Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
250815 13/6/03 3:35 pm Page 815 JOURNAL OF PLANKTON RESEARCH VOLUME NUMBER PAGES ‒ 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 250815 13/6/03 3:35 pm Page 816 JOURNAL OF PLANKTON RESEARCH VOLUME 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 NUMBER PAGES ‒ 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. 13/6/03 3:35 pm Page 817 P. MUNK ET AL. PLANKTON AND FISH LARVAE OFF W. GREENLAND Mesozooplankton biomass 71 70 Disko Baffin Bay 69 I Disko Banke Disko Bay 68 II Store Hellefiske Davis Banke 67 Latitude 250815 66 Cumber land West Greenland Strait III Lille Hellefiske Banke IV 65 Sukkertop Banke 64 Fyllas Banke 63 Labrador Sea 62 61 -64 -62 -60 -58 -56 -54 -52 -50 -48 Longitude (W) 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 13/6/03 3:35 pm Page 818 JOURNAL OF PLANKTON RESEARCH VOLUME 10 Proportion in WP-2 / FM nets 250815 1 Calanus finmarchicus Calanus glacialis Calanus hyperboreus Metridia longa Acartia spp. Pseudocalanus spp. Microcalanus spp. Oithona spp. Oncaea spp. Microsetella spp. 0.1 0.01 0.0 0.5 1.0 1.5 Length (mm) 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 NUMBER PAGES ‒ 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. 3:35 pm Page 819 P. MUNK ET AL. PLANKTON AND FISH LARVAE OFF W. GREENLAND a) b) I) Depth (m) Depth (m) I) 50 100 50 100 56 55 54 53 52 56 55 54 53 52 II) II) 3 Depth (m) Depth (m) 2.5 2 50 1.5 1 0.5 50 0 -0.5 100 100 -1 -1.5 57 56 55 54 III) Depth (m) III) Depth (m) 50 100 IV) 57 56 55 54 50 100 56 55 54 IV) 56 55 54 1.6 4 3.5 Depth (m) 13/6/03 Depth (m) 250815 3 50 2.5 2 1.3 50 0.9 0.6 1.5 100 100 1 0.3 0.5 54 53 54 Longitude 53 Longitude 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. 13/6/03 3:35 pm Page 820 JOURNAL OF PLANKTON RESEARCH VOLUME Depth (m) 50 100 56 55 54 53 52 II) Depth (m) 0 50 100 57 56 55 54 III) Depth (m) 0 50 100 56 55 NUMBER PAGES ‒ 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. I) 0 54 IV) 0 Depth (m) 250815 10 8 50 6 4 Zooplankton 2 100 0 54 53 Longitude 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 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 Page 821 P. MUNK ET AL. PLANKTON AND FISH LARVAE OFF W. GREENLAND ΙΙΙ) IV) Biomass in max. (mg C m-3 ) Ι) 200 150 100 50 0 80 50 40 30 20 10 0 55 54 53 52 57 ΙΙ) 200 150 100 50 0 58 57 56 55 54 ΙΙΙ) 50 40 30 20 10 0 56 55 54 50 Biomass in max. (mg C m-3 ) 56 30 20 10 0 54 55 54 57 56 55 53 52 30 20 10 0 54 35 30 20 15 10 5 0 55 54 35 30 Myrionecta rubra Autotroph. dinofl. Nanoplankton Haptophyceae Diatoms Heterotrophs 20 15 10 5 0 55 Longitude 53 40 56 IV) 40 55 56 80 50 58 Biomass in max. (mg C m-3 ) ΙΙ) b) 57 -2 Integrated chlorophyll a (mg m ) Ι) -2 Integrated chlorophyll a (mg m ) a) Biomass in max. (mg C m-3 ) 3:35 pm -2 Integrated chlorophyll a (mg m ) 13/6/03 -2 Integrated chlorophyll a (mg m ) 250815 54 53 Longitude 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. 13/6/03 3:35 pm Page 822 JOURNAL OF PLANKTON RESEARCH VOLUME a) NUMBER ‒ 478 519 601 Copepod biomass (mg C m -2 ) 4000 3000 2000 1000 100 0 ΙΙ) 55 54 53 52 ΙΙ) Copepod biomass (mg C m -2 ) 56 4000 3000 2000 1000 ΙΙΙ) 56 55 54 55 54 53 52 100 ΙΙΙ) 57 Copepod biomass (mg C m -2 ) 57 4000 3000 2000 1000 0 56 IV) 56 200 0 0 55 IV) 4000 3000 2000 C.finmarchicus C.glacialis C. hyperboreus 1000 0 54 56 54 100 0 55 54 200 Acartia spp. Pseudocalanus spp. Microcalanus spp. Oithona spp. Oncaea spp. Microsetella spp. Metridia longa 100 0 54 53 55 200 56 54 Copepod biomass (mg C m -2 ) Copepod biomass (mg C m -2 ) PAGES 200 0 Copepod biomass (mg C m -2 ) b) Ι) Copepod biomass (mg C m -2 ) Ι) Copepod biomass (mg C m -2 ) 250815 53 Longitude Longitude 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. Page 823 PLANKTON AND FISH LARVAE OFF W. GREENLAND 300 50 250 40 200 30 150 20 100 10 50 Eggs female 0 0 Eggs female -1 d -1 56 55 54 53 52 60 300 50 250 40 200 30 150 20 100 10 50 0 0 55 d -1 day 56 50 10 0 0 55 54 50 10 0 0 54 Copepod nauplii (10 3 m- 2 ) 56 54 Copepod nauplii (10 3 m- 2 ) 57 III) IV) Copepod nauplii (10 3 m- 2 ) II) Copepod nauplii (10 3 m- 2 ) 60 -1 d -1 I) -1 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 P. MUNK ET AL. d -1 3:35 pm -1 13/6/03 Eggs female 250815 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), 13/6/03 3:35 pm Page 824 JOURNAL OF PLANKTON RESEARCH I (BANK) Larval abundance (m -2 ) Biomass (mg C m -3 ) 4 2 PAGES 6 4 2 1000 100 Cephalothorax length (µm) 10000 Cephalothorax length (µm) 1.0 0.5 0.0 56 II) 6 Larval abundance (m -2 ) Biomass (mg C m -3 ) 4 2 10000 1000 0 100 2.0 55 Cephalothorax length (µm) 54 53 55 54 52 3.1 7.8 8 III) ‒ 0 10000 1000 0 100 I) 8 6 II) NUMBER 17 16 8 Biomass (mg C m -3 ) VOLUME I. (BAY) 29 1.0 0.5 0.0 4 2 1000 100 0 Cephalothorax length (µm) IV) 56 2.8 1.0 0.5 0.0 8 56 IV) 6 55 54 1.1 4 Larval abundance (m -2 ) Biomass (mg C m -3 ) 57 III) 6 Larval abundance (m -2 ) Biomass (mg C m -3 ) 8 2 10000 1000 0 100 250815 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 0.0 54 53 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 13/6/03 3:35 pm Page 825 P. MUNK ET AL. 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) 70 69 68 Latitude (N) 250815 67 66 65 64 -58 -57 -56 -55 -54 -53 -52 -51 -50 -58 -57 -56 -55 -54 -53 -52 -51 -50 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). 13/6/03 3:35 pm Page 826 JOURNAL OF PLANKTON RESEARCH VOLUME NUMBER 60 PAGES ‒ a) 3 Boreogadus saida 55 4 Liparis liparis Lumpenus maculatus 50 Lumpenus lampretaeformis 45 Temperature ( o C) Triglops pingelii Aspidophoroides olrikii 40 Leptagonus decagonus Reinharditus hippoglossoides 35 1 2 Stichaeus punctatus Mean length (mm) Hippoglossoides platessoides Ammodytes sp. 30 25 2 1 5 0 -1 20 3 15 -2 10 65 66 67 68 33 69 34 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) 250815 2 Polar Current 100 Mixing zone 150 1 NorthAtlantic/ Labrador Current 200 57 56 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 250815 13/6/03 3:35 pm Page 827 P. MUNK ET AL. 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; 250815 13/6/03 3:35 pm Page 828 JOURNAL OF PLANKTON RESEARCH VOLUME 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 NUMBER PAGES ‒ 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. 250815 13/6/03 3:35 pm Page 829 P. MUNK ET AL. PLANKTON AND FISH LARVAE OFF W. GREENLAND 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). REFERENCES Andriashev, A. P. (1986) Agonidae. In Whitehead, P. J. P., Bauchot, M.-L., Hureau, J.-C., Nielsen, J. and Tortonese, E. (eds), Fishes of the North-eastern Atlantic and the Mediterranean. UNESCO, Paris, Vol. 3, pp. 1266–1268. Berggreen, U. C., Hansen, B. and Kiørboe, T. (1988) Food size spectra, ingestion and growth of the copepod Acartia tonsa during development: implications for the determination of copepod production. Mar. Biol., 99, 341–352. Buch, E. (1990) A monograph on the physical environment of Greenland waters. Greenland Fisheries Research Institute Report, 405 pp. Cohen, D. M., Inada, T., Iwamoto, T. and Scialabba, N. (1990) Gadiform fishes of the world (Order Gadiformes). FAO Fish. Synop. 125, 10, pp. 1–442. Cowen, R. K., Hare, J. A. and Fahay, M. P. (1993) Beyond hydrography—can physical processes explain larval fish assemblages within the Middle Atlantic Bight. Bull. Mar. Sci., 53, 567–587. Diel, S. and Tande, K. S. (1992) Does the spawning of Calanus finmarchicus in high latitudes follow a reproduceable pattern? Mar. Biol., 113, 21–31. Edler, L. (1979) Recommendations for marine biological studies in the Baltic Sea. Phytoplankton and chlorophyll. Baltic Mar. Biol. Publ., 5, 1–38. Eschmeyer, W. N., Herald, E. S. and Hammann, H. (1983) A Field Guide to Pacific Coast Fishes of North America. Houghton Mifflin Company, Boston, pp. 1–336. Franks, P. J. S. (1992) Sink or swim—accumulation of biomass at fronts. Mar. Ecol. Prog. Ser., 82, 1–12. Franks, P. J. S. (1997) New models for the exploration of biological processes at fronts. ICES J. Mar. Sci., 54, 161–167. Govoni, C. B. and Grimes, C. B. (1992) The surface accumulation of larval fishes by hydrodynamic convergence within the Mississippi river plume front. Cont. Shelf Res., 12, 1265–1276. Govoni, J. J. and Spach, H. L. (1999) Exchange and flux of larval fishes across the western Gulf Stream front south of Cape Hatteras, USA, in winter. Fish. Oceanogr., 8(Suppl. 2), 77–92. Grasshof, K. (1976) Methods for Sea Water Analysis. Weinheim, New York, 600 pp. Hansen, B. W., Nielsen, T. G. and Levinsen, H. (1999) Plankton community structure and carbon cycling on the western coast of Greenland during the stratified summer situation. III. Mesozooplankton. Aquat. Microb. Ecol., 16, 233–249. Hassel, A. (1984) Seasonal changes in zooplankton composition in the Barents Sea, with special attention to Calanus spp. (Copepoda). J. Plankton Res., 8, 329–339. 250815 13/6/03 3:35 pm Page 830 JOURNAL OF PLANKTON RESEARCH VOLUME NUMBER PAGES ‒ Heburn, G. W. and Johnson, C. D. (1995) Simulations of the mesoscale circulations of the Greenland–Iceland–Norwegian seas. J. Geophys. Res. Oceans, 100, 4921–4941. The significance of food web structure for the condition and tracer lipid content of juvenile snail fish (Pisces: Liparis spp.) along 65–72°N off West Greenland. J. Plankton Res., 21, 1593–1611. Hirche, H. J. (1990) Egg production of Calanus finmarchicus at low temperature. Mar. Biol., 106, 53–58. Pedersen, S. A. (1998) Distribution and lipid composition of Pandalus shrimp larvae in relation to hydrography in west Greenland waters. J. Northw. Atl. Fish. Sci., 24, 39–60. Hirche, H. J. (1991) Distribution of dominant calanoid copepod species in the Greenland Sea during late fall. Polar Biol., 11, 351–362. Hirche, H. J. and Mumm, N. (1992) Distribution of dominant copepods in the Nansen Basin, Arctic Ocean, in summer. Deep-Sea Res., 39(Suppl. 2) 485–505. Jashnov, W. A. (1970) Distribution of Calanus species in the seas of the northern hemisphere. Int. Rev. Ges. Hydrobiol. Hydrogr., 55, 197–212. Klein Breteler, W. C. M., Fransz, H. G. and Gonzales, S. R. (1982) Growth and development of four calanoid copepod species under experimental and natural condition. Neth. J. Sea Res., 16, 195–207. Leis, J. M. (1982) Nearshore distributional gradients of larval fish (15 taxa) and planktonic crustaceans (6 taxa) in Hawaii. Mar. Biol., 72, 89–97. Levinsen, H. and Nielsen, T. G. (2002) The trophic role of marine pelagic ciliates and heterotrophic dinoflagellates in arctic and temperate coastal ecosystems: a cross-latitude comparison. Limnol. Oceanogr., 47, 227–439. Levinsen, H., Nielsen, T. G. and Hansen, B. W. (2000) Annual succession of marine pelagic protozoans in the arctic with emphasis on winter dynamics. Mar. Ecol. Prog. Ser., 206, 119–134. Madsen, S. D., Nielsen, T. G. and Hansen, B. W. (2001) Annual population development and production by Calanus finmarechicus, C. glacialis and C. hyperboreus in Disko Bay, western Greenland. Mar. Biol., 139, 75–93. Møller, E. F. and Nielsen, T. G. (2000) Plankton community structure and carbon cycling off the western coast of Greenland, with emphasis on sources of DOM for the bacterial community. Aquat. Microb. Ecol., 22, 13–25. Munk, P. (1997) Prey size spectra and prey availability of larval and small juvenile cod. J. Fish Biol., 51(Suppl. A), 340–351. Munk, P. and Nielsen, T. G. (1994) Trophodynamics of the plankton community at Dogger Bank: predatory impact by larval fish. J. Plankton Res., 16, 1225–1245. Munk, P., Larsson, P. O., Danielsen, D. and Moksness, E. (1995) Larval and small juvenile cod Gadus morhua concentrated in the highly productive areas of a shelf break front. Mar. Ecol. Prog. Ser., 125, 21–30. Narayanan, S., Colbourne, E. B. and Fitzpatrick, C. (1991) Frontal oscillations on the NE Newfoundland shelf. Atmos. Ocean, 29, 547–562. Newton, G. M. (1996) Estuarine ichthyoplankton ecology in relation to hydrology and zooplankton dynamics in a salt-wedge estuary. Mar. Freshwater Res., 47, 99–111. Pedersen, S. A. and Smidt, E. L. B. (2000) Zooplankton distribution and abundance in west Greenland waters, 1950–1984. J. Northw. Atl. Fish. Sci., 26, 45–102. Plourde, S. and Runge, J. A. (1993) Reproduction of the planktonic copepod Calanus finmarchicus in the lower St. Lawrence Estuary: relation to the cycle of phytoplankton production and evidence for a Calanus pump. Mar. Ecol. Prog. Ser., 102, 217–227. Poulsen, L. K. and Reuss, N. (2002) The plankton community on Sukkertop and Fylla Bank off West Greenland during a spring bloom and a post-bloom period. Hydrography, phytoplankton and protistplankton. Ophelia, 56, 69–85. Sabatini, M. and Kiørboe, T. (1994) Egg production, growth and development of the cyclopoid copepod Oithona similis. J. Plankton Res., 16, 1329–1351. Saito, K. and Taniguchi, A. (1978) Phytoplankton communities in the Bering Sea and adjacent seas. II Spring and summer communities in seasonal ice-covered areas. Astarte, 11, 27–35. Smith, K. A. and Suthers, I. M. (1999) Displacement of diverse ichthyoplankton assemblages by a coastal upwelling event on the Sydney shelf. Mar. Ecol. Prog. Ser., 176, 49–62. Smith, S. L., Lane, P. V. Z. and Schwarting, E. M. (1986) Zooplankton data report: the marginal ice zone experiment MITEX, 1984. Brookhaven National Laboratory, Upton, NY. Strickland, J. D. H. and Parsons, T. R. (1972) A Practical Handbook of Seawater Analysis. Bull. Fish. Res. Board Can., 167, 1–310. Thorrold, S. R. and Williams, D. M. (1996) Meso-scale distribution patterns of larval and pelagic juvenile fishes in the central Great Barrier Reef lagoon. Mar. Ecol. Prog. Ser., 145, 17–31. Utermöhl, H. (1958) Zur Vervollkommung der quantitativen Phytoplanktonmethodik. Mitt. Int. Ver. Limnol., 9, 1–38. Vernet, M. (1991) Phytoplankton dynamics in the Barents Sea estimated from chlorophyll budget models. Polar Res., 10, 129–145. von Qillfeldt, C. H. (1996) Ice algae and phytoplankton in North Norwegian and Arctic waters. Species composition, succession and distribution. PhD thesis, University of Tromsö, Norway. Werner, F. E., Page, F. H., Lynch, D. R., Loder, R. G., Lough, R. G., Perry, R. I., Greenberg, D. A. and Sinclair, M. M. (1993) Influences of mean advection and simple behaviour on the distribution of cod and haddock early life stages on Georges Bank. Fish. Oceanogr., 2, 43–64. Nielsen, T. G. and Hansen, B. (1995) Plankton community structure and carbon cycling in Arctic West Greenland during and after the sedimenting of a diatom bloom. Mar. Ecol. Prog. Ser., 125, 239–257. Werner, F. E., MacKenzie, B. R., Perry, R. I., Lough, R. G., Naimie, C. E., Blanton, B. O. and Quinlan, J. A. (2001) Larval trophodynamics, turbulence, and drift on Georges Bank: a sensitivity analysis of cod and haddock. Sci. Mar., 65, 99–115. Pedersen, L., Jensen, H. M., Burmeister, A. D. and Hansen, B. W. (1999) Received on June 22, 2002; accepted on March 28, 2003