Journal of Plankton Research Vol.21 no.7 pp.1249–1264, 1999

Spatial distribution and life-cycle timing of zooplankton in the marginal ice zone of the Barents Sea during the summer melt season in 1995 Stig Falk-Petersen, Gunnar Pedersen, Slawek Kwasniewski1, Else Nøst Hegseth2 and Haakon Hop Norwegian Polar Institute, N-9296 Tromsø, Norway, 1Institute of Oceanology, Polish Academy of Sciences, Powstancow Warszawy St 55, 81-712 Sopot, Poland and 2Norwegian College of Fishery Science, University of Tromsø, N-9037 Tromsø, Norway Abstract. The marginal ice zone (MIZ) in the northern Barents Sea is ecologically important because it represents a highly productive area in Arctic water masses north of the Polar Front. During a multidisciplinary cruise in 1995, ecological and oceanographic processes were investigated at four stations located in a north–south transect in the MIZ. This study was carried out in Arctic water masses north of the Polar Front where ice conditions varied from dense first-year pack ice to open water. Also, the phytoplankton development varied along the transect from a pre-bloom situation at the northernmost station to a post-bloom situation in the open water. This paper includes a study of the zooplankton community and population structure of some of the dominant copepod species. Numerically, the most important mesozooplankton components were the copepods Calanus glacialis, Pseudocalanus minutus and Oithona similis. Copepods of Atlantic origin, such as Calanus finmarchicus and Oithona atlantica, gave evidence of an advection of Atlantic water masses into the area. It is concluded that the occurrence of new cohorts of Arctic copepods coincides with the onset of the phytoplankton bloom in the MIZ, and, that therefore, the spawning relies on stored energy.

Introduction Marginal ice zones (MIZ), regions of major importance for biogenic production in high latitudes (Legendre et al., 1992), are some of the most dynamic areas in the world’s oceans. Spatial and temporal scales of the phenomena involved are many. For example, the location of the ice edge during summer in the Barents Sea can vary by hundreds of kilometres from year to year (Gloersen et al., 1992). On another scale, ice conditions vary within the ice zone. In the outer part of the ice margin, wave effects produce small ice floes of 10–50 m in diameter. Further inward from the active wave zone, the wave energy is rapidly dissipated and only long waves penetrate. Accordingly, the floe size increases from several hundred metres to several kilometres (Vinje and Kvambekk, 1991). The MIZ in the northern Barents Sea is important ecologically because it represents a highly productive area in Arctic water masses north of the Polar Front (Hegseth, 1992; Dayton et al., 1994; Slagstad and Stokke, 1994; Loeng et al., 1995). The phytoplankton blooms follow the receding ice edge as it melts during the spring and summer (Sakshaug and Slagstad, 1991), and intensive blooms also occur in leads as the MIZ opens up (Zenkevitch, 1963; personal observations). The onset of primary production is directly related to the availability of light, which is controlled by oscillation of the incident light in the northern hemisphere, and the melting of sea ice (Sakshaug and Slagstad, 1991). This intense production is grazed by herbivorous zooplankton and ice fauna, and biosynthesized with high © Oxford University Press

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efficiency into lipid stores (Sargent and Henderson, 1986; Falk-Petersen et al., 1987, 1997). Information about the zooplankton community inhabiting the part of the Barents Sea north of the Polar Front is limited to data on the genus Calanus (Eilertsen et al., 1989; Hansen et al., 1990). In the waters south of the Polar Front, the most important zooplankton components are herbivorous copepods, first of all of the genera Calanus (including C.finmarchicus, C.glacialis and C.hyperboreus) and Pseudocalanus (including P.minutus and P.acuspes), together with other copepods such as Metridia longa, Microcalanus pusillus, Microcalanus pygmaeus, Pareuchaeta norvegica, Pareuchaeta glacialis and Oithona similis (Loeng, 1989). The representatives of the genus Calanus constitute up to 90% of the zooplankton biomass in summer (Jashnov, 1939; Hassel, 1986). Among other organisms that contribute to zooplankton abundance and biomass are first of all euphausids (Thysanoessa inermis, Thysanoessa longicaudata, Thysanoessa raschii), but there are also medusae and siphonophores (Aglantha digitale, Sarsia princeps, Aeginopsis laurentii, Dimophyes arctica), ctenophores (Beroë cucumis, Martensia ovum), amphipods (Themisto abyssorum, Themisto libellula), chaetognaths (Sagitta elegans) and appendicularians (Oikopleura sp.) (Loeng, 1989; Dalpadado and Skjoldal, 1991). However, the roles of particular components of the zooplankton community, which change both in space and time, are not satisfactorily understood. Because of the importance of the MIZ, the Norwegian Polar Institute initiated an international, multidisciplinary, research programme (ICE-BAR) on the ecological and oceanographic processes in the MIZ of the northern Barents Sea. During the melting period in June 1995, a research cruise with R/V ‘Lance’ was performed to the northern Barents Sea. The cruise covered a transect from very dense first-year ice to open water. In this paper, we describe the zooplankton community and the population structure of some of the dominant copepods along this transect, together with a general overview of the basic environmental parameters. Method During the 1995 cruise, three ice stations and one open-water station were established in an area east of Svalbard (Figure 1). At the ice stations, the ship was anchored to ice floes, while at the open water station the ship was adrift. All sampling of the water column (CTD casts, chlorophyll, phytoplankton and zooplankton) was carried out from the ship deck. Detailed information about the cruise is given in Falk-Petersen and Hop (1996), and details on hydrographic measurements are given in Orvik and Kuznetsov (1996). Zooplankton sampling Zooplankton were sampled at four locations along a north–south transect close to 35°E in the Storbanken area (Table I). The sampling was performed with a WP-2 net (60 cm opening diameter, 180 µm mesh size; UNESCO, 1979), towed 1250

Zooplankton in the marginal ice zone of Barents Sea

Fig. 1. Map of the area with the locations of sampling stations. The average position of the Polar Front is indicated with a thick, grey line. Average ice concentrations are indicated (10/10 ice cover = solid pack ice).

vertically from 100 m depth to the surface. Two replicates were taken at each station. The zooplankton samples were preserved in 4% formalin solution buffered with borax. A bactericide, 1.2-propandiol (5% by volume), was added to the zooplankton preservative. Counting and sorting of the samples were carried out at the Institute of Oceanology, Polish Academy of Science, following standard procedures (e.g. Richter, 1994). Small-size zooplankters (most of Copepoda, Cirripedia, juvenile stages of Pteropoda, Euphausiacea, Amphipoda and Chaetognatha) were identified and counted in subsamples obtained from the fixed sample volume by an automatic pipette. Large zooplankters (big Copepoda, Euphausiacea, Amphipoda, Decapoda, Chaetognatha, Appendicularia and Pisces) were sorted out and identified from the whole sample. Calanus species were distinguished on the basis of prosome length (Unstad and Tande, 1991). Pseudocalanus species were distinguished on the basis of their morphological features as described by Frost (1987). A Zeiss stereomicroscope was used for routine identification and counting, and a compound microscope with more powerful magnification was also employed for detailed studies of taxonomic features. 1251

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Table I. Position and time of zooplankton sampling during ICE-BAR cruise, 1995 Station no.

Date (1995)

Time (CET)

Position

Depth (m)

Ice Station 1 Ice Station 2 Ice Station 3 Open Water Station

17 June 20 June 23 June 24 June

13:00 15:00 09:00 16:00

78°159N, 34°079E 77°499N, 34°389E 77°379N, 34°479E 76°569N, 34°159E

190 170 120 150

The net was not equipped with a flowmeter. Therefore, the amount of water volume filtered, used for the estimation of zooplankton abundance, has been calculated from the wire length and an assumed (theoretical) 90% filtration efficiency of the WP-2 net (UNESCO, 1979). The numbers of zooplankters at each station are mean values from the two replicate hauls which had been analysed for composition and abundance independently. Hydrography and chlorophyll sampling Hydrographic data were obtained with an OTS-1500 CTD sonde (Meerestechnik-elektronik GmbH). Pre-cruise calibration was carried out at the Geophysical Institute, University of Bergen. The CTD sonde was further calibrated against 25 water samples taken during the cruise with a Niskin water bottle and analysed for salinity by using the Portosal 8410A salinometer (Orvik and Kuznetsov, 1996). CTD casts were taken every 3 h at each station. Chlorophyll measurements were performed at each station in the upper 100 m from standard depths (0–5–10–20–30–50–100 m). Samples were taken with Niskin water bottles. Chlorophyll a was measured fluorometrically (Holm-Hansen et al., 1965) in a Turner Design fluorometer, using GF/C filters and methanol as extracting solvent. Cells were identified and counted using an inverted microscope technique. Results Abiotic factors and algal succession On a large scale, water currents and masses in the northern Barents Sea derive from north-east bound activity of the Norwegian Atlantic Current and southwest flow from the Arctic Ocean. Atlantic water from the Norwegian Atlantic Current enters the Barents Sea in an eastward direction along the southern edge of the Bear Island Trough. Part of this flow turns north-eastward into the Hopen trench, west of the Central Bank, where it submerges under lighter Arctic water. A minor inflow of Atlantic water also takes place through the Storfjord trench. The Atlantic water is characterized by temperatures above 2°C and salinity above 35 p.s.u. Arctic water is intruding into the Barents Sea, first of all between Spitsbergen and Franz Josef Land, and moves south-westward and westward (Loeng, 1989). The Arctic water at intermediate depths (between 20 and 150 m) is characterized by temperatures close to the freezing point (below –1.5°C) and salinities between 34.4 and 34.6 p.s.u. The transition zone between the Atlantic 1252

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and Arctic water masses is identified as the Polar Front. It follows the isobath separating the main features of the bottom topography, roughly at 250 m depth, because of the controlling influence of topography on steering the sea currents. A striking feature of the hydrography in the MIZ in spring/early summer is a 20m-thick layer of melt water due to the melting of first-year ice (Orvik and Kuznetsov, 1996). The present study was carried out well north of the Polar Front, in Arctic water masses, during the melting season. The ice conditions varied from dense first-year pack ice with large ice floes of ice concentration 8/10–9/10 at Ice Station 1, through first-year ice with small floes of ice concentration 8/10 and 6/10 at Ice Stations 2 and 3, respectively, to no ice at the Open Water Station (Figure 1). Ice Station 1 was situated in an area of dense first-year ice. The ship was anchored to a large ice floe of size 10 3 5 km. Ice cores showed that the ice thickness varied from 1.1 to 1.6 m. Profiles of water temperature and salinity indicated spring time conditions. A weak pycnocline at 15–20 m depth, resulting from the halocline produced due to dilution of the surface layer, gave evidence for ongoing ice melting (Figure 2A). Below, at intermediate depths, the water mass had properties related to winter convection with temperatures down to –1.8°C and salinities of 34.2–34.4 p.s.u. Under the seasonal thermocline, the water mass showed an increase in temperature up to 0.2°C at 140 m. The chlorophyll profile showed that the phytoplankton biomass was concentrated in the upper 20 m water layer (Figure 2C). The low chlorophyll values (maximum 0.53 µg l–1) and a plankton population dominated by a small dinoflagellate, Heterocapsa rotundata, along with other small flagellates, indicated a pre-bloom phase. Large amounts of nutrients supported this notion, although some silicate consumption could be observed, probably because of the ice diatoms which were abundant in the area (Falk-Petersen et al., 1998). Ice Station 2 was situated in thick first-year hummock ice, with an ice concentration of 7/10. The ice floes were much smaller than those at the former station, but the ice cores showed that the ice thickness was still remarkable, between 1.35 and 5 m. Water temperature and salinity profiles were similar to these recorded at Ice Station 1 (Figure 2A and B), except for the presence of a 3-m-thick top layer of slightly warmer (–1.4°C) and less saline (33.6 p.s.u.) water. The phytoplankton biomass was concentrated in the upper layer as on Ice Station 1, and the maximum chlorophyll value was still low (0.86 µg l–1) (Figure 2C). A dominance of the early spring diatom, Fragilariopsis oceanica, in the upper layer indicated a bloom in its early phase. Ice Station 3 was located in an area of first-year ice, with an ice concentration of 6/10, where small floes up to 50 m in diameter prevailed. Some of the ice floes were strongly affected by melting processes, so that the ice could by characterized as rotten. The station was located close to the ice edge, and the swells were penetrating from the open water. The layer of the melt water with salinity below 33.4 p.s.u. was prominent and reached down to 20 m (Figure 2A and B). Some modifications in the location of the seasonal thermocline occurred, probably as a result of an eddy system or wind forcing due to the proximity of the ice edge. The phytoplankton biomass distribution showed a small peak at 15 m (with 1253

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Fig. 2. Temperature (°C) (A), salinity (p.s.u.) (B) and chlorophyll a concentration (µg l–1) (C) in the water column at the sampling stations.

chlorophyll concentration 1.87 µg l–1), indicating a bloom just about to culminate and sink out of the euphotic zone (Figure 2C). This picture was further confirmed by the species composition, showing a phytoplankton community dominated by typical spring species such as Thalassiosira antarctica var. borealis and 1254

Zooplankton in the marginal ice zone of Barents Sea

Chaetoceros socialis, and an element of the chrysophyte, Dinobryon balticum, which is a typical summer species in the Barents Sea. Nutrients in the upper 20 m were low and silicate totally consumed. The Open Water Station was located a few hundred metres from the ice edge. The melt water was considerably warmer than inside the MIZ, with the temperature reaching 0.7°C at the surface. Thermocline and halocline coincided at ~10 m depth, which differed from the situation at the ice-covered stations (Figure 2A and B). The water below the transition layer was perfectly mixed and showed properties equivalent to those of water produced by winter convection, with temperatures down to –1.8°C (Orvik and Kuznetsov, 1996). The phytoplankton situation was clearly at a post-bloom stage with a well-developed chlorophyll maximum at 35–50 m depth (2–2.41 µg l–1) (Figure 2C), which is typical of a summer situation in large parts of the Barents Sea (Båmstedt et al., 1991). The chlorophyll maximum was dominated by Phaeocystis pouchetii and the diatoms T.antarctica var. borealis, Thalassiosira gravida and C.socialis, whereas D.balticum dominated in the upper 30 m. Zooplankton Abundance. Nearly 30 species were identified in the zooplankton samples (Table II). Copepods were the dominating group, ranging from 93 to 99% in abundance (Figure 3). Amphipods, chaetognaths and gastropods were also numerous. Of the Calanus species, C.glacialis was the most numerous with >22 000 individuals (ind.) m–2 at the Open Water Station, followed by C.finmarchicus with 2700 ind. m–2 at Ice Station 1 and C.hyperboreus with ~1000 ind. m–2 at the Open Water Station (Figure 4).

Fig. 3. Proportions of main zooplankton components at sampling stations.

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Table II. Abundance of zooplankton categories (ind. m–2) at sampling stations Zooplankton category

ICE 1

Calanus finmarchicus, male C.finmarchicus, female C.finmarchicus CV C.finmarchicus CIV C.finmarchicus CIII Calanus glacialis, male C.glacialis, female C.glacialis CV C.glacialis CIV C.glacialis CIII C.glacialis CII C.glacialis CI Calanus hyperboreus, female C.hyperboreus CV C.hyperboreus CIV C.hyperboreus CIII C.hyperboreus CII C.hyperboreus CI Metridia longa, male M.longa, female M.longa CV M.longa CIV M.longa CIII M.longa CII M.longa CI Pareuchaeta glacialis, male P.glacialis, female Pareuchaeta spp. CV Pareuchaeta spp. CIV Pareuchaeta spp. CII Pseudocalanus spp., male Pseudocalanus minutus, female P.minutus CV P.minutus CIV Pseudocalanus acuspes, female P.acuspes CV Pseudocalanus spp. CIII Pseudocalanus spp. CII Pseudocalanus spp. CI Microcalanus spp. Bradyidius similis, female Calanoida indet. Oithona atlantica Oithona similis Oncaea borealis Harpacticoida Copepoda nauplii Themisto abyssorum Themisto libellula Apherusa glacialis Thysanoessa inermis Thysanoessa longicaudata Euphausiacean larvae Decapoda larvae Cirripedia cypris Isopoda

9 181 666 1747 111 11 204 544 1747 774

1256

ICE 2

ICE 3

4 304 265 338 6

16 540 699 593

340 2319 3930

11 96 113 106 5

987 78 153 128 10

11 162 93 40 43 13

61 554 174 70

2 4

2 2 2

4 1557 5387 3291 372 3379 78 170 104 18 354 115 62 655 168 53

Open water 396 478 4 189 3508 53 203 5543 13 225 51 53 19 646 261 119 150 305 75 4

2 7 122 1487 863 288

7 5 2

2 283 1059 291 45 117 54 31 810 757 441

626 2099 1838 97

31 678 140 57 3265

37 509 18 106 28 043

11 2

5

23

2 7

2 2

44 92 2 4 5352 7 224

142 1575 195 27 513

389 2393 159 405 475 6 1899 202 53 356 2 5 44 8669 62 53 36 713 2 87 4 4 9

7 7

15

12

Zooplankton in the marginal ice zone of Barents Sea

Table II. continued Zooplankton category Aglantha digitale Aeginopsis laurentii Sarsia spp. Hydromedusae indet. Hydromedusae larvae Dimophyes arctica Ctenophora fragments Polychaeta indet. Clione limacina adult C.limacina larvae Limacina helicina adults L.helicina larvae Sagitta elegans Eukrohnia hamata Oikopleura spp. Pisces larvae Total

ICE 1

ICE 2

ICE 3

2 9 28

2 4

12

46

11 12 4 62 4 4 2 2 357 39 3030 103 2 48 2

4 2 216 23 39 115 5 30

166 35 174 127 16 30

15 705

49 378

94 128

Open water 27 7 5 4 67 2 58 41 274 117 7 180 78 064

Metridia longa was the most numerous (~900 ind. m–2) at Ice Stations 2 and 3. Also numerous were Pseudocalanus and O.similis with high numbers of nearly 7000 and 38 000 ind. m–2, respectively, recorded at Ice Station 3. Pareuchaeta glacialis was recorded in low numbers and only in ice-covered waters, whereas Bradyidius similis was found only at the Open Water Station. A few individuals of Microcalanus, Oithona atlantica and Oncaea borealis, as well as Harpacticoida, were also present, but were not sampled representatively in our survey. Copepod nauplii were recorded in very high numbers (37 000 ind. m–2) at Ice Station 3 and at the Open Water Station. Of the non-copepod zooplankton, Themisto libellula was recorded at all stations, but was the most numerous at the Open Water Station (~90 ind. m–2). The two pteropods, Clione limacina and Limacina helicina, were also found at all stations, with L.helicina being the most numerous. Limacina helicina veliger larvae were especially abundant at Ice Station 3 with 3000 ind. m–2. Of the hydromedusae, Aeginopsis laurentii and Sarsia spp. were the most abundant at all stations. Hydromedusae larvae were also recorded on all stations except for the Open Water Stations. Sagitta elegance and Oikopleura spp. were recorded at all stations. Population structure of selected copepods Calanus finmarchicus was most numerous at Ice Station 1 and the numbers recorded were considerably lower at the outer stations (Figure 4). At Ice Station 1, copepodite stages III, IV, V and females (F) were recorded, with CV being the most abundant stage. Only the older copepodite stages (IV, V, F) were recorded at Ice Station 3 and at the Open Water Station (Figure 5). 1257

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Fig. 4. Distribution of individual Calanus species at sampling stations.

Calanus glacialis was, on average, much more numerous than C.finmarchicus, increasing from ~3300 ind. m–2 at Ice Station 1 to >22 000 ind. m–2 at the Open Water Station (Figure 4). At Ice Station 1, the older stages were totally dominating (Figure 5), whereas at the outer station two cohorts were recorded, with the younger copepodite stages increasing in abundance towards open water. Stages CI and CII were by far the most numerous at the Open Water Station. There was also a marked shift in the structure of the older cohorts, indicated by an increase in the abundance of CV and F, and a decrease in CIII and CIV from Ice Station 1 towards the open water. As for C.glacialis, only the older stages of C.hyperboreus (CIV, CV, F) were recorded at the two northernmost stations, whereas the new generation, consisting of CI–CIII, was the most numerous at Ice Station 3 and the Open Water Station (Figure 5). There was also a shift in the structure of the older cohorts, indicated by a decrease in the relative abundance of CIV and an increase of CV and F towards open water. Metridia longa was recorded in moderate numbers at all stations (Table II). Females were dominating at ice-covered stations, but at the Open Water Station CV was the most numerous. Males were also abundant at all stations, whereas CII and CIII only were recorded on Ice Station 1 and a few CI were present at the Open Water Station (Figure 5). Two Pseudocalanus species were recorded (Table II). Older stages of P.minutus (CIV, CV, F) were by far the most numerous and were found at all stations, whereas older stages of P.acuspes were much less abundant and recorded only at Ice Stations 2 and 3 and at the Open Water Station. The new cohorts were recorded, with very high numbers of CI, CII and CIII, at Ice Station 3 (~4600 ind. m–2) and at the Open Water Station (2200 ind. m–2), but they were absent from 1258

Fig. 5. Demographic structure of selected copepod species at particular sampling stations.

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Ice Station 1. There was also an increase in the relative abundance of females and a decrease in CIV and CV towards open water. Discussion The transect studied during this mid-June cruise, from a station in dense first-year ice to open water, covered a time-series in phytoplankton development from a pre-bloom situation at Ice Station 1, via a clear bloom situation at Ice Station 3 to a post-bloom situation with a deep chlorophyll maximum at the Open Water Station. This is a classic picture of the development of algal production in the MIZ as described by Slagstad and Støle-Hansen (1991), Syvertsen (1991) and Sakshaug et al. (1992). The cohort development of the true Arctic herbivorous copepods, C.glacialis, C.hyperboreus and P.minutus, exhibited a clear gradient along the transect from ice-covered to open-water areas. There was a ‘maturing’ of the older cohort of C.glacialis indicated by an increase in the relative abundance of CV and females towards open water, accompanied by the appearance of the new cohort, the offspring of that year, which was visible at Ice Station 3 and which dominated the population at the Open Water Station. Water temperature was similar at all stations, hence the temperature did not, probably, generate this difference. The food conditions for the zooplankton, however, differed from the ice-covered to the open-water areas. At locations far from the ice edge, the chlorophyll concentrations were low. A small, but pronounced phytoplankton bloom was present in the ice edge zone. The phytoplankton biomass had developed into a deep chlorophyll maximum in the open water outside the ice. If we assume that C.glacialis needs ~70 days for development to CI at a temperature of –1°C (from Belehradek’s function; Corkett et al., 1986), this means that the new cohort observed in the MIZ originated from spawning which had taken place sometime in the second week of April, most likely in the waters covered by ice and prior to the spring activity of primary producers. Tande et al. (1985) and Slagstad and Tande (1990) suggested that the main spawning of C.glacialis in the central Barents Sea occurs in May and June. However, later, Tande (1991) and Pedersen et al. (1995) concluded from their data from the same area that spawning of C.glacialis takes place in the same period as the spawning of its Atlantic congener C.finmarchicus, i.e. in March–April (Hopkins et al., 1984). This corresponds with the observations from Fram Strait (Smith, 1990) and supports the assumption about a 1-year generation time of C.glacialis (MacLellan, 1967; Huntley et al., 1983; Runge et al., 1986; Smith, 1990), and contradicts the opinion of a 2-year generation time (Prygunkova, 1974; Tande et al., 1985; Slagstad and Tande, 1990). However, it is possible that separate populations of C.glacialis, inhabiting different parts of the Barents Sea, adjust their life cycles to local environmental conditions, primarily to the amount of food available. In a study of life strategy and lipid biochemistry of Arctic copepods, (C.Scott, S.Kwasniewski, S.Falk-Petersen, J.R.Millar and J.R.Sargent, unpublished) concluded that C.finmarchicus has a 1 year life cycle, that C.glacialis has a 1–2 year life cycle and C.hyperboreus has a 3–5 year life cycle 1260

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depending on the variation in food availability between years. The results from the present study support this view. Comparison of the data obtained during our study with previous research reveals pronounced variability in abundance of C.glacialis in the Barents Sea. Eilertsen et al. (1989) and Slagstad and Tande (1990), as well as Hassel (1986), report values of abundance of young generation copepodites CI, from northern waters of the Barents Sea, up to six times higher than we found. On the other hand, Pedersen et al. (1995) report densities an order of magnitude higher from almost the same area. However, Unstad and Tande (1991) and Hansen et al. (1990) found, in the central part of the Barents Sea south of the Polar Front, densities of C.glacialis younger stages very close to our estimations. Year-to-year changes in zooplankton abundance within the range of an order of magnitude are well documented for the Barents Sea (Hassel, 1986; Skjoldal et al., 1987). The demographic structure of C.hyperboreus suggests that this species reproduces earlier than C.glacialis in the MIZ in the Barents Sea, which agrees with what is known about C.hyperboreus biology (Smith, 1988; Diel, 1991). The low abundance of this species observed in the MIZ confirms the opinion of Conover (1988), Smith (1988) and Hirche (1991) that C.hyperboreus is an Arctic deepwater species, in contrast to C.glacialis, which appears to be an Arctic shelf water species. Both Pseudocalanus species found in the study area (P.minutus and P.acuspes) also occur in waters south of the Polar Front (Norrbin, 1991), but no data concerning their proportions are available. According to Norrbin (1991) and Koszteyn and Kwasniewski (1992), as well as own observations from the West Spitsbergen fjords, it is hypothesized that an open-sea environment and the presence of Arctic water masses are the reasons for the predominance of P.minutus in the area investigated. The abundance and demographic structure of the new cohort indicate well-developed reproduction of P.minutus, and it is postulated that it was this species which reproduced in the study area north of the Polar Front. There was no evidence for any development of a new cohort of the Atlantic copepod C.finmarchicus. This is in accordance with the study of, for example, Tande et al. (1985). For M.longa, the large number of females and the occurrence of CI at the Open Water Station could indicate the onset of the species’ reproduction in the study area. Spawning of M.longa after the onset of the phytoplankton bloom is in accordance with, for example, Grønvik and Hopkins (1984). The presence of C.finmarchicus, Oithona atlantica and Aglantha digitale, species of Atlantic origin (Jashnov, 1970; Shuvalov, 1980; Conover, 1988; Mumm, 1993), in water which exhibits Arctic temperature and salinity characteristics, suggests that the water mass investigated has passed over an Atlantic water mass, and that these organisms migrated upwards, or that it has an Atlantic origin but that its physical properties have been changed as a result of winter cooling and mixing with true Arctic water. Any of these events happened north or east of the area investigated, since no Atlantic water was found in the vicinity of sampling stations (Orvik and Kuznetsov, 1996), and Arctic water in the area proceeds in a southward to westward direction (Loeng, 1989). This scenario is possible since penetration of Atlantic water into the northern Barents Sea takes place not only 1261

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from between the Svalbard Bank and the Central Bank, but also from north of Spitsbergen, between Victoria Island and Franz Josef Land, as well as from east of our study area, between the Central Bank and Novaya Zemlya (HellandHansen and Nansen, 1909; Loeng, 1991; Løyning and Budgell, 1997). Identical to ours, biological indicators of the presence of Atlantic water north and east of our study area have already been found in those places [Sars, 1905; Bernstein (1932) in Shuvalov and Pavshtiks, 1977; Koszteyn and Kwasniewski, 1992; Mumm, 1993]. Conclusions The MIZ is the area where Arctic, cold water-adapted species predominate in the zooplankton community. During and after ice melting, numerically the most important mesozooplankton species are the copepods C.glacialis, P.minutus and O.similis. Expatriates of Atlantic origin, C.finmarchicus or O.atlantica, present in the MIZ gave evidence for an advection of an Atlantic water mass into the northern Barents Sea from various branches of the West Spitsbergen Current. The occurrence of the new cohorts of Arctic herbivorous copepods, C.glacialis, C.hyperboreus and P.minutus, coincides with the onset of the phytoplankton bloom in the MIZ. A development time of 70 days for C.glacialis indicates that spawning takes place in mid-April. This suggest that spawning relies on stored energy resources (Falk-Petersen et al., 1987). The expatriate of Atlantic origin, Calanus finmarchicus, is out of the temperature range favourable for spawning. Development of Arctic Calanus spp. seems to start out with C.hyperboreus, followed by C.glacialis and then M.longa. Acknowledgements The work is part of the ICE-BAR programme of the Norwegian Polar Institute and was supported by the Norwegian Research Council (project no. 112497/410) and Saga Petroleum (contract no. 9000 000 456). This is contribution no. 345 from the Norwegian Polar Institute. References Båmstedt,U., Eilertsen,H.C., Tande,K.S., Slagstad,D. and Skjoldal,H.R. (1991) Copepod grazing and its potential impact on the phytoplankton development in the Barents Sea. Polar Res., 10, 339–353. Conover,R.J. (1988) Comparative life histories in the genera Calanus and Neocalanus in high latitudes of the northern hemisphere. In Boxhall,G.A. and Schminke,H.K. (eds), Hydrobiologia, 167/168, 127–142. Corkett,C.J., McLaren,I.A. and Sevigny,J.-M. (1986) The rearing of the marine calanoid copepods Calanus finmarchicus (Gunnerus), C. glacialis Jashnov and C. hyperboreus Krøyer with comment on the equiproportional rule. Syllogenus (Natl Mus. Can.), 58, 539–546. Dalpadado,P. and Skjoldal,H.-R. (1991) Distribution and life history of krill from the Barents Sea. Polar Res., 10, 443–460. Dayton,P.K., Morida,B.J. and Bacon,F. (1994) Polar marine communities. Am. Zool., 34, 90–99. Diel,S., (1991) On the life history of dominant copepod species (Calanus finmarchicus, C. glacialis, C. hyperboreus and Metridia longa) in the Fram Strait. Rep. Polar Res., 88, 1–113. Eilertsen,H.C., Tande,K.S. and Taasen,J.P. (1989) Vertical distributions of primary production and

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