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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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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 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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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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Marked cross-shelf and along-shelf differences in phytoplankton composition and biomass were evident. Nearcoastal stations at the east end of Transect I were
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. 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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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
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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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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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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
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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
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Triglops pingelii Aspidophoroides olrikii
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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
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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 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
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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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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 (
