0
2.0
0
5000
0
1.8
TEMP Chl a
conc.
28.67
5.14
28.66
28.64
4.65
28.60
5.64
28.56
6.76
28.55
6.46
28.54
6.46
28.54
7.52
27.23
27.12
1.58
27.05
0.79
TABLES D26: 4-07-06
Depth
AF
Surface
0
1
30165
2
24832
5
5750
8
2833
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
18
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
TEMP Chl a
conc.
0
0
0
0
105.9 28.88
6.10
79164
0
0
0
78.9
29.39 11.08
217,658 2083
0
2083 77.5
29.35 13.78
26,666 14500
0
6250 73.2
29.31
3.82
20,833 11500
0
8333 70.1
29.31
6.62
0
4666 833 2083 70.1
29.29 0.315
2000
4000 417 2083 70.1
29.29
1.23
0
0
417
0
66.8
28.70
10.4
0
0
1667
0
3.5
27.36
0
0
2417
0
2.2
27.11
0
0
417
0
1.5
26.91
3.35
203
CH
HX
DO
TABLES D27: 15-7-05
Depth
AF
MM
BC
CP
NP
CH
Surface
0
0
6250
0
0
0
1
9833
0
2083 136,161 4250
2333
2
8083
0
0
143,244 6250
500
5
0
0
0
72,914 49998
667
8
0
1000
0
18,749 33332
0
10
0
0
0
16666 23,166
250
12.5
0
0
0
16666
4167
0
15
0
0
0
31499 47915
0
20
0
0
0
0
0
417
25
0
0
0
0
0
2917
30
0
0
0
0
0
17,499
HX
DO
TEMP
0
348
1126
0
0
0
0
0
0
0
0
112.5
101.7
101.7
99.0
90.1
71.9
78.0
72.8
41.2
7.7
3.3
29.01
28.44
28.44
28.02
27.89
27.87
27.86
27.86
27.76
27.12
27.03
TABLES D28: 17-07-06
Depth
AF
Surface
0
1
4167
2
6250
5
0
8
6500
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
21166
0
0
45998
0
0
35,415 4167
0
18479 29499 417
6250 12250
0
6250
6000 667
0
0
1250
0
0
500
0
0
4167
0
0
833
0
0
2917
204
HX
DO
0
0
10416
6250
8333
0
0
0
0
0
0
100.7
103.0
108.0
113.5
124.1
126.9
131.8
123.4
93.1
92.9
92.6
TEMP Chl a
conc.
29.34 14.63
29.23
9.48
29.02
8.30
28.91 10.01
28.90
8.97
28.89
28.87 14.11
28.81 25.71
27.35
9.55
27.03
26.93
0.80
Chl
a
conc.
8.31
8.24
6.73
6.00
2.58
1.27
0.89
-
TABLES D29: 29-07-05
Depth
AF
MM
BC
CP
NP
Surface
0
0
31249 51081 20833
1
8333
0
25000 26166 18749
2
0
0
4250 71164 16749
5
25332
0
11000 85,663 35499
8
0
0
2083 31249 10250
10
6250 2250 4167 84747 18749
12.5
1916
0
2083 16666 12500
15
1500
0
0
14583 14583
20
0
0
0
4167
4167
25
0
0
0
0
0
30
0
0
0
0
0
CH
HX
DO
0
0
0
0
0
0
0
0
0
0
0
4167
0
2083
2083
0
0
0
0
0
0
0
77.7
74.3
70.6
65.4
65.1
60.9
59.2
57.6
54.9
7.2
4.3
TEMP Chl a
conc.
27.53 12.98
27.51
5.79
27.46
5.53
27.44
5.81
27.41
3.70
27.41
8.17
27.40
27.40
6.98
27.33
4.83
27.25
27.04
1.06
TABLES D30: 28-07-06
Depth
AF
Surface
0
1
1500
2
0
5
0
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
27082
120,829
52081
25999
8333
4250
6000
0
0
0
0
NP
CH
HX
DO
2250
0
0
78.5
14749
0
4167 79.6
15332
0
10416 81.4
16166
0
0
84.1
20666
0
0
85.5
16499
0
0
86.2
13166
0
0
87.8
0
0
0
76.2
0
5416
0
52.5
0
10000
0
44.0
0
8333
0
40.6
205
TEMP Chl a
conc.
28.37
9.68
28.37
7.11
28.27 14.09
28.15
7.83
28.14
28.14
28.12
28.03
27.98
3.79
27.10
26.88
-
TABLES D31: 12-08-05
Depth
AF
MM
Surface
0
1
0
2
0
5
0
8
0
10
0
12.5
2000
15
0
20
0
25
0
30
2333
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
102,329 72,914 43748
0
79,830 56,498 27082
0
77,080 58664 20833
0
62,498 45832 24999
00
49,998 79164 31249
0
19,583 72997 20833
0
12,500 89496 18749
0
20,833 42665 20833
0
29,166 35665 8333 10250
31,249 20833 10416 3166
29,166 20833
0
1083
HX
DO
0
8333
0
10416
0
2083
2083
2083
6250
0
0
80.9
52.2
52.3
56.8
51.9
52.5
50.7
44.8
37.5
33.8
33.8
TEMP Chl a
conc.
27.03 16.41
27.59
27.05
8.44
27.02
6.39
26.97
7.05
26.95
6.85
26.90
7.05
26.88
6.85
26.86
7.97
26.87
26.85 12.05
TABLES D32: 11-08-06
Depth
Surface
1
2
5
8
10
12.5
15
20
25
30
AF MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
4250 2083
0
4250
0
0
16667 8333
0
22916 14583
6
4250 14583
0
8333 16667 20833
4167 6250
0
2083
0
0
0
0
7083
0
0
8832
0
0
10000
206
HX
DO
2083
2083
0
0
0
0
0
0
0
0
0
75.9
76.1
76.5
63.1
42.4
45.0
26.9
30.1
19.4
3.9
6.9
TEMP Chl a
conc.
28.10 22.14
28.08 21.74
28.02 20.50
27.75 21.81
27.66 10.48
27.64 15.19
27.61 12.11
27.60
6.61
27.50 12.57
27.36
26.90
1
TABLES D33: 26-08-05
Depth
AF
MM
Surface
1
2
5
8
10
12.5
15
20
25
30
2333
2167
2000
0
2083
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
HX
DO
60414 27082 4167
0
66664 98746 31498
0
24999 33332 31332
0
26249 6166 6166
0
10416
0
6000
0
6166
0
1916
0
4167
0
0
0
0
0
0
167
0
0
0
83
0
0
0
1250
0
0
0
1250
2083
6250
8333
2083
0
0
0
0
0
0
0
62.4
48.1
48.1
43.1
40.6
35.8
26.1
14.7
4.5
3.0
2.9
TEMP Chl a
conc.
26.56 32.12
27.04 22.00
27.01 29.12
26.99 17.46
26.98 27.04
26.95
9.81
26.88 16.40
26.86
5.51
26.85
1.90
26.84
26.84 29.51
TABLES D34: 25-08-06
Depth
AF
Surface
0
1
0
2
0
5
0
8
0
10
1583
12.5
0
15
0
20
0
25
0
30
0
MM
BC
0
0
0
0
0
0
0
0
0
0
0
6250
4167
2083
6250
2083
0
0
1500
0
0
0
CP
NP
CH
21749
0
0
93996
0
0
77080
0
0
29499 9333
0
17665 14833
0
8166 6166
0
2167
0
83
3000
0
167
0
0
1250
0
0
12500
0
0
583
207
HX
DO
83
164
0
0
0
0
0
0
0
0
0
126.2
125.2
129.8
114.5
98.9
67.2
37.1
29.1
9.4
6.5
2.6
TEMP Chl a
conc.
28.49 28.75
28.30 32.95
28.07 21.42
27.88 25.08
27.73 18.86
27.54 24.04
27.41 12.97
27.40
9.89
27.35
7.27
27.32
27.02
4.53
TABLES D35: 8-09-05
Depth
AF
MM BC
Surface
0
1
6250
2
6250
5
29166
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
18749
0
81497 17666
68831 47915
39582 2333
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CH
HX
DO
TEMP
83
83
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
41.1
41.4
41.7
33.2
21.9
15.5
14.5
14.3
15.2
10.5
10.3
27.57
27.50
27.26
26.93
26.84
26.83
26.82
26.82
26.82
26.81
26.81
Chl a
conc.
No Data
TABLES D36: 8-09-06
Depth
AF
Surface 2083
1
0
2
0
5
0
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
HX
DO
0
4250 10416
0
2083 127.9
4666 4000 6250 500 6250 129.4
0
10416 16667 417 16666 129.6
0
2083 22916 333 6250 112.6
4750 6250 10416
0
0
60.0
0
4167 10416 250
0
54.9
0
0
0
333
0
20.4
0
0
0
667
0
4.9
0
0
0
2917
0
1.6
0
0
0
2917
0
1.3
0
0
0
5000
0
1.2
208
TEMP Chl a
conc.
28.53 14.60
28.53 15.72
28.44 12.83
27.92 19.97
27.56 20.89
27.48 19.65
27.39
9.17
27.35
7.40
27.32
8.18
27.27
27.24
3.50
TABLES D37: 23-09-05
Depth
AF
Surface
1
2
5
8
10
12.5
15
20
25
30
16999
18749
45998
16666
14583
12500
62331
29499
4167
18749
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH HX
102079 6250
0
174993 20833
0
214575 31499
0
270823 41665
0
58748 177076 0
57831 49998
0
26000 20833 750
191659 37748 167
46165
4167
0
158327 1916
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
DO
TEMP
69.5
63.5
57.9
70.7
64.5
50.4
19.8
9.5
2.6
1.9
1.7
29.06
28.47
28.24
27.78
27.48
27.31
27.03
26.97
26.84
26.82
26.84
Chl a
conc.
No Data
TABLES D38: 22-09-06
Depth
AF
Surface
0
1
0
2
2333
5
0
8
2000
10
1583
12.5
0
15
0
20
0
25
0
30
0
MM
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
HX
DO
17166
0
0
0
0
127.9
19499 10416
0
0
1283 129.4
0
35415
0
0
2083 129.6
0
18833 14583
0
6250 112.6
2083 2083 58664
0
0
60.0
0
2083 36332
0
0
54.9
0
0
0
0
0
20.4
0
0
0
3333
0
4.9
0
0
0
3750
0
1.6
0
0
0
4167
0
1.3
0
0
0
2500
0
1.2
209
TEMP Chl a
conc.
28.53 12.77
28.53 13.56
28.44 14.41
27.92 13.42
27.56 11.46
27.48
7.46
27.36
6.15
27.35
4.06
27.32
3.93
27.27
27.24
2.88
TABLES D39: 7-10-05
Depth
AF
Surface
1
2
5
8
10
12.5
15
20
25
30
43748
25000
31249
16667
0
0
0
0
0
0
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
TEMP
313154 33332
0
217658 54831 333
229324 35582 1000
93746 343820 1000
35415
833
3000
18749
2083 2250
0
0
1749
0
0
750
0
0
250
0
0
0
0
0
0
0
2417
5916
5856
2417
0
0
0
0
0
0
104.9
114.1
97.7
86.9
12.0
4.6
3.1
3.4
3.2
3.1
2.4
30.79
30.78
29.05
28.42
27.48
27.21
26.94
26.86
26.83
26.81
26.81
Chl a
conc.
No Data
TABLES D40: 6-10-06
Depth
AF
MM
Surface
1
2
5
8
10
12.5
15
20
25
30
1583
1500
2083
2000
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
9333 18749
0
167
0
47915 2083
83
6500 148161 22916 2917
2333 17666 14583 833
1500 14500 6166 917
0
4500
6333 583
0
0
2083
0
0
0
0
0
0
0
0
1000
0
0
0
3750
0
0
0
500
210
HX
DO
6250
6250
6250
2083
16666
0
6250
0
0
0
0
125.4
130.2
125.7
123.5
75.2
54.5
43.4
8.5
2.0
1.6
1.5
TEMP Chl a
conc.
29.93
8.91
29.51 11.92
29.29
7.66
28.82 11.46
28.03
9.95
27.93
7.66
27.76
7.01
27.46
2.81
27.34
3.60
27.17
27.05
1.28
TABLES D41: 20-10-05
Depth
Surface
1
2
5
8
10
12.5
15
20
25
30
AF MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
6166 6250
0
27082 8750
0
16667 17749
0
0
0
0
0
0
0
0
0
0
0
0
6083
0
0
15166
0
0
4333
0
0
333
0
0
0
HX
DO
TEMP
0
2083
5567
6354
0
0
0
0
0
0
0
35.8
36.6
36.1
34.7
22.1
10.0
6.2
4.0
3.1
2.6
1.9
30.59
30.18
30.00
29.46
28.09
27.56
27.17
27.02
26.86
26.83
26.82
Chl a
conc.
No Data
TABLES 42: 20-10-06
Depth
AF
Surface
0
1
0
2
2083
5
0
8
1916
10
1749
12.5
0
15
0
20
0
25
0
30
0
MM
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
HX
DO
0
0
0
0
6500 37582
0
0
1916 68747
0
0
0
10333 8333
0
0
8500 10333
0
0
6500 14500 1250
0
0
0
0
0
0
0
1250
0
0
0
1167
0
0
0
1000
0
0
0
167
0
0
8654
6657
0
0
0
0
0
0
0
106.4
118.1
119.9
117.9
42.6
8.4
3.3
2.6
2.0
1.8
1.7
211
TEMP Chl a
conc.
30.87
7.79
30.15 10.74
29.97
8.38
29.76
9.36
28.19 10.35
27.89 10.61
27.76
7.01
27.49
3.52
27.32
2.84
27.19
27.02
1.36
TABLES D43: 4-11-05
Depth
Surface
1
2
5
8
10
12.5
15
20
25
30
AF MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
2417
0
0
8666 8333
2
6250 10250
0
5750 6250
0
0
0
0
0
0
250
0
0
750
0
0
10500
0
0
12666
0
0
83
0
0
1333
HX
DO TEMP
2493
6671
2023
154
0
0
0
0
0
0
0
No
Data
Chl a
conc.
No Data
TABLES D44: 3-11-06
Depth
AF
Surface
0
1
0
2
2083
5
0
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM
BC
0
0
0
0
0
0
0
0
0
0
0
2333
2250
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
DO
23166
0
6250
58998
0
2083
58331 2083 6250
6250
8500 2083
10082 141994
0
2083 35082
0
0
0
1916
0
0
2500
0
0
2500
0
0
2083
0
0
1667
6500
2333
6789
2316
0
0
0
0
0
0
0
111.7
120.2
121.8
123.2
91.8
37.8
30.5
26.1
14.3
6.3
5.7
212
TEMP Chl a
conc.
30.69
9.49
30.57 12.38
30.35 15.06
30.22 10.67
29.41 12.11
27.83 10.87
27.61
6.48
27.48
1.13
27.37
2.84
27.27
27.11
1.81
TABLES D45: 19-11-05
Depth
AF
Surface
0
1
9333
2
1583
5
2250
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
8500 4583 2083
0
6250 42832 2000
0
4167 24832 9166
0
1916 9250 10250
0
0
0
0
0
0
0
1167
0
0
0
0
0
0
0
0
417
0
0
0
7750
0
0
0
10250
0
0
0
7083
HX
6250
26164
12441
2083
0
0
0
0
0
0
0
DO TEMP
No
Data
Chl a
conc.
No Data
TABLES D46: 17-11-06
Depth
AF
Surface
0
1
0
2
4000
5
0
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
20999
0
0
6453
62831
0
0
19879
58664 2083
0
13465
10666 7416
0
5687
14583 148327
0
1346
25
42665
0
0
0
0
2167
0
0
0
2500
0
0
0
2500
0
0
0
2167
0
0
0
1749
0
213
DO
117.1
117.6
120.1
117.9
117.1
15.4
6.1
4.3
2.7
2.3
1.8
TEMP Chl a
conc.
31.37
7.33
31.34
7.66
30.96
6.61
30.61
7.33
29.59 11.33
28.05
8.83
27.81
4.33
27.76
3.36
27.50
2.33
27.28
27.00
1.25
TABLES D47: 2-12-05
Depth
AF
Surface
0
1
0
2
4333
5
0
8
0
10
2083
12.5
0
15
0
20
0
25
0
30
0
MM
0
0
0
0
0
0
0
0
0
0
0
BC
CP
NP
CH
7250 1500 481480
0
0
7000 241907
0
0
14666 212825
0
4250 20916 322820
0
1417
0
0
0
0
0
0
583
0
0
0
417
0
0
0
333
0
0
0
1333
0
0
0
4416
0
0
0
3500
HX
DO
TEMP
4687
12897
24364
8345
234
0
0
0
0
0
0
38.2
38.0
38.7
38.9
37.2
24.5
12.9
8.4
5.6
3.6
3.0
31.38
31.13
30.53
30.26
29.11
27.61
27.27
27.02
26.84
26.82
26.81
HX
DO
TEMP
2134
6548
8729
10416
1462
0
0
0
0
0
0
38.2
38.0
38.7
38.9
37.2
28.5
12.9
8.4
5.6
3.6
3.0
31.38
31.13
30.53
30.26
29.11
27.61
27.77
27.02
26.84
26.82
26.81
Chl a
conc.
No Data
TABLES D48: 15-12-05
Depth
AF
MM
BC
Surface
1
2
5
8
10
12.5
15
20
25
30
0
3417
3167
9500
8166
0
0
3583
0
0
0
4500
4749
0
1250
0
1916
0
0
0
0
0
4833
1500
0
0
0
1000
0
0
1167
0
0
CP
NP
CH
18749 1916
0
10416 4167
0
29665 16667
0
6250 6250
0
12583 8416
0
1833 4167
0
2333
0
1583
10333 4167 1333
0
0
5416
0
0
3833
0
0
417
214
Chl a
conc.
No Data
TABLES D49: 16-12-06
Depth
AF
Surface
0
1
0
2
6166
5
4333
8
0
10
0
12.5
0
15
0
20
0
25
0
30
0
MM BC
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
CP
NP
CH
HX
47915 1083
0
93663
0
0
60498
0
0
18749 29248
0
6250 23165
5
0
14583 417
0
0
833
0
0
2583
0
0
4167
0
0
3749
0
0
2917
5788
6350
9785
234
0
0
0
0
0
0
0
215
DO TEMP Chl a
conc.
No
Data
8.58
6.48
8.84
8.84
10.08
7.66
4.51
1.71
0.99
9.17
TABLE E
Community Abundance Data (Volumetric)
(Measured in number of individuals per cubic meter of lake water)
Table E 1: Bi-weekly abundance data of all zooplankton taxa during January to June 2005.
Species
14-1
28-1
11-2
26-2
11-3
25-3
8-4
22-4
7-5
20-5
2-6
17-6
30-6
Adult females
Adult males
M. micrura
B.calyciflorus
B. dimidiatus
H.intermedia
K. cochlearis
F.pejleri
F.camascela
C.ceratopogones
Copepodites
Nauplii
Ovigerous
Females
4278
4261
0
742
0
25412
0
0
0
7741
63671
43941
0
6926
3342
0
710
0
0
0
0
0
11640
11640
14317
0
4307
306
0
371
0
9845
0
0
0
3899
33058
26656
233
5005
986
0
0
0
10121
0
0
0
3870
24560
8381
873
2735
15426
0
1116
0
13284
0
0
0
6286
54126
41089
0
2406
13678
504
629
0
6024
0
0
0
6693
58840
47084
0
4511
26845
87
1106
0
90486
0
0
0
7421
41322
34862
0
2173
4281
1067
0
0
24624
0
0
0
10185
28402
33698
563
29100
15218
11640
2541
0
18426
0
0
0
698
32592
23280
873
17460
3929
6518
956
0
582
146
0
0
349
17460
15947
291
14550
35124
727
32068
0
1231
0
0
0
3201
14550
6169
2406
17460
50285
393
2774
0
0
0
0
0
3492
23047
7973
1746
4831
6077
956
8905
0
10814
0
0
4656
4074
23105
3288
87
216
Table E 2: Bi-weekly abundance data of all zooplankton taxa during July to December 2005.
Species
15-7
29-7
12-8
26-8
8-9
23-9
7-10
20-10
4-11
19-11
2-12
15-12
Adult females
Adult males
M. micrura
B.calyciflorus
B. dimidiatus
H.intermedia
K. cochlearis
F.pejleri
F.camascela
C.ceratopogones
Copepodites
Nauplii
Ovigerous
Females
3492
25702
233
771
0
2734
0
0
0
6984
58840
45629
58
22291
22174
291
24502
0
698
0
0
0
4656
116458
8206
582
1135
1465
0
62584
125169
1465
0
0
0
4365
37597
102374
466
1292
1833
0
36957
0
5238
0
0
0
495
9254
4772
314
1304
29100
0
0
0
0
0
0
0
4074
14434
3521
210
12338
43737
0
576
0
0
0
0
0
2910
106564
25433
1222
23222
34920
0
0
0
10476
0
0
0
2910
105866
62274
3841
1006
1426
0
0
0
1426
0
0
0
6402
13968
10592
0
0
2037
0
80
0
43650
0
0
0
11058
4074
5325
0
1315
4656
0
3783
0
121871
6169
5238
0
12222
15016
7159
198
4743
3929
174
4743
0
104760
4656
0
0
10767
14841
41904
437
2076
13676
0
0
0
38701
0
0
0
8730
143928
28809
15331
217
Table E 3: Bi-weekly abundance data of all zooplankton taxa during January to early July 2006.
Species
14-1
28-1
10-2
24-2
10-3
24-3
7-4
21-4
6-5
19-5
16-6
4-7
Adult females
Adult males
M. micrura
B.calyciflorus
B. dimidiatus
H.intermedia
K. cochlearis
F.pejleri
F.camascela
C.ceratopogones
Copepodites
Nauplii
Ovigerous Females
6111
5328
0
0
0
0
0
0
0
36898
39285
42602
2794
1048
4621
0
0
0
233
0
0
0
39576
14375
2444
0
1484
291
0
0
0
11058
291
0
0
3899
33058
26656
233
3870
1046
5995
0
0
10168
0
0
0
873
15074
5936
1193
2018
17757
897
0
0
15132
2328
11640
0
5936
28052
8963
349
2153
11640
1557
0
0
6693
0
12222
0
11640
50925
12804
291
3929
30846
4598
0
0
12603
0
0
0
3347
82586
33814
786
1432
5820
1116
756
0
30846
0
0
0
3725
49761
49412
0
2386
15423
0
0
0
23280
0
0
0
17460
29915
28751
737
2697
23280
1077
0
0
10185
0
0
0
7275
103829
42952
2406
1385
2037
652
0
0
73623
0
0
1745
2910
27412
22465
105
4423
29088
800
0
0
108500
0
0
1454
2910
60121
18391
407
218
Table E 4: Bi-weekly abundance data of all zooplankton taxa during mid-July to December 2006. Because sampling did
not occur during the first week of December, data is subsequently not available.
Species
17-7
28-7
11-8
25-8
8-9
22-9
6-10
20-10
3-11
19-11
2-12
Adult females
2270
3056
1139
2597
829
3841
3085
3026
2910
3376
No
Data
Adult males
M. micrura
B.calyciflorus
B. dimidiatus
H.intermedia
K. cochlearis
F.pejleri
F.camascela
C.ceratopogones
Copepodites
Nauplii
Ovigerous
Females
15417
401
0
0
15417
0
0
0
11640
39343
25084
776
9599
0
0
0
7854
0
0
0
12804
118030
70131
320
2037
254
0
0
7857
0
0
0
18915
21767
11000
93
23280
652
5820
0
2037
0
0
0
5238
134035
48364
175
7854
0
8730
0
42760
0
0
1454
8730
18624
79792
73
19489
19963
0
0
54395
0
0
1454
7566
76417
69491
553
31125
0
16703
0
3781
0
0
1745
18624
184727
40798
524
11635
0
3550
0
7854
0
0
0
20457
88871
10185
480
15417
0
407
0
0
38688
4363
0
16587
99347
57909
349
17441
0
0
0
40482
0
0
4418
8032
83633
33756
378
219
16-12
6929
3201
284
174
0
78570
3218
0
0
11756
13270
15132
291
TABLE F
Community Biomass Data (Volumetric) 2005
(measured in milligram dry weight per meter cube)
Table F1
Date
Copepodites
Male
Copepods
19.80
15.69
1.28
3.33
63.13
58.43
115.92
14.82
65.71
15.90
133.87
192.72
37.01
97.42
96.26
6.38
11.71
153.61
202.23
164.85
7.37
9.71
22.43
Nauplii
137.26
23.93
55.77
103.31
110.94
120.74
80.80
60.96
63.71
35.31
30.12
48.87
49.47
135.05
281.55
140.04
19.18
34.96
269.35
229.00
32.18
10.80
42.12
Adult Female
Copepods
25.29
41.45
28.61
27.66
14.45
14.42
25.56
17.16
181.43
100.88
99.80
109.09
27.86
20.74
139.50
11.91
12.07
10.88
81.25
181.40
6.421
0.00
9.229
14-1
28-1
11-2
26-2
11-3
25-3
8-4
22-4
7-5
20-5
2-6
17-6
30-6
15-7
29-7
12-8
26-8
8-9
23-9
7-10
20-10
4-11
19-11
2-12
16-12
30.17
28.05
32.88
46.83
17.60
15.03
8.43
3.07
9.06
3.05
1.99
1.78
9.87
9.26
8.19
7.18
5.07
3.36
4.99
1.73
0.70
9.63
1.55
23.17
1.22
0.70
4.51
11.33
3.15
1.16
1.54
220
M.micrura Chaoborus B.
calyciflorus
0.00
184.67
0.20
0.00
441.09
0.19
0.00
120.11
0.12
0.00
71.16
0.00
0.00
126.37
0.34
2.24
202.28
0.17
0.43
232.78
0.34
5.14
310.22
0.00
59.85
67.86
0.83
30.22
5.08
0.31
3.19
55.96
9.87
1.73
71.50
0.85
4.42
112.42
0.36
1.10
241.33
0.27
1.39
173.80
8.74
0.00
187.29
17.14
0.00
15.74
10.12
0.00
82.12
0.00
0.00
53.20
0.12
0.00
59.11
0.00
0.00
187.19
0.00
0.00
401.08
0.01
0.00
402.77
0.80
1.083
1.494
342.68
305.94
0.86
0.03
H.intermedia
0.97
0.00
0.30
0.34
0.50
0.18
4.12
0.73
0.85
0.02
0.04
0.00
0.42
0.13
0.03
0.07
0.17
0.00
0.00
0.34
0.02
1.81
5.07
5.12
3.33
Table F2
Community Biomass Data (Volumetric)
2006
Date
Copepodites
14-1
28-1
10-2
24-2
10-3
24-3
7-4
21-4
6-5
19-5
5-6
16-6
4-7
17-7
28-7
11-8
25-8
8-9
22-9
6-10
20-10
3-11
15-11
15-12
84.58
32.02
71.79
30.42
57.22
99.10
156.24
102.47
61.15
208.88
190.61
54.34
127.09
77.18
252.29
43.41
276.72
34.64
166.69
391.15
194.42
177.25
151.32
209.20
Adult Female
Copepods
63.75
6.49
9.18
34.57
13.69
14.49
28.78
7.88
19.38
35.78
19.16
9.17
27.85
20.22
23.68
8.70
17.45
5.58
27.97
21.61
21.78
21.15
23.92
157.32
Male
Copepods
26.20
21.59
1.41
4.81
77.08
53.26
135.33
28.62
63.46
105.97
63.74
9.81
129.65
56.51
48.93
8.98
127.86
37.65
99.32
131.56
50.77
66.92
74.51
61.28
Nauplii
M.micrura
Chaoborus
B. calyciflorus
H.intermedia
8.98
0.50
6.44
1.10
1.68
2.43
7.10
10.10
6.26
9.23
5.30
4.89
4.28
5.66
17.94
2.58
11.26
16.32
15.14
9.30
2.23
10.81
6.25
6.43
0.00
0.00
0.00
32.99
3.73
6.29
19.07
4.98
0.00
4.99
1.69
3.13
3.09
1.67
0.00
1.05
2.77
0.00
94.89
0.00
0.00
0.00
0.00
0.00
460.30
841.10
130.18
34.17
196.89
508.60
94.66
111.58
436.46
277.67
218.14
94.63
84.692
308.80
410.18
477.37
176.85
244.73
251.01
469.25
406.70
178.60
214.45
223.10
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.20
1.54
2.23
0.00
3.69
0.83
0.00
0.00
0.00
0.03
0.01
0.39
0.35
0.57
0.20
4.06
1.31
0.68
0.29
1.42
3.81
4.25
0.64
0.32
0.33
0.08
1.92
2.02
0.16
0.31
1.47
1.55
1.37
221
TABLE G 1
Community Production Data (Volumetric)
2005
(measured in milligram dry weight per meter cube per day)
Date
Copepodites
Females
Nauplii
M.micrura
Chaoborus
14-1
28-1
11-2
26-2
11-3
25-3
8-4
22-4
7-5
20-5
2-6
17-6
30-6
15-7
29-7
12-8
26-8
8-9
23-9
7-10
20-10
4-11
19-11
2-12
16-12
57.35
10.12
13.65
13.48
36.48
52.02
32.54
21.09
18.40
11.50
11.49
19.76
19.44
50.80
96.15
18.39
5.94
12.96
105.62
80.70
8.53
3.62
13.72
11.16
8.84
0.00
0.00
1.02
0.00
0.00
0.00
0.00
1.13
0.12
0.07
0.74
0.44
0.07
0.06
0.10
1.48
0.82
0.56
0.41
0.63
0.00
0.00
0.64
0.36
0.15
2.60
1.03
0.47
0.51
2.77
3.49
2.52
2.50
1.38
0.95
1.23
0.41
0.17
1.95
0.35
5.95
0.27
0.19
1.55
4.19
0.60
0.35
0.48
2.77
1.00
0.00
0.00
0.00
0.00
0.00
0.27
0.05
0.62
7.27
3.90
0.38
0.20
0.61
0.13
0.18
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.13
0.19
25.03
49.71
8.80
4.14
8.67
10.09
10.64
25.00
5.66
0.26
1.14
1.69
6.24
12.98
10.73
176.08
0.70
2.44
1.53
1.29
7.55
0.79
23.56
21.31
17.07
222
TABLE G 2
Community Production Data (Volumetric)
2006
Date
14-1
28-1
10-2
24-2
10-3
24-3
7-4
21-4
6-5
19-5
5-6
16-6
4-7
17-7
28-7
11-8
25-8
8-9
22-9
6-10
20-10
3-11
15-11
15-12
Copepodites
Females
Nauplii
M.micrura
Chaoborus
34.51
11.09
20.54
9.88
19.46
40.91
65.04
37.43
25.39
86.00
72.82
14.39
51.95
23.25
75.92
15.11
126.66
11.37
52.20
133.71
71.35
80.46
69.22
107.25
1.78
0.00
0.79
1.22
0.80
0.59
0.87
0.00
1.41
2.98
1.27
0.30
0.40
1.43
0.36
0.28
0.26
0.35
0.57
0.75
0.66
0.46
0.44
32.07
2.52
0.14
2.07
0.36
0.60
0.95
2.44
3.67
1.94
3.90
1.30
1.46
1.13
1.62
4.84
0.68
3.52
4.66
3.89
2.55
0.58
4.69
2.73
1.93
0.00
0.00
0.00
4.07
0.46
0.77
2.42
0.62
0.00
0.64
0.21
0.39
0.40
0.21
0.00
0.13
0.35
0.00
11.08
0.00
0.00
0.00
0.00
0.00
39.96
78.34
10.34
2.28
14.77
25.85
5.53
9.04
32.12
24.52
13.27
6.48
7.72
29.94
26.19
50.79
14.23
18.91
8.23
24.51
21.85
11.00
12.72
18.70
223