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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 …………………….. Signature …………….......... Date Prof. Emmanuel Frempong (Supervisor) ………………………. Signature ………………. … Date Dr. Samuel Aikins (Co-supervisor) ……………………….. Signature ………………….. Date Dr. P. K Baidoo (Head of Department) …………………………. Signature ii ………………..... 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 iii 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. iv 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 v 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 vi 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 vii 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 viii 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 ix 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 x 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 xi 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 xii 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 xiii 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 xiv 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 xv 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. 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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