Download Changes in plankton and fish larvae communities

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Changes in plankton and fish larvae
communities across hydrographic fronts
off West Greenland
PETER MUNK*, BENNI W. HANSEN1, TORKEL G. NIELSEN2 AND HELGE A. THOMSEN
,
, PO BOX , DK- ROSKILDE, DENMARK
DANISH INSTITUTE FOR FISHERIES RESEARCH, CHARLOTTENLUND CASTLE, DK- CHARLOTTENLUND, 1ROSKILDE UNIVERSITY, PO BOX
DK- ROSKILDE AND 2NATIONAL ENVIRONMENTAL RESEARCH INSTITUTE, FREDERIKSBORGVEJ
*CORRESPONDING AUTHOR: [email protected]
The variability in plankton community structure was studied in Disko Bay and across important
fishing banks off the west coast of Greenland. The primary goal of the study was to investigate
possible linkages between hydrographical processes and plankton structures, hypothesizing that hydrographic fronts would be present in the area, and that these to a large extent determine plankton distribution, composition and productivity. We sampled along four cross-shelf transects, one covering Disko
Bay and Disko Bank, while the other three covered Store Hellefiske Bank, Lille Hellefiske Bank and
Sukkertop Bank. The hydrography was examined by CTD profiling, the phytoplankton by fluorescence profiling and water bottle sampling, while mesozooplankton and ichthyoplankton were sampled
by vertical or oblique net hauls, respectively. We observed distinct along-shelf flowing currents in the
area (e.g. the West Greenland Current, the Polar Current and the Irminger Current), and the physical
characteristics indicated frontogenesis at the shelf slope, in regions of 80–100 m water depth. Phytoplankton and ichthyoplankton showed a cross-shelf structuring with apparent linkages to frontal
characteristics, while a more diverse pattern was observed for the mesozooplankton which were dominated by Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus. The relationship
between hydrographic characteristics and plankton distribution differed among species, and apparently specific plankton communities were established in different areas of the shelf. For example the
larvae of Boreogadus saida, Ammodytes sp., Reinhardtius hippoglossoides and Stichaeus punctatus
differed markedly in distributional characteristics. In addition to the cross-shelf structuring, marked
differences in species composition and total plankton abundance were observed in the along-shelf
(north–south) direction. The latitudinal differences in the unicellular plankton communities are interpreted largely within a seasonal successional framework (i.e. an early dominance of diatoms followed
by increasing importance of smaller unicellular plankton), while the ichthyo- and zooplankton
communities also differed by the respective dominance of species with polar versus temperate origin.
Our findings suggest that the flow of major currents and the establishment of hydrographical fronts
are of primary importance to the plankton communities in the West Greenland shelf area, influencing the early life of fish and the recruitment to the important fisheries resources.
I N T RO D U C T I O N
Plankton and fish larvae communities are often structured
in assemblages with a close relationship to environmental
characteristics (Cowen et al., 1993). The background and
adaptive potential of the hydrographical–biological
linkages have attracted great interest, and are addressed
in a number of ecological investigations. One approach
has been to link specific hydrographical characteristics to
the spatial structure of plankton communities, including
fish larval distribution patterns (Munk and Nielsen, 1994;
Thorrold and Williams, 1996; Smith and Suthers, 1999),
while other studies examine the physical processes which
act to concentrate and transport the plankton organisms
(Govoni and Grimes, 1992; Franks, 1997). Yet another
approach considers the reproductive strategies, whether
timing and location of spawning are correlated to various
physical features of importance to the offspring (Leis,
Journal of Plankton Research 25(7), © Oxford University Press; all rights reserved
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1982; Newton, 1996). Lately, investigations of plankton
distribution patterns and ecological dynamics in shelf
areas have been focusing on physical processes related to
hydrographical frontal phenomena. Spatial overlap
between frontal hydrography and plankton distribution is
demonstrated during field studies [e.g. (Munk et al., 1995;
Govoni and Spach, 1999)] when the physical and behavioural processes of plankton aggregation and structuring
are examined by modelling exercises (Franks, 1992;
Werner et al., 1993, 2001).
A majority of the studies on linkages between frontal
hydrography and plankton ecosystems are carried out in
temperate waters, while little is known about the significance of frontal hydrography with respect to ecosystem
functioning in the Arctic, notwithstanding prominent
frontal zones are found in Arctic marine regions
(Narayanan et al., 1991; Heburn and Johnson, 1995).
In the present study we examine the hydrographical–biological linkages in an area that encompasses
regions of partly polar (Arctic) and partly temperate
characteristics. The area, which covers the shelf off West
Greenland (from 65 to 69°N), has a characteristic
hydrography dominated by major currents which originate in the North Atlantic and the Arctic Basin (Buch,
1990). These currents flow along-shelf, offshore of a
coastal water mass influenced by local run-off, and the
variability in regimes and conditions across the shelf indicates the existence of hydrographical fronts. The shelf
area includes a number of shallow banks, inhabited by a
variety of zooplankton species, and is known as the
nursery site of a number of fish species, for example
Boreogadus saida, Ammodytes sp., Liparis liparis, Reinhardtius
hippoglossoides, Hippoglossoides platessoides and Stichaeus punctatus (Pedersen and Smidt, 2000). The richness of the area
is evident from three decades of annual surveys described
by Pedersen and Smidt (Pedersen and Smidt, 2000),
which illustrate the variability in hydrography and
plankton distribution along a series of cross-shelf
transects.
Detailed information on the plankton food web structure in this region is available for Disko Bay, which is an
embayment within the northern part of the present
investigation area (Møller and Nielsen, 2000; Levinsen
and Nielsen, 2002). In the Disko area, the phytoplankton
(mostly diatoms) peaks markedly during a short spring
bloom event and is succeeded by a more diverse subsurface bloom of smaller magnitude. The zooplankton
community is dominated by large bodied Calanus spp.
during May–June (Madsen et al., 2001), followed by protozoans in July–August (Levinsen et al., 2000), and finally by
small bodied copepods in September–May (D. Madsen,
T. G. Nielsen and B. W. Hansen, unpublished results).
It is the aim of the study to explore the major
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physical/biological linkages and plankton community
patterns in the area covering the major west Greenland
fishing banks, hypothesizing that fronts between the
prevailing major currents exert a predominant influence
on plankton distribution and dynamics. Our specific goals
are (i) to identify frontal characteristics and their influence
on the entrainment of nutrients, the abundance of
primary producers and the productivity of secondary
consumers, (ii) to describe the plankton assemblages at all
trophic levels and ascertain their distributional overlap,
and (iii) to ascertain cross-shelf and along-shelf differences in plankton abundances and the influence of frontal
hydrography in the formation of plankton communities.
METHOD
The study was carried out on board RV ‘Adolf Jensen’
(Greenland Institute of Natural Resources) in an area off
the west coast of Greenland during the period June
25–July 7, 1996. Sampling took place along four transects,
each following a given latitude. We crossed Disko Bank
and Disko Bay along 69°08N, Store Hellefiske Bank
along 67°35N, Lille Hellefiske Bank along 65°56N and
Sukkertop Bank along 65°00N (Figure 1). The bathymetry varied markedly along the transects, the sea floor
declining gradually from 30 to 50 m depth at the most
shallow part of the banks to ~150 m at the shelf break,
and declining more steeply from the break to the ~500 m
depth measured at the westernmost stations. In Disko Bay,
the sea bottom declined to ~800 m depth in the central
parts of the Bay.
Station distances along the transects were either 20 or
10 min longitude (~14 or 7 km). At each station, the
sampling was initiated by a CTD cast (Seabird 25-01,
with mounted Chelsea fluorometer), which profiled
temperature, conductivity and fluorescence of the water
column to ~10 m above the bottom. At every second
station along each transect, the CTD cast was followed by
water sampling for nutrients, chlorophyll a (Chl a) and
phytoplankton, and vertical net hauls for zooplankton. At
every station an oblique net haul was carried out for
ichthyoplankton sampling.
Nutrients
The depths 5, 20, 40, 80, 120 and 200 m and the depth
of maximal fluorescence were sampled by 5 l Niskin water
bottles. Samples for determination of nutrients (NO2–,
NO3–, PO43–, SiO43–) were taken from the Niskin water
sample, frozen on board, and later analysed by an automatic nutrient analyser following the procedures in
Grasshof (Grasshof, 1976). All samples were analysed in
duplicate with a precision of 0.06, 0.09 and 0.12 µM for
nitrate, phosphate and silicate, respectively.
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Mesozooplankton biomass
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Fig. 1. The bathymetry of the area off West Greenland, with indication of the four cross-shelf transects. The transects cover Disko Bay
and Disko Bank (I), Store Hellefiske Bank (II), Lille Hellefiske Bank (III)
and Sukkertop Bank (IV). Isobaths of 200 m (dotted line) and 500 m
(full, thin line) are indicated on the map.
Chlorophyll a and phytoplankton
On board, 1–2 l of each sample were filtered on GF/F
filters, extracted in 96% ethanol and measured on a spectrophotometer (Strickland and Parsons, 1972). The in situ
fluorometer measurements were calibrated against the
spectrophotometer determined Chl a by a linear regression of respective measurements from the same depth.
Measurements based on the calibrated in situ fluorometer
were subsequently used in the characterization of Chl a
distribution. Samples of 300 ml from the 5 m depth and
from the fluorescence maximum were preserved by
addition of 6 ml of acid Lugol. In the laboratory the
abundance and biomass of dominating phytoplankton
taxa were determined by inverted microscopy (Utermöhl,
1958) of 50 ml sedimented samples. Phytoplankton
biomass was calculated from volume estimation using a
carbon conversion factor of 0.13 pg C µm–3 for dinoflagellates and 0.11 pg C µm–3 for all other groups (Edler,
1979).
Mesozooplankton abundance was estimated from two
vertical net hauls. One net had an opening of 22 cm
diameter and a mesh size of 50 µm (Fine Meshed net,
FM), the other had an opening of 58 cm diameter and a
mesh size of 200 µm (WP-2 net). Each net was lowered to
60 m depth (or 2 m above bottom at shallower stations),
and retrieved the given distance at either 5 m min–1 (FM)
or 10 m min–1 (WP-2) assuming 100% filtration efficiency.
The samples were preserved in 4% Borax buffered
formalin (final concentration). In the laboratory, subsamples of ~300 copepodites were identified to species and
stage. Within each copepodite stage up to 10 specimens
were length measured (cephalothorax length). From the
same subsample, nauplii stages were identified either to
Calanus spp., Pseudocalanus sp. or ‘others’ and up to 200
nauplii were length measured. Copepod eggs were
enumerated and their diameter measured. Abundance
and length information was used to estimate copepod
biomass within taxonomic groups. Length to carbonweight relationships were obtained from the literature:
Calanus (all three species) and Metridia longa from Hirche
and Mumm (Hirche and Mumm, 1992), Acartia spp. and
all nauplii from Berggreen et al. (Berggreen et al., 1988),
Pseudocalanus sp. from Klein Breteler et al. (Klein Breteler
et al., 1982), while for the taxons Oithona spp., Microcalanus
spp., Oncaea spp. and Microsetella spp., the relationship for
Oithona spp. in Sabatini and Kiørboe (Sabatini and
Kiørboe, 1994) was used.
Compared with the WP-2 net, the FM net undersampled copepods >500 µm (cephalothorax length) because
of the smaller opening and the slower towing speed. On
the other hand, the coarser WP-2 net undersampled the
copepods <500 µm. In order to average abundance estimates made by the two nets, we first calculated the relative
catchability of each net in a comparison of the sum of
catches (within copepod stages) from all parallel FM and
WP-2 net hauls (Figure 2). Each comparison considered
catchability of copepodites at species and stage and catchability of nauplii at length (50 µm length intervals), and
was subsequently used to calibrate the undersampled
intervals of each FM and WP-2 haul. After this relative
calibration procedure, the FM and WP-2 estimates at a
given station were considered replicates, with the same
catchability across the range of copepod sizes, and the
average of the two inter-calibrated hauls was used as the
final estimate at this station.
Mesozooplankton egg production
Live specimens of large copepods were collected by a
gentle WP-2 net haul in the depth stratum of maximal
fluorescence. The copepods were brought to a laboratory
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Proportion in WP-2 / FM nets
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Calanus finmarchicus
Calanus glacialis
Calanus hyperboreus
Metridia longa
Acartia spp.
Pseudocalanus spp.
Microcalanus spp.
Oithona spp.
Oncaea spp.
Microsetella spp.
0.1
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Fig. 2. Comparison of catchability between the two gears used for
sampling of copepods (WP-2 and FM). Each value indicates the relative
size of the catch in the WP-2 compared with the FM for each copepod
species and copepodite stage (see text). Species are indicated by symbols;
not all stages are represented for each species. Arrows indicate the level
of equal catchability, and the copepod size where gear superiorities
change.
on board, and, if available, individual ripe females of the
three Calanus species were pipetted into 600 ml polycarbonate bottles filled with 45 µm screened surface water.
The bottles were incubated at surface water temperature
for 48 h, and thereafter females were length measured and
spawned eggs were counted.
Ichthyoplankton abundance
For ichthyoplankton sampling we used a ring net of 2 m
diameter, equipped with a black net of 16 m length and
1 mm mesh size. Oblique hauls were carried out to 10 m
above sea bottom (maximal depth 90 m) at a ship’s speed
of 3 knots. The net was deployed and retrieved at speeds
of ~20 and 10 m min–1, respectively. A calibrated flowmeter in the opening of the net was used to estimate filtered
volume of water. The fish larvae were immediately sorted
from the sample and preserved in 96% ethanol. In the
laboratory, the larvae were identified to species, and up to
50 per species and station were length measured to
standard length.
R E S U LT S
Hydrography, nutrients and phytoplankton
Figure 3a and b shows the vertical profiles for the upper
120 m of the water column, as observed along the four
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transects by our CTD profiling. Figure 3a illustrates variation in temperature and salinity, while Figure 3b illustrates variation in water density and Chl a content.
Considerable cross-shelf variation was found in all
parameters. Specific water masses colder than both
surface and bottom water were found either at intermediate depths (Transects I and II) or at the shelf slope
(Transects III and IV). The colder water is less saline than
the neighbouring water masses at the same depths, visible
by the inclining halolines between the water masses
(Figure 3a).
The buoyancy effect of temperature is not fully
compensated by the salinity effect, and we observed a
doming of isopycnals at the upper part of the shelf slope
(at Transects II and IV an offshore inclination of isopycnals indicates another doming, not fully covered by these
transects). The surface concentration of nitrate was
depleted to below detection level and silicate to below
2 µM except where doming of the pycnocline brought
nutrient-rich water to the surface. This is clearly seen in
Figure 4 where the concentrations of nitrate and silicate
are illustrated by shading and isolines, respectively. The
distribution patterns of these two nutrients followed each
other closely. The surface concentration of phosphorus
was >0.1 µM and was not depleted throughout the area
investigated (data not shown).
The abundance of phytoplankton tended to peak close
to interfaces between water masses (Figure 3b), either at
the shelf slope between 50 and 100 m depth, or in the
vicinity of the inclining isopycnals further off-bank.
Measurements at the Disko Bank/Bay transect were of
outstanding magnitude, showing very high chlorophyll
values at 20–50 m depth. Based on the profiles of Chl a
we calculated the total amount of Chl a below a square
metre sea surface to 120 m depth (Figure 5a). Marked
differences in Chl a concentration were evident, both in
cross-shelf and along-shelf directions. Enhanced phytoplankton abundance appeared in the vicinity of, but
slightly displaced from, the doming in nutrient-rich water;
for example, the two significant peaks in Chl a at
Transect I (Figure 5a) were found close to two conspicuous domes in the nutrient profiles.
Information on phytoplankton is available both from
the calibrated fluorometer measurements described above
(Figures 3b and 5a) and from the water bottle sampling in
the depth layer of fluorescence maximum from which
phytoplankton were identified, counted and size
measured (Figure 5b). The stacked bars in Figure 5b show
the biomass of autotrophic plankton within major taxonomic and/or functional groups, while inserted curves
illustrate abundances of heterotrophic ciliates and dinoflagellates. The ‘nanoplankton’ group as defined here
excludes diatoms, dinoflagellates and haptophytes.
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Fig. 3. Contouring of physical parameters and Chl a concentration along Transects I–IV. Arrows denote the 80 m depth at the shelf slope. (a)
Temperature and salinity from surface to 120 m depth. Temperature is illustrated by different intensity of shading, values (in °C) are denoted by
inserted bars (Transects I + II, upper bar; Transects III + IV, lower bar). Halolines are shown in intervals of 0.1 p.p.t. (b) Water density and
Chl a from surface to 120 m depth. Chl a is illustrated by different intensity of shading, the inserted bars show levels in mg m–3. Isopycnals are
shown in intervals of 0.1 kg m–3.
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dominated by diatoms (in particular Thalassiosira bulbosa
and Detonula confervacea), whereas the haptophyte Phaeocystis pouchetii (colonial stage) was the single most abundant
organism at the westernmost stations. Dinoflagellates
contributed significantly to biomass levels at the easternmost half of Transect I and at some stations along
Transect II. Large, athecate heterotrophic forms were
particularly abundant, e.g. Gyrodinium spirale, Gyrodinium
crassua and Gymnodinium rhomboides. Diatoms were
contributing significantly to overall biomass levels at most
stations, and core species were Corethron criophilum at the
shelf break and Actinocyclus cf. octonarius and Thalassiosira
spp. at near coastal stations. Haptophytes were only
abundant at the most nearshore station on Transect II
(P. pouchetii, colonial stage). The photosynthetic ciliate
Myrionecta rubra was the dominant single organism at two
stations. Biomass levels further decrease at Transects III
and IV reaching levels which are approximately one order
of magnitude lower than those observed at peak stations
along Transect I. Small, athecate dinoflagellates (10–20
µm) were abundant at all stations along Transect III.
Centric diatoms (Thalassiosira spp.) and P. pouchetii (colonial
form) were abundant towards the eastern end of the
transect, whereas M. rubra and Chrysochromulina spp.
(Haptophyceae) contribute to the build-up of unicellular
biomass at the most offshore station. Unidentified flagellates comprise the bulk of the nanoplankton fractions at
both Transects III and IV. Small, athecate dinoflagellates
(10–20 µm), and haptophytes (Chrysochromulina spp.),
dominated at the shelf front biomass maximum station,
and at the coastal stations (P. pouchetii, flagellate stage). It
is important when searching for correlations between data
sets presented here also to emphasize that the data shown
in Figure 5b represent values obtained from the subsurface chlorophyll maximum depth, which in most cases
was 20–30 m during the late June, early July period. This
means that the group-specific protistplankton data only
reflect features of the upper water mass.
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Fig. 4. Contouring of nutrient concentration along Transects I–IV. The
concentrations of nitrite and nitrate (NO2–+ NO3–) are illustrated by
different shading intensity; the inserted bar illustrates the intervals used
(in µM). Isolines illustrate distribution of silicate (SiO43–).
Marked cross-shelf and along-shelf differences in phytoplankton composition and biomass were evident. Nearcoastal stations at the east end of Transect I were
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Copepods dominated the zooplankton at all four transects. The following species were identified: Calanus
finmarchicus, Calanus hyperboreus, Calanus glacialis, Metridia
longa, Pseudocalanus elongatus, Acartia longiremis, Oithona similis
and Microcalanus pusillus. In the following, the four latter
species are considered by genus name, together with
unidentified species of these taxa. In addition, Microsetella
spp. and Oncaea spp. were found in the area.
Figure 6a and b illustrates the distribution and relative
importance of the different taxonomic groups (copepodite stages). The three Calanus species were by far the
most important group with respect to biomass (Figure 6a).
Among the species, C. finmarchicus was abundant at all four
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Fig. 5. Measurements of algal abundance along Transects I–IV. (a) Integrated Chl a (mg m–2). (b) Composition of phytoplankton biomass in the
water layer of maximal chlorophyll. Biomasses in mg C m–3 are illustrated for major taxonomic groups of autotrophic organisms by different shading
of bars and for heterotrophic organisms by connected symbols, see inserted legend beside graph IV. Note the change in scales between Transects
I + II and III + IV.
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Ι)
Copepod biomass (mg C m -2 )
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Fig. 6. Composition of copepod biomass in the upper 60 m (mg C m–2) along Transects I–IV. (a) Biomass of three Calanus species. The three
species are indicated by variable shading of stacked bars, see legend beside graph IV of this series. (b) Biomass of the copepod species other than
Calanus spp. The different species are indicated by variable shading of stacked bars, see legend beside graph IV of this series.
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PLANKTON AND FISH LARVAE OFF W. GREENLAND
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transects while C. glacialis and C. hyperboreus dominated at
Transect I. The biomass of the other taxa (Figure 6b) was
an order of magnitude less than for the Calanus spp.—
note the change in biomass scale from Figure 6a to b. Of
these other taxa M. longa and Pseudocalanus spp. were by far
the most important. However, M. longa only dominated at
the Disko Bay part of Transect I. Pseudocalanus spp. were
most abundant at Transects I and II, high biomasses were
observed in Disko Bay and at the shelf break at Transects
II and IV. Compared with these values, the biomass of the
remaining taxa was of little significance; Acartia spp. were
found at the shoremost stations (Transects II and III),
while Oithona spp. and Microsetella spp. were found at the
most offshore stations.
Of the nauplii stages of copepods, only the Calanus spp.
were caught in quantifiable numbers. The abundance of
these nauplii showed great variation, both between and
along transects (Figure 7). The overall abundance
declined more than an order of magnitude from the
northernmost to the southernmost transect, while the
along-transect variation also reached an order of magnitude. Except for Transect II, which showed a peak in
nauplii abundance close to the coast, the maximal abundance of nauplii was observed at the shelf break. In
Figure 7 we also compare the abundance of nauplii with
measured copepod egg production for C. fimarchicus and
C. glacialis. Both egg production and abundance of nauplii
decline from north to south, but apparently the egg
production and nauplii abundance are not related along
the transects. The variation in Calanus spp. egg production
along Transect I follows to some extent the variation in
phytoplankton abundance (compare with Figure 5a and
b); however, there is no such tendency along the other
transects.
The variable taxa and stage composition of the
copepod communities is reflected in the biomass spectra
of the copepods. In Figure 8 biomass spectra from the
different transects are illustrated. The spectra are
averaged for all stations along the given transect (at
Transect I for two sections). The Calanus nauplii are
found in the size classes below 500 µm and the modest
abundance of Calanus nauplii at Transects III and IV is
reflected in the low biomass in this smaller size range.
The Pseudocalanus spp. have a marked influence on the
accumulated biomass for size classes between 500 and
1200 µm, while C. finmarchicus and C. glacialis dominate
the size range 1000–3000 µm. Calanus hyperboreus is by far
the largest copepod and dominates classes above 3000
µm. Field investigations indicate that larvae prefer
copepod prey whose length is in the order of 3–5% of
larval length (Munk, 1997), and in order to illustrate the
prey sizes of relevance as potential prey to the fish larvae,
Figure 8 includes curves of size distributions where
Eggs female
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Calanus spp. nauplii
Egg production
C. finmarchicus
Egg production
C. glacialis
53
Longitude
Fig. 7. Calanus spp. measurements along Transects I–IV. Egg production estimates (eggs female–1 day–1) and naupliar abundance (no. 103
m–2). Abundance of Calanus spp. nauplii is illustrated by open circles, the
egg production by C. finmarchicus or C. glacialis is illustrated by closed or
open triangles, respectively.
size corresponds to 5% of respective larval length
distributions.
Fish larvae
The following species of fish larvae were caught during
our survey: B. saida (Arctic cod), Ammodytes sp. (sandeel),
L. liparis (striped seasnail), R. hippoglossoides (Greenland
halibut), S. punctatus (Arctic shanny), Lumpenus maculatus
(daubed shanny), Lumpenus lumpretaeformis (snake blenny),
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Cephalothorax length (µm)
Fig. 8. Spectra of the copepod biomass calculated for Transects I
(Disko Bank and Disko Bay), II, III and IV. The biomass (mg C m–3) is
accumulated within log-scaled intervals of copepod lengths (see text).
Inserted curves illustrate relative prey size preference (no scale used on
y-axis) by the fish larval communities along the respective transects (fit
of larval length distributions to log–normal curves, lengths multiplied by
0.05).
0.5
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Boreogadus saida
Liparis liparis
Lumpenus maculatus
Lumpenus lampretaeformis
Stichaeus punctatus
Triglops pingelii
Ulcina olrikii
Leptagonus decagonus
Reinhardtius hippoglossoides
Hippoglossoides platessoides
Ammodytes sp.
Longitude
Fig. 9. Abundance of fish larvae along Transects I–IV (no. m–2). The
different species are indicated by variable shading of stacked bars, see
inserted legend. Arrows denote the position of the 80 m water depth at
the shelf slope.
H. platessoides (American plaice), Leptagonus decagonus
(Atlantic poacher), Ulcina olrikii (Arctic alligatorfish),
Triglops spp. (ribbed sculpin a.o.), Myoxocephalus scorpius
(common sculpin) and Anarhicas lupus (Atlantic wolffish).
The relative abundance of these larvae, as well as their
spatial distribution, varied significantly in our area of
investigation. Figure 9 shows the changes in larval abundance (no. m–2) along the four transects as stacked bars,
while Figure 10a and b illustrates the spatial variation in

abundance of the six more important species in relation
to geography and bathymetry.
Larvae were in general at highest abundances along
Transect I; along the other transects, high abundance estimates were mainly due to the large quantities of
Ammodytes sp. The species B. saida and L. liparis were
predominantly found along Transect I, while the species
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PLANKTON AND FISH LARVAE OFF W. GREENLAND
R. hippoglossoides and H. platessoides were mainly found in
the southern part of the area, along Transects III and IV
(Figures 9 and 10). A marked cross-shelf change in species
composition was observed, for example when S. punctatus
was found at the shoremost stations only (Figure 10b), the
Lumpenus spp. were found further offshore and species
such as R. hippoglossoides and H. platessoides were found at
the outermost stations (Figure 10a and b). Larval lengths
showed no systematic variation along transects, but the
mean lengths of most species declined from south to
north. Figure 11 illustrates the variation in larval lengths
averaged for each species and transect; the decline in
length from Transect IV to Transect II (2.5º latitude,
~275 km) is in the order of 30% for many of the larval
species.
DISCUSSION
Hydrography
Our study showed a distinct structuring of hydrographical and biological parameters in the cross-shelf direction,
implying the existence of hydrographic frontal phenomena with associated assemblages of plankton organisms at
the shelf slope. The hydrography was obviously strongly
influenced by major along-shelf flowing currents. We
found cold water masses at mid-depth, and low saline
water of relatively high temperature in the upper and
coastal water layers. We characterize the water masses
following the specifications given by Buch (Buch, 1990)
and use the hydrographic measurements along
Transect II to exemplify this interpretation. The
a)
b)
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Fig. 10. Illustration of horizontal distribution of abundant larval fish species. Each symbol increases in area from zero to the maximal abundance
of the given species. (a) Stars, B. saida (max. 0.5 m–2); squares, Ammodytes sp. (max. 7.7 m–2); and triangles (upwards), H. platessoides (max. 0.3 m–2).
(b) Circles, S. punctatus (max. 0.3 m–2); diamonds, L. liparis (max. 1.0 m–2); and triangles (downwards), R. hippoglossoides (max. 0.1 m–2).
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Boreogadus saida
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Liparis liparis
Lumpenus maculatus
50
Lumpenus lampretaeformis
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Temperature ( o C)
Triglops pingelii
Aspidophoroides olrikii
40
Leptagonus decagonus
Reinharditus hippoglossoides
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1
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Stichaeus punctatus
Mean length (mm)
Hippoglossoides platessoides
Ammodytes sp.
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Salinity (ppt)
Latitude
b)
Fig. 11. Latitudinal changes in mean length of fish larvae. Larval
mean length (in mm) is illustrated against the latitude of each transect.
Larval species are indicated by symbols, see inserted legend.
5 Ice water
3 Baffin
Current
respective temperature and salinity values are shown in a
T/S plot (Figure 12a), and the interpretation of
currents/water masses is illustrated on a hydrographic
profile for Transect II (Figure 12b). The north-flowing
Polar Current exerts a major influence on the hydrography at the shelf slope (Figure 12b). At its core, this current
is of a temperature <1°C and a salinity <33.4 p.p.t.
(Buch, 1990) (see Figure 12a). It becomes colder and less
saline when we proceed from south to north (from
Transect IV to I, Figure 3a), but it could be recognized at
the shelf slope along all four transects. Above and below
the Polar Current we find two warmer water masses, of
salinities either <33.4 p.p.t. (West Greenland Current) or
>34.0 p.p.t. (North Atlantic/Labrador Current) (see
Figure 12a). At their core, these water masses are of
temperatures >4°C, but we observe only the mixing zones
of intermediate temperatures between these and the
Polar Current. The North Atlantic/Labrador Current is
found in deeper parts of >200 m depth, and might
include a component of the Irminger Current (Buch,
1990). Beside these currents, indicated at all transects, we
also find two other water masses at the two northerly transects. As illustrated in Figure 12a and b, another cold
water mass can be distinguished at mid-depth along
Transect II; at its core, this water mass has a temperature
below zero, and is of lower temperature and salinity than
the Polar Current. Along Transects I and II, sampling
extends so far west that we meet the Baffin Current, which
is the returning Polar Current that has been round Baffin
Bay and flows south in the western parts of Baffin Bay
and Davis Strait (Figure 1). The remaining water masses
seen at Transects I and II are the cold and fresh ice melt
4 West Greenland
Current
Mixing zone
50
Depth (m)
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Current
100
Mixing zone
150
1 NorthAtlantic/
Labrador
Current
200
57
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55
54
Longitude
Fig. 12. Interpretation of water characteristics. (a) T/S plot of corresponding measurements of temperature (°C) and salinity (p.p.t.) along
Transect II. Numbers refer to deduced water masses. (b) Contouring of
combined salinity/temperature characteristics along Transect II with
indication of deduced water masses. Numbers correspond to indication
in (a).

water (Figure 12a and b). These we find at the surface at
the westernmost sections where we approach the West-Ice
off the Canadian Coast, and in the eastern part of
Transect I where we meet the glacier water in inner
Disko Bay.
Obviously a number of frontal zones could be distinguished between these different water masses. The
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PLANKTON AND FISH LARVAE OFF W. GREENLAND
temperature and salinity fronts are to some extent density
compensated; however, major frontal patterns can be
deduced from the water density structure (Figure 3b). In
the upper 40 m we observed distinct interfaces, either
between the Polar Current and the West Greenland
Current as evident in Transect IV and the eastern part of
Transect II, or between Polar/Baffin Currents and Ice
Water as evident along Transect I and the western part of
Transect II. Frontal phenomena at mid-depth are evident
from the inclination of isopycnals between the Polar
Current and the Baffin Current (Transect II), or the Polar
Current and the West Greenland Current. The latter
zone is found at the shelf slope of ~80 m water depth (this
section indicated by arrows in Figure 3). The inclining
isopycnals point to frontal processes leading to convergent/divergent flow and upwelling of deeper water and
nutrients. Such upwelling of nutrients is apparent at the
shelf slope along Transects II–III, at the skerry midpart
of Transect I, and between the Polar and Baffin Currents
at Transect II (Figure 4).
Phytoplankton
We found indications of enhancement of phytoplankton
biomass in the identified frontal zones (i.e. in the surface
pycnocline, at the shelf slope and further offshore in the
vicinity of the shelf break). This is seen from phytoplankton patches measured during our fluorescence
profiling (Figure 3b), and from the cross-shelf abundance
analysis (Figure 5a). When examining the taxonomic
patterns of the protistplankton (Figure 5b) a number of
station-specific differences emerge when analysing individual transects in addition to distinct north–south differences between transects. In most cases it appears that
differences observed in relation to taxonomic and functional groups reflect successional patterns rather than
clear-cut responses to frontal zones for example.
Marked latitudinal trends in phytoplankton characteristics were (i) a north to south decrease in biomass levels
by approximately an order of magnitude, (ii) a distinct
north–south change in protistplankton size spectra from
a microplankton (>20 µm) dominance at the northernmost transects to a nanoplankton (<20 µm) dominated
system at the southernmost transects, and (iii) a shift from
dominance by diatoms and colonial stages of P. pouchetii at
Transects I and II towards nanoflagellate dominated
communities at Transects III and IV. The latter regional
shift in community composition is also paralleled by
distinct changes in the composition of the unicellular
community of phytoplankton grazers, e.g. exemplified by
a north–south shift in the heterotrophic dinoflagellate
community from large-sized diatom grazing forms such as
G. spirale and G. rhomboides at the northernmost transect,
towards an athecate dinoflagellate community at
southernmost transects dominated by nanoplankton sized
dinoflagellates (10–20 µm).
It is apparent that the differences listed above are most
convincingly explained when relating these to successional events reflecting seasonal changes in forcing
environmental factors such as ice coverage, water column
stability, turbulence, light, temperature, nutrients and
grazers. At this stage only circumstantial evidence exists,
corroborating the interpretation of the present data as
‘waves of succession’ moving in a northward direction
along the Greenland coast. This will have to be further
substantiated by examining time series of collected data
from the entire region. However, additional support for
the interpretation is provided by Poulsen and Reuss
(Poulsen and Reuss, 2002) and a time-series study undertaken at a fixed station in Disco Bay (H. A. Thomsen,
unpublished results). In this scenario the southernmost
transects are representing a post-spring bloom scenario
where nutrients are exhausted in the photic zone and
small nano-sized forms prevail throughout the water
column. The Sukkertop and Fylla Bank transects were
sampled in May 2000 during the peak of the spring
bloom (Poulsen and Reuss, 2002). The average biomass
level for the diatom dominated autotrophic phytoplankton was 92 ± 45 µg C l–1, while the heterotrophic
community reached 17 ± 7 µg C l–1, which mirrors the
biomass levels reported here for Transect I. The southernmost Transects III and IV are thus, despite the low
overall protist biomasses presented here, in a seasonal
perspective no different from the northernmost transects.
While nutrients are also partly exhausted at the northernmost stations (Figure 4), the species composition and
overall biomass levels indicate that these particular transects were sampled during the latter part of the regional
spring bloom event. Redfield ratios of nutrients indicate
N-limitation in all samples from both the surface and the
fluorescence maximum depth. A typical sequence of
species in an Arctic spring bloom is pennate diatoms, e.g.
Fossula and Fragilariopsis, followed by Thalassiosira spp., D.
confervacea and Chaetoceros spp. (Saito and Taniguchi, 1978;
von Qillfeldt, 1996). This is consistent with our interpretation of stations along Transect I as representing late
diatom bloom successional phases.
The most conspicuous along-transect (cross-shelf )
difference in species composition was seen at Transect I.
While stations sampled within the West Greenland
Current are dominated by diatoms, the westernmost part
of the transect features the colonial stage of P. pouchetii as
the single most prominent organism. It is known from
other high latitude, northern hemispheric regions that
P. pouchetii is an integral part of the spring bloom event,
however, with a tendency to peak immediately following
the decline of a diatom spring bloom [e.g. (Vernet, 1991;
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von Quillfeldt, 1996)], and it is therefore tempting to
interpret our observations accordingly. A prolonged ice
coverage of the easternmost coastal stations located
within Disko Bay proper, in combination with likely
initially increased levels of silicon in coastal waters, can
explain the different timing of spring bloom events along
the Transect I stations and the developmental delay
observed at nearshore stations.
Zooplankton
Our observations on copepod characteristics showed a
quite diverse pattern; however, some linkage to hydrographical patterns and distribution of other plankton
organisms was indicated. We found marked differences in
species composition and total abundance both in the
cross-shelf and in the along-shelf direction, and species
which dominate in polar regions were numerous at the
northernmost transects while more temperate species
dominated at the southernmost transects.
Other studies at the west coast or in Disko Bay also
revealed a major dominance by the three Calanus species
(Nielsen and Hansen, 1995; Pedersen et al., 1999; Madsen
et al., 2001). Calanus finmarchicus and C. glacialis are
regarded as Atlantic and Arctic species, respectively
( Jashnov, 1970; Hassel, 1984), whereas C. hyperboreus has
its main distribution in subarctic areas from where it
spreads into the Arctic Ocean (Hirche, 1991; Hirche and
Mumm, 1992). However, C. finmarchicus is also found in
high densities in the Greenland Sea at sub-zero temperatures (Smith et al., 1986) and is reported to reproduce
here, thus temperature itself does not restrict the zoogeographical range due to reproductive failure (Hirche,
1990). The biomass registered during earlier studies in
Disko Bay was somewhat higher than that found during
the present study, while the egg production rates were of
the same order (Nielsen and Hansen, 1995; Hansen et al.,
1999). Annual studies during 1996–1997 in Disko Bay
included measurements of the biomass of smaller bodied
copepods (S. D. Madsen, T. G. Nielsen and B. W. Hansen,
unpublished results). These revealed biomass estimates of
0.5 mg C m–3, hence an order of magnitude lower than
the Calanus spp. biomass. As also observed during the
present study, the Pseudocalanus spp. and Oithona spp. were
the dominant taxa during the June–July period.
In our study the egg production of C. finmarchicus and
C. glacialis peaked in areas of enhanced phytoplankton
concentration (at maximum ~3 mg Chl a m–3) corresponding to previously reported studies, e.g. Diel and
Tande (Diel and Tande, 1992) and Nielsen and Hansen
(Nielsen and Hansen, 1995). The lower egg production
rates in areas of chlorophyll concentration <1 mg m–3 are
in accordance with findings of food limitation in such
areas (Nielsen and Hansen, 1995). In fact, food-limited
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egg production is reported below 0.15 µg C l–1 (Plourde
and Runge, 1993) and a linear ingestion response is found
up to 10 mg Chl a l–1 (Hansen et al., 1999). Hence, our
findings indicated a latitudinal progression in the productivity of the Calanus population, from a highly productive
population in the northern part of the study area, to
populations of decreasing productivity in the southern
parts.
Pedersen (Pedersen, 1998) has published concurrent
observations of the abundance and condition of shrimp
larvae (Pandalus borealis). The area off west Greenland
supports a large stock of this species, and the larval (zoea)
stages were included in the plankton community. Shrimp
larvae were not present in the northern part of our
investigation area (at this time of year), but larval stages
in the size range 1.5–3 mm were found in densities up to
40 m–2 in the southern part (Pedersen, 1998). This corresponds to a shrimp larval biomass of ~20 mg C m–2;
hence, their biomass is quite minor compared with the
Calanus spp. biomass in the area, which is in the order of
2000 mg C m–2. The variation in shrimp abundance
along the transects [described by Pedersen (Pedersen,
1998)] showed peaks either in the frontal zone area of
~80 m water depth, or in nearshore areas.
Fish larvae
The fish larval assemblages were distinctly structured crossshelf and along-shelf. The species-characteristic distribution pattern reflects well known habitat preferences,
either climate (polar/temperate) or coastal/offshore. The
species B. saida, L. maculatus, S. punctatus, U. olriki and
L. decagonus are reported as ‘polar’ species (Eschmeyer et al.,
1983; Andriashev, 1986; Cohen et al., 1990), which is in
accordance with our findings of a more northerly distribution of these species. The great latitudinal difference
across our sampling area is also apparent in the variation
of mean lengths within species. We observed a marked
decline in mean lengths with increasing latitude, which
most probably reflects a seasonal progression of larval
emergence, i.e. due to an earlier enhancement of light
conditions, spawning will take place at an earlier time at the
lower latitudes. However, our findings also indicate that
larval prey was not as plentiful at the higher latitudes, which
could (relatively) depress their growth rates in these areas.
This is suggested by the comparison of the ascertained
ranges of preferred prey sizes and the accumulated
copepod biomasses within these ranges (Figure 8). Obviously, the relatively large fractions of medium sized
copepods (~0.8–1.6 mm in length) at the two southernmost
transects are relatively high and of great value to these
communities of fish larvae, while less biomass is available
within the prey size ranges of the fish larval communities
at the northernmost transects.
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At Transects II–IV, the total abundance of fish larvae
tends to peak at the shelf slope, at positions where the
water depths are ~80 m. This corresponds to the position
of the frontal zone between the Polar Current and the
West Greenland Current (positions indicated by arrows
both on Figure 3 and Figure 9). Along Transect I, the
peaks in larval distribution appear connected to the
frontal zones between Baffin and Polar Currents, either at
Disko Bank (predominantly L. liparis), or in Disko Bay
(predominantly B. saida). An additional importance of the
off-shelf interface between ice melt water and the underlying water masses is indicated by the observed increase
in abundance of L. liparis and B. saida at the westernmost
station of Transect I and of Ammodytes sp. at Transect II.
Hence, when considering the major trends in distribution of the plankton groups (i.e. the position of notable
peaks in abundance) our results signify a great influence
from frontal hydrography on plankton distribution
patterns and their assembly in communities. The crossshelf structuring in abundance and composition was
definite in the cases of phytoplankton and ichthyoplankton, while our mesozooplankton (copepod) observations
did not show cross-shelf patterns that could be unconditionally traced to frontal hydrography. Our analysis of
spatial patterns is to some extent constrained by the use
of ‘point’ sampling (with a relatively coarse station grid)
in the very heterogeneous and dynamic environment.
Plankton organisms will be patchily distributed at the
metre scale of our phyto- and zooplankton sampling, and
such small-scale variability would be embedded in our
description of the major kilometre-scale variation. Here,
it is worth noting that our ichthyoplankton sampling, integrating distribution patterns across much wider horizontal scales, affords the most coherent picture of changes
within (ichthyo-)plankton assemblages along transects.
Our findings of relationships between frontal hydrography and plankton communities in this transition area
between temperate and Arctic regions add to the accumulating evidence of a universal set of mechanisms that
shape the plankton ecosystems in shelf areas. As a transition area between the (shallow and freshwater influenced) coastal zone and the ocean, the shelf includes
water masses of different physical/chemical characteristics and, consequently, a series of frontal zones. Because
of the exceptional physical processes connected to frontal
zones, the frontal activity affects plankton organisms in a
number of aspects, among these are nutrient entrainment, primary/secondary production and plankton
distribution/aggregation. Hence, the fate of plankton
communities is attached to frontal variability, and marked
deviations in frontal activity are likely to influence the
ecosystem and the survival probabilities of the different
plankton organisms.
AC K N O W L E D G E M E N T S
Thanks to the crew on RV ‘Adolf Jensen’, the Greenland
Institute for Natural Resources, and to B. Søborg, L.
Riemann, C. M. Andersen and S. A. Pedersen. Special
thanks to H. Trier for access to selected data from his
master thesis. The project was supported by the Danish
Research Council (no. 9501038).
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Received on June 22, 2002; accepted on March 28, 2003
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