ORIGINAL PAPER

Haakon Hop á Michael PoltermannOle Jùrgen Lùnne á Stig Falk-PetersenReinert Korsnes á William Paul Budgell

Ice amphipod distribution relative to ice density and under-icetopography in the northern Barents Sea

Accepted: 27 November 1999

Abstract Arctic ice amphipods are part of the sympagicmacrofauna in the Marginal Ice Zone of the northernBarents Sea and represent an important link from lowerto higher trophic levels in some Arctic marine foodchains. The species diversity in this area (1995/1996)consisted of four species: Gammarus wilkitzkii, Apherusaglacialis, Onisimus nanseni and Onisimus glacialis. Thelarger ice amphipod, G. wilkitzkii, was the mostabundant with the highest biomass (>90%), whereasA. glacialis was abundant, but contributed little to thetotal biomass (<4%). The other two species were foundonly in small numbers. Both abundance and biomass ofice amphipods decreased along a latitudinal gradientfrom north to south across the Marginal Ice Zone. Theirdistribution was also related to the under-ice topographywith regard to mesoscale structures (edge, ¯at area,dome and ridge). Overall, the abundance and biomasson ridges were much higher in comparison to othermesoscale structures, although edges also showed highabundance, but low biomass. The large G. wilkitzkii wasconsistently abundant on ridges. The small A. glacialiswas predominately associated with edges, but alsoshowed high numbers in dome-shaped areas. TheOnisimus species were present in low numbers at allstructures, and their biomass contributed <10% on anyone structure. The reasons for di�erent distributionpatterns of the dominant amphipod species under Arctic

sea ice are probably related to di�erent requirements ofthe species, especially for food, shelter and physiologicalconditions.

Introduction

Arctic sea ice, which covers at most 5% of the northernhemisphere, is a dominant environmental feature of thenorthern Barents Sea (Maykut 1985). The margins of theice pack, referred to as the Marginal Ice Zone (MIZ), arevery dynamic systems with strong inter-annual andseasonal variation in extent and thickness of ice cover.The location of the ice edge during summer in theBarents Sea can vary by hundreds of kilometres fromyear to year (Vinje and Kvambekk 1991; Gloersen et al.1992; Johannessen et al. 1995), and the seasonal varia-tion ranges from almost ice-free conditions in Septemberto complete ice cover south to the Polar Front in March(Sakshaug et al. 1994). This has important implicationsfor the distribution of plankton blooms and ice fauna, aswell as the upper trophic levels, represented by marinemammals and sea birds. The ice serves as habitat for alarge size range of animals from microscopic protozoansto polar bears, distributed on very di�erent spatial scalesfrom square centimetres to square kilometres. Withinthe mesoscale (square metres) of individual ice ¯oes,assemblages of ice algae and ice amphipods are thedominant features of Arctic biodiversity.

Four amphipod species are known as autochthonousice macrofauna in the Arctic: Gammarus wilkitzkii,Apherusa glacialis, Onisimus nanseni and O. glacialis(Melnikov and Kulikov 1980; Lùnne and Gulliksen1989; Pike and Welch 1990; Lùnne 1992; Melnikov 1997;Poltermann 1997). These species use all food resourcesavailable under the sea ice, and also represent an im-portant food for other ice-associated organisms such asthe polar cod, sea birds and seals (Bradstreet 1980;Bradstreet and Cross 1982; Cross 1982; Gjertz andLydersen 1986; Lùnne and Gulliksen 1989; Lùnne and

Polar Biol (2000) 23: 357±367 Ó Springer-Verlag 2000

H. Hop (&) á M. Poltermann á S. Falk-PetersenR. Korsnes á W. P. Budgell1

Norwegian Polar Institute,The Polar Environmental Centre,9296 Tromsù, Norwaye-mail: Haakon.Hop@npolar.no; Fax: +47-77-750501

O. J. LùnneUniversity Courses on Svalbard,9171 Longyearbyen, Norway

Present address:1Institute of Marine Research,Marine Environment Centre,P.O. Box 1870 Nordnes,5024 Bergen, Norway

Gabrielsen 1992; Nilssen et al. 1995; Barrett et al. 1997;Poltermann 1997; Werner 1997a). Ice amphipods aretherefore an important link from lower to higher trophiclevels in Arctic marine food chains.

Several studies of ice-amphipod distribution underArctic sea ice have been conducted (e.g. Cross 1982;Gulliksen and Lùnne 1989; Pike and Welch 1990; Lùnneand Gulliksen 1991a, c; Averintzev 1993; Werner 1997a;Poltermann 1998), but none of these studies quantita-tively investigated the relationship between amphipoddistribution and under-ice topography. The approach ofthis study was therefore to identify typical mesoscaleunder-ice structures and relate the distribution of iceamphipods to these structures. In addition, the interestwas focused on their distribution on a larger scale,across the Marginal Ice Zone of the northern Barentssea. The present multidisciplinary study was part of theinternational ICE-BAR research program, administeredby the Norwegian Polar Institute (Falk-Petersen et al.,in press). The overall goal of this program was to in-crease the understanding of the importance of the MIZfor the productivity and biodiversity in the northernBarents Sea.

Materials and methods

The sampling area in the northern Barents Sea is characterised byin¯ux of cold Arctic water from the north. The currents and icedrift, which bring ice fauna into the northern Barents Sea, enter

between Svalbard and Franz Josef Land, and in addition there is anin¯ux from the east from the Kara Sea (Fig. 1). At the Polar Frontthe cold Arctic water meets warm Atlantic water, which subductsbelow the less saline Arctic water masses. During the winter, themaximum extent of the ice may coincide with the Polar Front.During the summer melt period, when our sampling was per-formed, the ice edge retreats northwards because of melting andthe ice zone may also undergo rapid changes in ice extent becauseof changing wind directions.

Ice cover

Sampling of ice amphipods was done during the period 27 July to8 August 1996, during the peak of the melt season when the icezone underwent rapid changes in ice extent (Fig. 2a, b). The dailymean sea-ice concentration in the Marginal Ice Zone was deter-mined for the start and the middle of the sampling period, on 28July and 3 August 1996. Images of 25-km resolution werecomputed from Special Sensor Microwave Imager (SSM/I) data,from the Defence Meteorological Satellite Program (DMSP) F-13satellite. SSM/I-based sea-ice concentrations were provided by theEarth Observation System (EOS) Distributed Active ArchiveCentre (DAAC) at the National Snow and Ice Data Centre,University of Colorado, USA. The NASA Team Algorithm(Cavalieri et al. 1992) was used in the computation of daily meansea-ice concentrations from brightness temperatures. For moredetailed interpretations of ice concentrations, we used satellite datawith higher resolution: Advanced Very High Resolution Radiom-eter (AVHRR) with 1.1-km resolution, and Synthetic ApertureRadar (SAR) with 16-m resolution.

The consolidated pack ice was located north of Kvitùya andSvalbard during late July/early August 1996. During the start ofour sampling period, drifting ice at 20±40% concentrations waspresent in the northern part of the Barents Sea (Fig. 2a). The iceretreated rapidly northwards and most of the Barents Sea was open

Fig. 1 Map of the Barents Seawith main currents, bottomtopography and the PolarFront. The ice stations sampledin 1996 are marked

358

water during the middle of the sampling period. Ice persistedmainly in the vicinity of Kvitùya and Kong Karsl Land, and atongue of ice extended into the Barents Sea between Kvitùya andFranz Josef Land (Fig. 2b).

Ice stations of 2±3 days sampling duration were selected in solidpack ice (Ice-0 and Ice-1), broken-up ice at intermediate ice con-centration (Ice-2), and near the southern extent of the MIZ (Ice-3).The northernmost ice stations, Ice-0 (81.31°N 33.92°E) and Ice-1(81.29°N 34.36°E), were both in 7/10 ice cover. More detailedimages showed that the pack ice was in an early stage of break-up.A few large ice ¯oes, in the range 200±300 km2, several ¯oes in therange 50±75 km2, and many small ¯oes less than 5 km2 could beidenti®ed. Station Ice-2 (79.30°N 32.79°E) was located south ofKvitùya relatively close to open water. The ice in this area was inan advanced stage of break-up and melting. The more rounded¯oes were relatively small, not exceeding 10±30 km2 in extent.Large areas of open water were present around the concentrated ice®eld where the station was located. Station Ice-3 (78.35°N 25.40°E)was located west of Kong Karls Land, where the ice was more

concentrated due to the proximity to the archipelago. The ice wasin an advanced stage of break-up and melting at this location withmany small ¯oes.

Sampling of ice amphipods was also performed during 14±23June 1995, when 7/10 ice cover in the northern Barents Seaextended as far south as the outer margins of the ice zone in lateJuly 1996 (Fig. 2a). The southern extent of the ice pack was thenat 77°N, and our three ice stations were located close together(77.5±78.1°N). Data from 1995 on relative abundance and biomassof ice amphipods were included for comparison with our 1996 data,to show that consistent patterns in diversity, relative abundanceand biomass were present in the Marginal Ice Zone.

Under-ice topography

The mesoscale topography of the underside of selected ice ¯oes wasmapped by a side-scanning sonar. A 250-m cable linked the sonarto a computer-controlled data logger on board R/V ``Lance''. Thetechnique was used earlier to determine the draft of ice in the FramStrait and the Barents Sea (Kvambekk and Vinje 1992). Themeasuring system included a Mesotech 971 side-scanning sonaroperating at 675 kHz on the tip of a vertical 20-m metal bar in-serted through a hole in the ice. The sonar had a conical beamwidth of 1.7° and provided localised ice-thickness data around thehole by scanning linear pro®les below the ice for each 5° horizontal(Kvambekk 1989). The resolution of the mapped under-ice areawas in the range 0.6±1.2 m, at a scanning distance of 20±40 m. Icedraft was determined from the water surface down, but ice less than1-m thickness was considered negligible in the frequency distribu-tions. This was con®rmed by visual observations from divers.

Both topographical maps and three-dimensional views wereproduced after the data had been processed. Topographical mapswere produced in xy co-ordinate systems. The horizontal extentof each scanned section was determined from these maps. Eachresulting map represented a 100- to 140-m-diameter section of alarger ice ¯oe, including the edge. The scanned areas covered from6620 to 9570 m2, depending on how many circular scans wereperformed.

Visual classi®cation of the underside of sea ice was attemptedbased on a pre-made classi®cation scheme used by divers, photo-graphic techniques and video images taken by a remotely operatedvehicle (ROV). However, complex three-dimensional structureswere di�cult to interpret objectively using these techniques and theside-scanning sonar provided the best topographical descriptions.Initial sonar images could be produced on site and were used bydivers to identify di�erent structures. Four identi®able mesoscalestructures were sampled (Fig. 3):

1. Edge ± the complex borders of the ice ¯oe with melting holesand crevices, and pushed up/down ice pieces resulting fromcollisions between ice ¯oes.

2. Flat area ± the general horizontally smooth under-surface ofthe ice.

3. Dome ± dome-shaped areas characterised by advanced snowmelt often with melt ponds on top that increased light pene-tration.

4. Ridge ± keels protruding down from the ¯at ice under-surfacein the range of several metres.

Fig. 2a±b Daily mean sea-ice concentrations in the Marginal IceZone of the northern Barents Sea, as 25-km resolution imagescomputed from Special Sensor Microwave Imager (SSM/I) data.Sea-ice concentrations are indicated on the coloured scale and theapproximate positions of the ice stations are shown. Images are forthe following dates in 1996: a 28 July, b 3 August

Fig. 3 Mesoscale under-ice structures identi®ed for Arctic pack ice.Ice draft was measured from the sea surface down

359

Sampling and analyses

Ice amphipods were sampled by electrical suction pumps operatedby SCUBA divers (Lùnne 1988). Sampling in 1995 was semi-quantitative, done as 5-min index sampling, without reference tounit area or structures. In 1996, quantitative sampling was carriedout by means of 50 ´ 50 cm standard frames. Samples were takenfrom a set area (2.5 m2) on a single mesoscale structure by placingthese frames ten times (=one replicate sample) from a randomstarting point. When vacuuming animals from inside the frame, thediver did not exhale, to avoid disturbance and loss of organisms byair bubbles. After vacuuming one frame, it was moved about ametre forward to an undisturbed area where the procedure wasrepeated. Four replicate samples from the same structure weretaken by a single diver to avoid repeated sampling of the same area.For safety reasons the maximum diving range under the ice was40 m, measured from a dive site (edge or hole in the ice), whichimplies that a half circle with this radius (2500 m2) was generallyavailable to the diver. To get better coverage of the scanned areas,the sampling procedure was sometimes repeated at two to threedive sites on the same ice ¯oe. Therefore, the total number ofreplicates for each mesoscale structure ranged from 4 to 12.

All ice amphipods taken by suction pumps were preserved in4% bu�ered formaldehyde solution immediately after sampling.Samples (n � 12 in 1995, and n � 114 in 1996) were analysedfor species composition, abundance and biomass. Species weredetermined according to Gurjanova (1951), Holmquist (1965) andPoltermann (1997). The biomass (wet weight) was obtained fromformaldehyde-preserved organisms blotted on ®lter paper andweighed to the nearest milligram.

Statistically signi®cant di�erences in abundance and biomassbetween di�erent ice ¯oes and under-ice structures were determinedby analyses of variance, with the signi®cance level set at P � 0.05.All data were subjected to a Box-Cox transformation to achievenormality and homogeneity in the data (Box and Cox 1964).

Results

Mesoscale ice structures were identi®ed based on thesonar images of the undersides of ice ¯oes (Fig. 4). Themeasured sections of the ice ¯oes (Ice-0, 1, 2, 3) showeddi�erent thickness frequencies. However, all ice ¯oeswere probably multi-year ice, since the mode thicknessof each ¯oe was 2±4 m and the maximum thickness ofridges was 6±9 m. Some of the ¯oes could also be acombination of thick multi-year ice and ®rst-year ice,which had been rafted and frozen in place. The promi-nent ridges and other mesoscale structures were solidand rounded by past melting and freezing. This was verydi�erent from the stacked ice blocks often found underpressure ridges in ®rst-year ice.

The ice ¯oes sampled all had the four identi®ed me-soscale structures in common but they were also di�er-ent, partly because they were in di�erent stages ofmelting, with Ice-2 and 3 being the most advanced(Fig. 4). This was con®rmed by analyses of ice crystal-lography, brine ratio and other physical characteristicsof ice cores taken at each station. Ice-0 was about 3.5 mthick (mode draft). About half of the scanned undersideof the ice ¯oe was nearly ¯at (2±3 m), whereas the otherhalf contained two large ridge structures extending downto 9 m depth. Small domes were present between theseridges. Ice-1 was the thinnest (2.5 m thick mode) and

the most uniform among the surveyed ice ¯oes. Thelower surface was relatively ¯at except for two ridgesnear the edge. Several large domes were present withinthe scanned area. Ice-2 was about 3.5 m thick (mode)and relatively varied. It had a thin part of less than 4 mwith a smaller ridge, and thick part as a ridge extendingdown to 9 m depth. A large dome-shaped area waspresent between these ridges. Ice-3 had a relatively roughunder-ice surface with a mode thickness of about 3.5 m.A large ridge was situated in the centre, and a pointedvertical structure down to 9 m depth was located nearthe edge. The area between these structures was identi-®ed as a dome.

Species diversity of ice amphipods in the samplingarea was low, both in 1995 (n � 1869 individuals) and1996 (n � 16,784 individuals), consisting of 4 species:G. wilkitzkii, A. glacialis, O. nanseni and O. glacialis. Thelarger ice amphipod species, G. wilkitzkii, was the mostabundant with the highest biomass (Fig. 5). The relativebiomass of G. wilkitzkii was similar in 1995 and 1996; itsrelative abundance in 1995 was about 20% lower thanin 1996. Of the three smaller amphipod species, onlyA. glacialis was abundant, but its contribution to thetotal biomass was relatively small. In 1995, the relativeabundance of this species was about 16% higher than in1996, but the relative biomass was nearly the same. TheOnisimus species (O. nanseni and O. glacialis) were onlyfound in small numbers in both years and contributedless than 5% to the total biomass.

Ice amphipod abundance and biomass varied amongthe di�erent ice ¯oes and mesoscale structures. Bothabundance and biomass were related to the large-scaleice distribution in the Marginal Ice Zone, sampledalong a latitudinal gradient. The highest relative valueswere at the northernmost stations close to the polarpack ice, and both abundance and biomass decreasedtowards the southern extent of the MIZ (Fig. 6a). Themean total abundance of ice amphipods for all stationscombined ranged from 16 (dome) to 110 ind. m)2

(ridge) and the biomass from 0.35 (edge) to 4.16 g m)2

(ridge) (Table 1). The maximum number found in sin-gle samples was 518 ind. m)2 with a maximum biomassof 15.22 g m)2. The mean abundance and biomass forall amphipod samples were 59 ind. m)2 and 1.96 g m)2,respectively.

Ridges and edges generally showed the highest am-phipod densities of the four investigated under-icestructures (Fig. 6b). Overall, the abundance and bio-mass on ridges were much higher than those of othermesoscale structures. G. wilkitzkii was consistentlyabundant on ridges, although it could be relativelyabundant on other structures (Fig. 7). At Ice-0 thisspecies was equally abundant in ridges and domes; atIce-2 it was equally abundant in ridges and ¯ats, whereasat Ice-3 it was about equally abundant in ¯ats, domesand ridges. A. glacialis was predominantly associatedwith edges, but also showed high numbers in dome-shaped areas (Ice-1, Ice-2). The biomass of ice amphi-pods was, at nearly all structures, dominated by

360

G. wilkitzkii, except at the edges where A. glacialisgenerally had the highest biomass. The Onisimus specieswere present in low numbers at all structures, and theirbiomass contributed less than 10% on any one structure(Fig. 7). They were, therefore, not included in the sig-ni®cance matrix (Tables 2, 3).

The analysis of variance for di�erent under-icestructures (Table 2) showed signi®cant di�erences(P � 0.05) in the occupation of identi®ed mesoscale

structures. The abundance of G. wilkitzkii on ridges wasin most cases di�erent from that of other structures. Thesame was valid for the abundance of A. glacialis at ¯oeedges. Edges, ridges and ¯at areas from di�erent icestations showed no signi®cant di�erences in amphipodabundance (except ridges Ice-0/Ice-2 for A. glacialis)(Table 3). The observed di�erences in abundance forG. wilkitzkii and A. glacialis between ice stations weremainly attributed to domes.

Fig. 4 Ice bottom topographywith corresponding ice draftfrequencies (%) of sections ofice ¯oes in the Marginal IceZone of the northern BarentsSea, 1996. The topography im-ages are from the SCUBA div-er's perspective, showing ridges(green), ¯at areas and domes(yellow to orange). The hori-zontal extent (m2 ) of eachscanned section is indicated,and the horizontal measurementbars apply to the centre ofimages

361

Discussion

Sampling methods and abundance estimates

The sampling methodology has varied widely amongstudies (Table 4). In early studies, ice amphipods weremost often caught by hand-held sweep nets. However,for quantitative sampling such nets could only be usedunder smooth ice, such as fast ice, and their e�ciencyhad to be calibrated (Pike and Welch 1990; Lùnne andGulliksen 1991a). In multi-year sea ice the swept areacould not be accurately determined because of theroughness of the ice under-surface (Lùnne and Gulliksen1991c). The amphipods often stay inside brine channelsand their number determined by sweep nets will there-fore be underestimated. Ice cores have also been used forabundance and biomass estimations (Grainger et al.1985), but larger and motile amphipods will be missedusing this technique. Photography (video and still pho-tos) tends to underestimate small or poorly visible

organisms, and it is often di�cult or impossible toidentify organisms from photographs (Pike and Welch1990; Werner 1997a). However, it was shown that atleast high numbers of A. glacialis can be registered withthis technique (Lùnne and Gulliksen 1991c). After thedevelopment of diver-operated electrical suction pumps(Lùnne 1988), it became possible to get more realisticquantitative samples of ice amphipods. Most of thesesampling e�orts have been based on time index samplingrather than unit area sampling (Table 4). Because of thepatchy distribution of ice amphipods, divers will usuallytend to concentrate general sampling in areas whereorganisms are visible and abundant (which causesoverestimation).

The present study used suction pumps combined withstandard frames in a replicate sampling design. Thissampling technique has previously been used by Lùnneand Gulliksen (1991a, c) to obtain quantitative estimatesand calibrate time index sampling. Our abundance andbiomass estimates are, therefore, comparable to theirstudies.

Diversity and distribution

The four amphipod species ± G. wilkitzkii, A. glacialis,O. nanseni and O. glacialis ± found in this study areautochthonous sympagic species already reported fromother studies in high Arctic areas (e.g. Gulliksen andLùnne 1989; Melnikov 1997; Werner 1997a; Poltermann1998). The complete absence of allochthonous amphi-pod species indicates that all investigated ice ¯oes orig-inated in the open sea and were never part of fast ice orice that had drifted over shallow waters. The presence ofonly four amphipod species in drifting sea ice representsa low diversity for this taxonomic group and shows, inan evolutionary sense, that only a few species have beenable to adapt to such a dynamic and extreme habitat asdrifting Arctic pack ice.

Abundance and biomass estimates of ice-associatedamphipods under fast as well as under pack ice havebeen made in di�erent geographic areas such as theBarents Sea east of Svalbard and near Franz Josef Land,the Greenland Sea including the Fram Strait, the LaptevSea, the central Arctic Ocean and the Canadian Arctic(Table 4). Because of di�erent sampling methods applied,as mentioned earlier, the resulting estimates are of onlylimited comparability. However, the mean abundanceand biomass of ice-associated amphipods found in thisstudy are in the same order of magnitude as those ofother investigations (Bradstreet and Cross 1982; Cross1982; Gulliksen 1984; Werner 1997a). The wide range inabundance and biomass (Table 4) is probably partlycaused by the patchy distribution of these animals underArctic sea ice, and partly by seasonal and annual vari-ation (Lùnne and Gulliksen 1991a, b).

In our study, G. wilkitzkii was the dominantamphipod species in the MIZ of the northern BarentsSea, both in 1995 and 1996. Lùnne and Gulliksen

Fig. 5 Relative abundance and biomass of ice amphipods in theMarginal Ice Zone of the northern Barents Sea in 1996 (n � 16,784individuals)

Fig. 6 Relative abundance and biomass of ice amphipods (n � 16,784individuals), in the Marginal Ice Zone of the northern Barents Sea(1996) on: a ice stations (Ice-0 to Ice-3) on a N-S transect across MIZ;b mesoscale under-ice structures, all stations combined: edge, ¯at,dome and ridge (percentage is based on summarised means for allsampling stations)

362

Table

1Ice-amphipodabundance

andbiomass

(means�

SD)in

theMarginalIceZone,

northernBarents

Sea,1996.Sampleswereobtained

from

fourice¯oes

andfourstructures

(edge,

¯at,dome,

ridge),andnsets

often50

´50m

quadrants

weresampledbydiversusingelectricalsuctionpumps

Station

Structure

Gammaruswilkitzkii

Apherusa

glacialis

Onisim

usnanseni

Onisim

usglacialis

Allspeciescombined

Abundance

Biomass

Abundance

Biomass

Abundance

Biomass

Abundance

Biomass

Abundance

Biomass

(ind.ám

)2)SD

(gám

)2)SD

(ind.ám

)2)SD

(gám

)2)SD

(ind.ám

)2)SD

(gám

)2)SD

(ind.ám

)2)SD

(gám

)2)SD

(ind.ám

)2)SD

(gám

)2)SD

Ice-0

Edge(n=

8)

4.8

4.2

0.211

0.323

61.3

32.7

0.387

0.156

0.0

0.0

0.000

0.000

0.0

0.0

0.000

0.000

66.1

35.2

0.599

0.440

Flat(n=

8)

21.0

23.9

0.758

0.776

11.7

13.6

0.058

0.073

0.7

0.7

0.027

0.036

0.0

0.0

0.000

0.000

33.4

27.2

0.842

0.746

Dome(n=4)

30.9

12.3

0.830

0.414

1.4

0.8

0.011

0.009

0.0

0.0

0.000

0.000

0.0

0.0

0.000

0.000

32.3

12.7

0.840

0.412

Ridge(n=

12)

190.4

162.3

6.232

3.539

3.5

4.5

0.045

0.064

0.3

0.4

0.023

0.036

0.0

0.0

0.000

0.000

194.2

163.6

6.300

3.545

Ice-1

Edge(n=

4)

8.8

16.3

0.176

0.203

82.1

47.9

0.243

0.125

0.0

0.0

0.000

0.000

0.1

0.2

0.003

0.007

91.0

47.0

0.422

0.241

Flat(n=

9)

16.2

16.3

1.211

1.730

9.2

10.9

0.077

0.047

0.2

0.5

0.010

0.024

0.6

0.5

0.013

0.013

26.2

21.4

1.311

1.739

Dome(n=8)

2.4

1.8

0.203

0.310

5.9

2.3

0.075

0.031

0.0

0.0

0.000

0.000

0.4

0.7

0.006

0.012

8.7

3.9

0.284

0.301

Ridge(n=

12)

169.1

130.1

6.417

4.689

3.7

3.3

0.050

0.037

0.7

1.3

0.085

0.166

0.0

0.0

0.000

0.000

173.4

132.4

6.552

4.756

Ice-2

Edge(n=

4)

1.1

1.0

0.034

0.054

22.6

18.9

0.081

0.08

0.1

0.2

0.011

0.023

0.0

0.0

0.000

0.000

23.8

18.6

0.126

0.054

Flat(n=

5)

24.7

13.2

1.354

0.480

2.4

2.0

0.015

0.019

0.8

1.2

0.097

0.163

0.0

0.0

0.000

0.000

27.9

12.6

1.466

0.434

Dome(n=4)

2.9

1.4

0.183

0.181

11.8

15.6

0.084

0.108

0.0

0.0

0.000

0.000

0.0

0.0

0.000

0.000

14.7

15.4

0.267

0.200

Ridge(n=

8)

21.6

26.2

1.715

1.007

1.8

1.0

0.028

0.021

1.1

2.1

0.139

0.240

0.0

0.0

0.000

0.000

24.4

25.8

1.882

1.029

Ice-3

Edge(n=

4)

0.2

0.2

0.000

0.000

0.1

0.2

0.000

0.000

0.0

0.0

0.000

0.000

0.0

0.0

0.000

0.000

0.3

0.2

0.000

0.000

Flat(n=

8)

5.7

6.5

0.415

0.531

0.5

0.6

0.013

0.017

0.4

0.6

0.001

0.002

0.1

0.2

0.003

0.007

6.6

6.7

0.432

0.534

Dome(n=4)

3.7

6.0

0.002

0.003

0.2

0.2

0.002

0.003

0.1

0.2

0.000

0.000

0.0

0.0

0.000

0.000

4.0

6.3

0.004

0.006

Ridge(n=

12)

11.1

13.4

0.886

0.785

0.9

1.1

0.016

0.019

0.5

0.9

0.008

0.026

0.1

0.2

0.004

0.007

12.5

4.3

0.914

0.785

Allstn.Edge(n=

20)

3.9

7.6

0.127

0.233

45.5

41.8

0.220

0.195

0.0

0.1

0.002

0.010

0.0

0.1

0.001

0.003

49.5

44.6

0.349

0.380

Flat(n=

30)

14.6

16.7

0.796

1.071

5.7

9.6

0.040

0.052

0.4

0.7

0.024

0.069

0.2

0.3

0.004

0.009

20.9

21.0

0.863

1.086

Dome(n=20)

9.6

13.9

0.354

0.408

6.3

8.1

0.061

0.061

0.0

0.0

0.000

0.000

0.2

0.5

0.003

0.009

16.1

13.6

0.419

0.384

Ridge(n=

40)

107.3

135.5

4.066

4.006

2.5

3.2

0.034

0.042

0.6

1.2

0.058

0.141

0.0

0.1

0.001

0.004

110.4

137.1

4.160

4.038

363

(1991a, b,c), who did earlier investigations in the BarentsSea, stated that A. glacialis is the most abundant ice-associated amphipod. They consider, in general, bothG. wilkitzkii and A. glacialis to be characteristic speciesfor multi-year pack ice and A. glacialis to be morecommon in ®rst-year ice. Since our investigated ice ¯oes

mainly represented multi-year ice, the dominance of G.wilkitzkii was not unexpected. Interestingly, the relativeabundance of G. wilkitzkii was about 20% lower in 1995than in 1996 in spite of nearly the same biomass in bothyears. The population of G. wilkitzkii in June (1995) wasprobably much smaller than in July/August (1996) be-cause most of the females had not yet released theiryoung out of their brood pouches so early in the season(Poltermann 1997). However, the relative abundance ofA. glacialis was lower in the later season (1996). Onereason could be attributed to prolonged predationpressure by ice-associated polar cod, which preferen-tially prey on A. glacialis (Lùnne and Gulliksen 1989).

The decrease in abundance and biomass of ice am-phipods from north to south in 1996 contradicts theassumption that the organisms colonise the remainingice when their habitat is lost because of melting. Ourresults may indicate increased loss to predators such asthe polar cod, sea birds and seals, which are mostabundant in areas of broken ice in the southern part ofthe MIZ. In addition, the pumping e�ect caused by theswell from the open Barents Sea increases near the outermargins of the MIZ. The animals will, therefore, besucked out of the brine channels into the water column.During the 1996 cruise, sympagic amphipods were foundin the water column down to 200 m depth at thesouthernmost station, and only a few inhabited the seaice. Pelagic occurrence of G. wilkitzkii has also beendescribed in other studies (Werner et al. 1999). Whilemoving freely in the water column the amphipods areeasily available to predators. The observed pattern ofdecrease in abundance and biomass of ice amphipodsalong the latitudinal gradient supports the view ofa transport of ice amphipods into the seasonally ice-covered Barents Sea from areas further north that arepermanently covered with sea ice (Lùnne and Gulliksen1991b; Lùnne 1992).

Table 3 Comparing ice ¯oes signi®cance matrix for analysis ofvariance of abundance (ind. m)2) and biomass (g m)2) of the iceamphipods Gammarus wilkitzkii (Gw) and Apherusa glacialis (Ag)from the Marginal Ice Zone, northern Barents Sea, 1996 (seeMaterials and methods regarding sampling). Signi®cant di�er-ences (P = 0.05) are indicated between ice ¯oes for same struc-ture (e.g. Ice-0 vs Ice-1, for edge)

Abundance Biomass

Ice-0 Ice-1 Ice-2 Ice-0 Ice-1 Ice-2

Edge Ice-1 ± ±Ice-2 ± ± Ag ±Ice-3 ± ± ± Ag ± ±

Flat Ice-1 ± ±Ice-2 ± ± ± AgIce-3 ± ± ± ± Ag ±

Dome Ice-1 Ag Gw Ag GwIce-2 Ag Gw ± ± ±Ice-3 ± ± ± Gw Ag Gw

Ridge Ice-1 ± ±Ice-2 Ag ± Gw ±Ice-3 ± ± ± Gw Gw ±

Fig. 7 Relative abundance and biomass of ice-amphipod species atfour ice stations and four under-ice structures on ice ¯oes in theMarginal Ice Zone of the northern Barents Sea, 1996

Table 2 Comparing structures signi®cance matrix for analysis ofvariance of abundance (ind. m)2) and biomass (g m)2) of the iceamphipods Gammarus wilkitzkii (Gw) and Apherusa glacialis (Ag)from the Marginal Ice Zone, northern Barents Sea, 1996 (seeMaterials and methods regarding sampling). Signi®cant di�erences(P = 0.05) are indicated between structures within each ice ¯oe(e.g. edge vs ¯at for Ice-0)

Abundance Biomass

Edge Flat Dome Edge Flat Dome

Ice-0 Flat Ag AgDome Ag Gw ± Ag Gw ±Ridge Ag Gw Gw Gw Ag Gw Gw Gw

Ice-1 Flat Ag ±Dome Ag ± ± ±Ridge Ag Gw Gw Gw ± Gw

Ice-2 Flat ± GwDome ± Gw ± GwRidge Ag ± Gw Gw ± Gw

Ice-3 Flat ± GwDome ± ± ± GwRidge ± ± ± Gw ± Gw

All stn. Flat Ag Gw GwDome ± ± Ag GwRidge Ag Gw Gw Gw Ag Gw Gw Gw

364

Table

4Abundance

andbiomass

oficeamphipodsin

Arcticseaice,determined

indi�erentstudiesondi�erenticetypes

(FY®rstyear,MYmulti-year,-F

fastice,-P

pack

ice).Data

are

means(andranges)

Source

Area

Ice

type

Gear

SpeciesGammarus

wilkitzkii

Apherusa

glacialis

Onisim

us

nanseni

Onisim

us

glacialis

Onisim

us

spp.

Speciescombined

Abundance

(no.m

)2)

Bradstreet

andCross

(1982)

CanadianArctic

FY-F

Handnet

5±

±±

±±

21.3

(1±72)b

Cross

(1982)

CanadianArctic

FY-F

Handnet

8±

±±

±±

32(0.3±163)

Grainger

etal.(1985)

CanadianArctic

FY-F

Core

±±

±±

±(0±140)

±Gulliksen(1984)

Barents

Sea

MY-P

Handnet

4(0±14)

(0±118)

±±

±20(0±124)

GulliksenandLùnne(1989)

Barents

Sea

MY-P

Pump

4(±200)

(0±2488)

±±

±±

GulliksenandLùnne(1991)

Barents

Sea

MY-P

Pump

4±

(0±2488)

±±

±±

Presentstudy

Barents

Sea

MY-P

Frame/

pump

447(0±517)

12(0±142)

0.4

(0±6)

0.1

(0±2)

±59(0±518)

LùnneandGulliksen(1991a)Barents

Sea

FY/

MY-P

Handnet

4(0±1)

(1±25)

±±

±0.001±0.179

FY-P

Pumpa

4(1±18)

(6±102)

±±

(4±25)

±LùnneandGulliksen(1991c)

Barents

Sea

MY-P

Photo

4(21±54)

(8±2196)

±±

(0±2)

728(54±2223)

MY-P

insitu

count

4(13±89)

±±

±±

±

MY-P

Pumpa

4(2±113)

(2±263)

±±

(0±26)

±Melnikov(1997)

CentralArctic

MY-P

Handnet

6(10±15)

±(14±21)

±±

±PikeandWelch

(1990)

CanadianArctic

FY-F

Handnet

10

±±

±±

±(±500)b

Poltermann(1998)

FranzJosefLandFY-F

Frame/

pump

4368(0±1888)

34(0±272)

8(0±80)

7(0±48)

(0±144)

420(0±1888)

Werner

(1997a)

Fram

Strait/

LaptevSea

FY-P

Video

4(0±19)

(0±11)

±±

(0±44)

2(0±44)

GreenlandSea

MY-P

Video

4(0±100)

(0±500)

±±

(0±200)

27(0±800)

Biomass

(gww

m±2)

Barnard

(1959)

CentralArctic

MY-P

Trap

7±

±±

±±

1.0

Bradstreet

andCross

(1982)

CanadianArctic

FY-F

Handnet

5±

±±

±±

0.516(0.010±1.584)c

Cross

(1982)

CanadianArctic

FY-F

Handnet

8±

±±

±±

0.127(0.001±0.620)

GolikovandScarlato

(1973)

FranzJosefLandMY-P

Handnet

4±

24.0

±±

±36.0

Grainger

etal.(1985)

CanadianArctic

FY-F

Core

±±

±±

±(0.0±0.24)

±Gulliksen(1984)

Barents

Sea

MY-P

Handnet

4(0.0±1.26)

(0.0±0.81)

±±

±±

GulliksenandLùnne(1989)

Barents

Sea

MY-P

Pump

4±

±±

±±

9.6

(1.6±25.2)

GulliksenandLùnne(1991)

Barents

Sea

MY-P

Pump

4±

6.7

±±

±±

Presentstudy

Barents

Sea

MY-P

Frame/

pump

41.854(0.0±6.417)0.073(0.0±0.243)0.029(0.0±0.139)0.002(0.0±0.013)±

1.958(0.0±2.860)

LùnneandGulliksen(1991a)Barents

Sea

FY/

MY-P

Handnet

4(0.0±0.179)

(0.001±0.073)

±±

(0.0±0.121)

(0.001±0.179)

FY-P

Pumpa

4(0.043±0.762)

(0.016±0.540)

±±

(0.056±0.355)(0.339±1.381)

LùnneandGulliksen(1991c)

Barents

Sea

MY-P

Pumpa

4(0.954±9.816)

±±

±±

±MY-P

Pumpa

4(0.025±13.857)

(0.003±1.384)

±±

(0.0±0.667)

±PikeandWelch

(1990)

CanadianArctic

FY-F

Handnet

10

0.0001

0.0005

0.0001

0.0004

±5.0

b

Poltermann(1998)

FranzJosefLandFY-F

Frame/

pump

410.12(0.0±63.91)0.099(0.0±0.581)0.668(0.0±3.434)0.042(0.0±0.518)±

10.61(0.0±63.92)

aFiveminute

index

sampling

bDetermined

from

diagrams

cRecalculatedfrom

dry

weight±conv.factor3.37(H

.Hop,unpublished

work)

365

Mesoscale habitat

The present study showed that ridges were the mostimportant habitat for occupation by ice amphipods. Thesecond most important habitat was edges, with regard toabundance, and ¯at areas with regard to biomass. Thisre¯ected the dominance of the large G. wilkitzkii onridges and ¯ats, whereas the much smaller A. glacialiswas consistently abundant on edges. Despite its smallindividual weight, the biomass of A. glacialis exceededthe biomass of G. wilkitzkii on edges. In domes, oftenone or the other species was dominant. However, thismay be explained by the morphology of this structure,which contains elements of edges, ridges and ¯at areas.The uppermost part is ¯at, whereas the walls are moresimilar to ridges. The high light penetration in domes,usually caused by melt ponds on the top of ice ¯oes, ismore typical for ¯oe edges. Dome-shaped areas aretherefore more di�cult to separate from the other threeinvestigated mesoscale structures.

The most abundant species, G. wilkitzkii and A. gla-cialis, showed signi®cant di�erences in distribution ondi�erent under-ice structures, as well as between ice¯oes. The inconsistent pattern of abundance/biomass inrelation to ice structures between di�erent ice ¯oes mayre¯ect the very di�erent topography of individual ice¯oes; for example, ridges sometimes occurred very closeto the edge while at other times they were remote fromthe edges. However, we suppose that other factors, suchas microhabitat, food and physiological conditions arealso responsible for the speci®c distribution patternsobserved. Although these factors were not speci®callyinvestigated during this study, we discuss them becauseof their obvious connection to the under-ice topography.

Microhabitat

To escape predators and ®nd protection against strongwater currents, poor swimmers such as G. wilkitzkii usebrine channels and melting holes for shelter. Thesestructures are most abundant on ridges and older ¯oeedges where melting processes have formed a three-di-mensional ice habitat (personal observation; Poltermann1998). We found the highest abundance and biomass ofthis species on ridges supporting this type of micro-habitat.

A. glacialis is a much better swimmer than G. wi-lkitzkii (Poltermann 1998) and can therefore escape frompredators by quick movements. This species has lessneed for brine channels or melting holes as shelter, andindividuals are often seen moving around on the under-ice surface and at ¯oe edges (personal observation;Poltermann 1997). Because of its small body size andwhitish colour, this species is well camou¯aged onsmooth ice surfaces such as ¯at areas, in domes and onthe exposed and highly accessible edges (Poltermann1998). This may partly explain why we found the highestabundance and biomass of this species on edges.

Food

Ice algae, an important food item of sympagic amphi-pods, especially for A. glacialis (Werner 1997b), growbest in areas with a high light penetration such as ¯oeedges (Melnikov 1997). Detritus, as a further importantfood source for ice amphipods (Poltermann 1997), oftenaccumulates in brine channels close to ¯oe edges andridges where water currents form recirculation regionsand stagnation zones (Melnikov 1997). High concen-trations of ice algae and detritus attract grazers andomnivorous ice amphipods, which are potential prey forG. wilkitzkii. Ridges are more exposed to water currents,which may facilitate ®lter feeding by G. wilkitzkii onpelagic organisms (Poltermann 1997).

Physiological conditions

The formation of thin water layers (<0.5 m) withstrongly reduced salinity is typical under Arctic sea iceduring the summer melt period (Eicken 1994). As wasshown by Aarset and Aunaas (1990), individuals ofG. wilkitzkii subjected to such conditions show muchhigher energy expenses caused by osmotic stress. Sincethe ridges where animals were sampled during this studyprotruded deeper than 1 m into the underlying watercolumn, the amphipods on these structures were not in-¯uenced by the low salinity. This could therefore explainthe preferred occupation of this structure byG. wilkitzkii.

In conclusion, it should be emphasised that ridgesand ¯oe edges of Arctic pack ice represent the mostimportant mesoscale structures for occupation by sym-pagic amphipods. The chosen sampling method in thepresent study showed the distribution patterns on a scaleof metres, but it is evident from our observations thatsmall and microscale structures are also important forhabitat choice. Further studies should attempt to samplea range of scales (centimetres to hundreds of metres) toidentify the most important scale for amphipod distri-bution under Arctic sea ice. Proper data on seasonal andannual variability are an important prerequisite formonitoring amphipods in ``the Arctic's shrinking seaice'' (Johannessen et al. 1995). For monitoring purposes,sampling needs to be quantitative and standardised withregard to mesoscale ice structures. We suggest replicatedsampling with standard frames of two of the identi®edstructures, ¯at areas and ridges, in Arctic sea ice. Thevariation in abundance and biomass of ice amphipodsthen needs to be correlated with changes in ice distribu-tion, and ultimately with regard to possible climatechanges in the Arctic.

Acknowledgements We thank Lars Henrik Smedsrud for the re-cording and processing of sonar images, Dr. Katrin Iken for divingand assisting with collections of amphipods, Dr. Bo BergstroÈ m andJan-Otto Pettersson for ROV video images that aided in the in-terpretation of under-ice structures, and Dr. Gunnar Pedersen forassisting with cruise planning, logistics and reporting. Generalthanks go to other cruise participants who helped out in thiswork. We thank the captain and crew of RV ``Lance'' for their

366

professional assistance on the cruise and at ice stations. Finally, wewish to thank Harvey Goodwin, Audun Jgeound and Tone Vollenfor help with the ®gures and Anne Estoppey for map presentation.This study was partially supported by the Norwegian ResearchCouncil (Project no. 112497/410) and the partners of the BarentsSea Production Licences 182, 225, 228; Norsk Hydro, Statoil,Chevron, Enterprise, Neste, Agip and SDéE. This is contributionno. 351 from the Norwegian Polar Institute.

References

Aarset AV, Aunaas T (1990) E�ects of osmotic stress on oxygenconsumption and ammonia excretion of the Arctic sympagicamphipod Gammarus wilkitzkii. Mar Ecol Prog Ser 58: 217±224

Averintzev VG (1993) Cryopelagic life at Franz-Josef-Land. In:Matishov GG, Galaktionov KV, Denisov VV, Drabysheva SS,Chinarina AD, Timofeeva SV (eds) Environment and ecosys-tems of the Franz-Josef-Land (archipelago and shelf). RussianAcademy of Science, Apatity, pp 171±187

Barnard JL (1959) Epipelagic and under-ice Amphipoda of theCentral Arctic Basin. Scienti®c studies at Fletcher's Ice IslandT-3, 1952±1955. Geophys Res Pap 63: 115±153

Barrett RT, Bakken V, Krasnov JV (1997) The diets of commonand BruÈ nnich's guillemots Uria aalge and U. lomvia in theBarents Sea region. Polar Res 16: 73±84

Bradstreet MSW (1980) Thick-billed murres and black guillemotsin the Barrow Strait area, N.W.T., during spring: diets and foodavailability along ice edges. Can J Zool 58: 2120±2140

Bradstreet MSW, Cross WE (1982) Trophic relationships at high-arctic ice edges. Arctic 35: 1±12

Box GEP, Cox DR (1964) An analysis of transformations. J R StatSoc Ser B 26: 211±243

Cavalieri DJ, Crawford J, Drinkwater M, Emery WJ, Eppler DT,Framer LD, Goodberlet M, Jentz R, Milman A, Morris C,Onstott R, Schweiger A, Shuchman R, Ste�an K, Swift CT,Wackerman C, Werner RL, Weaver RL (1992) NASA sea icevalidation program for the DMSP SSM/I: ®nal report. NASATechnical Memorandum 104559. National Aeronautics andSpace Administration, Washington, DC

Cross WE (1982) Under-ice biota at the Pond Inlet ice edge and inadjacent fast ice areas during spring. Arctic 35: 13±27

Eicken H (1994) Structure of under-ice melt ponds in the centralArctic and their e�ect on the sea-ice cover. Limnol Oceanogr39: 682±694

Falk-Petersen S, Hop H, Budgell WP, Korsnes R, Lùyning TB,érbñk JB, Hegseth EN, Kawamura T, Shirasawa K (in press).Variability in ecological and physical processes in the MarginalIce Zone of the northern Barents Sea (ICE-BAR) during thesummer melt period. Programme for the investigation, studyarea and physical environment. J Mar Systems

Gjertz I, Lydersen C (1986) The ringed seal (Phoca hispida) springdiet in north-western Spitsbergen, Svalbard. Polar Res 4: 53±56

Gloersen P, Campbell WJ, Cavaliere DJ, Comaso JC, ParkinsonCL, Zwalley HJ (1992) Arctic and Antarctic sea ice, 1978±1987.Satellite passes: microwave observations and analysis. Scienti®cand technical program. NASA, Washington, DC

Golikov AN, Scarlato OA (1973) Comparative characteristics ofsome ecosystems of the upper regions of the shelf in tropical,temperate and Arctic waters. Helgol Wiss Meeresunters 24:219±234

Grainger EH, Mohammed AA, Lovrity JE (1985) The sea ice faunaof Frobisher Bay, Arctic Canada. Arctic 38: 23±30

Gulliksen B (1984) Under-ice fauna from Svalbard waters. Sarsia69: 17±23

Gulliksen B, Lùnne OJ (1989) Distribution, abundance, and eco-logical importance of marine sympagic fauna in the Arctic.Rapp PV Re un Cons Perm Int Explor Mer 188: 133±138

Gulliksen B, Lùnne OJ (1991) Sea ice macrofauna in the Antarcticand Arctic. J Mar Systems 2: 53±61

Gurjanova EF (1951) Beach hoppers of the seas of the USSR andadjacent waters (Amphipoda-Gammaridea) (in Russian).Academy of Science of the USSR, Moscow

Holmquist C (1965) The amphipod genus Pseudalibrotus. Zool SystEvolut Forsch 3: 19±46

Johannessen OM, Miles M, Bjùrgo E (1995) The Arctic's shrinkingsea ice. Nature 376: 126±127

Kvambekk AÊ S (1989) Relation between top and bottom icetopography using a scanning sonar. In: Axelson KBE, FranssonLAÊ (eds) POAC'89 ± the 10th International Conference on portand ocean engineering under Arctic conditions, vol 1. LuleaÊUniversity of Technology, LuleaÊ , pp 77±86

Kvambekk AÊ S, Vinje T (1992) Ice draft recordings from UpwardLooking Sonars (ULSs) in the Fram Strait and the Barents Seain 1990/91. Technical Report 79. Norwegian Polar Institute,Oslo/St. Petersburg

Lùnne OJ (1988) A diver-operated electric suction sampler forsympagic (�under-ice) invertebrates. Polar Res 6: 135±136

Lùnne OJ (1992) Distribution, abundance and trophic role of thesympagic macro-fauna in Svalbard waters. Thesis, NorwegianCollege of Fishery Science, University of Tromsù

Lùnne OJ, Gabrielsen GW (1992) Summer diet of seabirds feed-ing in sea-ice-covered waters near Svalbard. Polar Biol 12:685±692

Lùnne OJ, Gulliksen B (1989) Size, age and diet of polar cod,Boreogadus saida (Lepechin 1773), in ice covered waters. PolarBiol 9: 187±191

Lùnne OJ, Gulliksen B (1991a) On the distribution of sympagicmacro-fauna in the seasonally ice covered Barents Sea. PolarBiol 11: 457±469

Lùnne OJ, Gulliksen B (1991b) Source, density and composition ofsympagic fauna in the Barents Sea. In: Sakshaug ECC, HopkinsCC, éritsland NA (eds) Proceedings of the Pro Mare Sympo-sium on Polar Marine Ecology, Trondheim, 12±16 May 1990.Polar Res 10: 289±294

Lùnne OJ, Gulliksen B (1991c) Sympagic macro-fauna frommultiyear sea-ice near Svalbard. Polar Biol 11: 471±477

Maykut GA (1985) The ice environment. In: Horner RA (ed) Seaice biota. CRC Press, Fla, pp 21±82

Melnikov I (1997) The Arctic sea ice system. Gordon and Beach,Amsterdam

Melnikov IA, Kulikov AS (1980) Cryopelagic fauna of the cen-tral Arctic Basin. In: Vinogradov ME, Melnikov IA (eds)Biology of the central Arctic Basin (Canadian Translations ofFisheries and Aquatic Sciences 4910). Nauka, Moskow,pp 97±111

Nilssen KT, Haug T, Potelov V, Timoshenko YK (1995) Feedinghabits of harp seals (Phoca groenlandica) during early summerand autumn in the northern Barents Sea. Polar Biol 15: 485±493

Pike D, Welch HE (1990) Spatial and temporal distribution ofsub-ice macrofauna in the Barrow Strait area, NorthwestTerritories. Can J Fish Aquat Sci 47: 81±91

Poltermann M (1997) Biology and ecology of cryopelagic amphi-pods from Arctic sea ice. PhD Thesis, University of Bremen BerPolarforsch 225, Bremerhaven, pp 1±170

Poltermann M (1998) Abundance, biomass and small-scale distri-bution of cryopelagic amphipods in the Franz Josef Land area(Arctic). Polar Biol 20: 134±138

Sakshaug E, Bjùrge A, Gulliksen B, Loeng H, Mehlum F (1994)Ecosystem Barents Sea (in Norwegian). Oslo

Vinje T, Kvambekk AÊ S (1991) Barents Sea drift ice characteristics.Polar Res 10: 59±68

Werner I (1997a) Ecological studies of the Arctic under-ice habitat-colonisation and processes at the ice-water interface. BerSonderforschungbereich, University of Kiel

Werner I (1997b) Grazing of Arctic under-ice amphipods on sea-icealgae. Mar Ecol Prog Ser 160: 93±99

Werner I, Auel H, Garrity C, Hagen W (1999) Pelagic occurrenceof the sympagic amphipod Gammarus wilkitzkii in ice-freewaters of the Greenland Sea ± dead end or part of life cycle?Polar Biol 22: 56±60

367