Abundance, biomass and small-scale distribution of cryopelagic amphipods in the Franz Josef Land area (Arctic)

Arctic sea ice is inhabited by several amphipod species. Abundance, biomass and small-scale distribution of these cryopelagic (=ice associated) amphipods were investigated near Franz Josef Land in summer 1994. The mean abundance of all species was 420 ind./m²; the mean biomass was 10.61 g ww/m². Gammarus wilkitzkii was the dominant species, whereas Apherusa glacialis, Onisimus nanseni and O. glacialis were only scarcely found. Amphipods were concentrated at the edges of ice floes and were less frequent in areas further away under the ice. The relationship between the distribution and ecological/physiological requirements of cryopelagic amphipods, as well as the small-scale morphology of Arctic sea ice, are discussed. Received: 14 January 1998 / Accepted 14 April 1998     

Arctic sea ice as feeding ground for amphipods – food sources and strategies

The amphipod species Gammarus wilkitzkii, Apherusa glacialis, Onisimus nanseni and O. glacialis live permanently associated with the Arctic sea ice. Qualitative and semi-quantitative investigations of gut contents and faeces showed that all four species use detritus as the main food source. Detrital lumps from the underside of sea ice had the same item composition as amphipod gut contents and faeces. Crustacean remains and ice algae were additional food items, but overall they were quantitatively less important. All species are omnivorous; however, differences in gut contents, behavioural observations and functional–morphological studies of the mandibles suggest a differentiation within this feeding strategy. G. wilkitzkii is a detritivorous-carnivorous-necrophagous-suspension-feeding species and shows the most complex feeding strategy. O. nanseni and O. glacialis are predominantly detritivorous-necrophagous, whereas A. glacialis is characterised as a more herbivorous-detritivorous species. By using a variety of the available food sources under Arctic sea ice, the amphipods are well adapted to the under-ice habitat and are less influenced by temporal and spatial variations. Furthermore, the wide food spectrum of all four species reduces the intra- and interspecific competition in a habitat where certain food sources are limited or only seasonally available. Accepted: 30 June 2000     

Lipids and trophic interactions of ice fauna and pelagic zooplankton in the marginal ice zone of the Barents Sea

Gammarus wilkitzkii, Apherusa glacialis, Onismus nanseni, Onismus glacialis, Boreogadus saida, Parathemisto libellula and Calanus hyperboreus, collected in late June in the Barents Sea marginal ice zone, contained substantial levels (28–51% of the dry mass) of total lipid, the highest levels (51% and 41% respectively) being in  A. glacialis and  C. hyperboreus. Neutral lipids were present in greater amounts than polar lipids in all species. Triacylglycerols were major neutral lipids in A. glacialis, G. wilkitzkii and O. nanseni; triacylglycerols and wax esters were present in similar amounts in O. glacialis; higher levels of wax esters than triacylglycerols occurred in P. libellula; wax esters greatly exceeded triacylglycerols in C. hyperboreus, the opposite being true for B. saida. Diatom fatty acid markers were prominent in the triacylglycerols of G. wilkitzkii, O. nanseni, O. glacialis and, particularly, of  A. glacialis; 20:1(n-9) and 22:1(n-11) moieties were abundant in wax esters of G. wilkitzkii, O. nanseni, O. glacialis, P. libellula and  C. hyperboreus, and in triacylglycerols of B. saida. We deduce that  A. glacialis feeds mainly on ice algae and phytodetritus, G. wilkitzkii and the Onismus spp. feed on calanoid copepods as well as ice algae, whereas P. libellula and especially B. saida feed extensively on calanoid copepods. Accepted: 17 May 1998     

Zooplankton boom and ice amphipod bust below melting sea ice in the Amundsen Gulf, Arctic Canada

Early summer in the Arctic with extensive ice melt and break-up represents a dramatic change for sympagic–pelagic fauna below seasonal sea ice. As part of the International Polar Year-Circumpolar Flaw Lead system study (IPY-CFL), this investigation quantified zooplankton in the meltwater layer below landfast ice and remaining ice fauna below melting ice during June (2008) in Franklin Bay and Darnley Bay, Amundsen Gulf, Canada. The ice was in a state of advanced melt, with fully developed melt ponds. Intense melting resulted in a 0.3- to 0.5-m-thick meltwater layer below the ice, with a strong halocline to the Arctic water below. Zooplankton under the ice, in and below the meltwater layer, was sampled by SCUBA divers. Dense concentrations (max. 1,400 ind. m⁻³) of Calanus glacialis were associated with the meltwater layer, with dominant copepodid stages CIV and CV and high abundance of nauplii. Less abundant species included Pseudocalanus spp., Oithona similis and C. hyperboreus. The copepods were likely feeding on phytoplankton (0.5–2.3 mg Chl-a m⁻³) in the meltwater layer. Ice amphipods were present at low abundance (<10 ind. m⁻²) and wet biomass (<0.2 g m⁻²). Onisimus glacialis and Apherusa glacialis made up 64 and 51% of the total ice faunal abundance in Darnley Bay and Franklin Bay, respectively. During early summer, the autochthonous ice fauna becomes gradually replaced by allochthonous zooplankton, with an abundance boom near the meltwater layer. The ice amphipod bust occurs during late stages of melting and break-up, when their sympagic habitat is diminished then lost.     

Molecular relationships of gammaridean amphipods from Arctic sea ice

The information on the biology and ecology of the Arctic sea ice-associated amphipods (Apherusa glacialis, Gammarus wilkitzkii, Onisimus glacialis, and O. nanseni) has increased, but their molecular taxonomic information still remains undisclosed. In the present study, we investigated long-range DNA sequences spanning 18S to 28S rDNA of these four sea ice-associated amphipods and analyzed their genetic relationships with other amphipod taxa. Variations of rDNA within the individuals of the same species were not detected. Phylogenetic analyses showed that each ice amphipod was separated, forming clusters with other conspecifics. Pairwise comparisons led to similar phylogenetic results, showing that the molecular taxonomy of the ice amphipods was in accordance with morphological systematics. In addition, these findings suggest that all four amphipods have little genetic variation compared with their morphologically defined conspecifics from temperate regions. Based on DNA taxonomy, G. wilkitzkii was supported as a species in good standing, refuting a recent synonymization with Gammarus duebeni. Considerably low genetic divergences of O. glacialis and O. nanseni in 18S, ITS, and 28S rDNA suggest the presence of population distinctions within species.     

Sympagic amphipods in the Arctic pack ice: redescriptions of <Emphasis Type="Italic">Eusirus holmii</Emphasis> Hansen, 1887 and <Emphasis Type="Italic">Pleusymtes karstensi</Emphasis> (Barnard, 1959)

During an expedition into the Arctic Ocean, in September 2004, six different species of amphipods were collected in the ice above 82°N. All six species (Apherusa glacialis, Gammarus wilkitzkii, Onisimus nanseni, O. glacialis, Pleusymtes karstensi and Eusirus holmii) were observed to be living adjacent to the sea ice or partly within its brine channels. The nature of the association with the ice for the last two species is uncertain, but the finding raises important questions regarding our knowledge of the sympagic fauna. Based on the obtained material, the two species E. holmii and P. karstensi are redescribed, and their association with the sea ice is discussed.     

Osmotic and ionic regulation in response to salinity variations and cold resistance in the Arctic under-ice amphipod <Emphasis Type="Italic">Apherusa glacialis</Emphasis>

Amphipods living at the underside of Arctic sea ice are exposed to varying salinities due to freezing and melting, and have to cope with the resulting osmotic stress. Extracellular osmotic and ionic regulation at different salinities, thermal hysteresis, and supercooling points (SCPs) were studied in the under-ice amphipod Apherusa glacialis. The species is euryhaline, capable to regulate hyperosmotically at salinities S _R < 30 g/kg, and osmoconforms at salinities S _R ≥ 30 g/kg. Hyperosmotic regulation is an adaptation to thrive in low-salinity meltwater below the ice. Conforming to the ambient salinity during freezing reduces the risk of internal ice formation. Thermal hysteresis was not observed in the haemolymph of A. glacialis. The SCP of the species was −7.8 ± 1.9°C. Several ions were specifically downregulated ([Mg²⁺], [SO₄ ²⁻]), or upregulated ([K⁺], [Ca²⁺]) in comparison to the medium. Strong downregulation of [Mg²⁺], is probably necessary to avoid an anaesthetic effect at low temperatures.     

Observations on the relationship between the Antarctic coastal diatoms Thalassiosira antarctica Comber and Porosira glacialis (Grunow) Jørgensen and sea ice concentrations during the late Quaternary

The available ecological and palaeoecological information for two sea ice-related marine diatoms (Bacillariophyceae), Thalassiosira antarctica Comber and Porosira glacialis (Grunow) Jørgensen, suggests that these two species have similar sea surface temperature (SST), sea surface salinity (SSS) and sea ice proximity preferences. From phytoplankton observations, both are described as summer or autumn bloom species, commonly found in low SST waters associated with sea ice, although rarely within the ice. Both species form resting spores (RS) as irradiance decreases, SST falls and SSS increases in response to freezing ice in autumn. Recent work analysing late Quaternary seasonally laminated diatom ooze from coastal Antarctic sites has revealed that sub-laminae dominated either by T. antarctica RS, or by P. glacialis RS, are nearly always deposited as the last sediment increment of the year, interpreted as representing autumn flux. In this study, we focus on sites from the East Antarctic margin and show that there is a spatial and temporal separation in whether T. antarctica RS or P. glacialis RS form the autumnal sub-laminae. For instance, in deglacial sediments from the Mertz Ninnis Trough (George V Coast) P. glacialis RS form the sub-laminae whereas in similar age sediments from Iceberg Alley (Mac.Robertson Shelf) T. antarctica RS dominate the autumn sub-lamina. In the Dumont d'Urville Trough (Adélie Land), mid-Holocene (Hypsithermal warm period) autumnal sub-laminae are dominated by T. antarctica RS whereas late Holocene (Neoglacial cool period) sub-laminae are dominated by P. glacialis RS. These observations from late Quaternary seasonally laminated sediments would appear to indicate that P. glacialis prefers slightly cooler ocean–climate conditions than T. antarctica. We test this relationship against two down-core Holocene quantitative diatom abundance records from Dumont d'Urville Trough and Svenner Channel (Princess Elizabeth Land) and compare the results with SST and sea ice concentration results of an Antarctic and Southern Ocean Holocene climate simulation that used a coupled atmosphere–sea ice–vegation model forced with orbital parameters and greenhouse gas concentrations. We find that abundance of P. glacialis RS is favoured by higher winter and spring sea ice concentrations and that a climatically-sensitive threshold exists between the abundance of P. glacialis RS and T. antarctica RS in the sediments. An increase to > 0.1 for the ratio of P. glacialis RS:T. antarctica RS indicates a change to increased winter sea ice concentration (to >80% concentration), cooler spring seasons with increased sea ice, slightly warmer autumn seasons with less sea ice and a change from ~ 7.5 months annual sea ice cover at a site to much greater than 7.5 months. In the East Antarctic sediment record, an increase in the ratio from <0.1 to above 0.1 occurs at the transition from the warmer Hypsithermal climate into the cooler Neoglacial climate (~ 4 cal kyr) indicating that the ratio between these two diatoms has the potential to be used as a semi-quantitative climate proxy.     

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

Arctic ice amphipods are part of the sympagic macrofauna in the Marginal Ice Zone of the northern Barents Sea and represent an important link from lower to higher trophic levels in some Arctic marine food chains. The species diversity in this area (1995/1996) consisted of four species: Gammarus wilkitzkii, Apherusa glacialis, Onisimus nanseni and Onisimus glacialis. The larger ice amphipod, G. wilkitzkii, was the most abundant with the highest biomass (>90%), whereas A. glacialis was abundant, but contributed little to the total biomass (<4%). The other two species were found only in small numbers. Both abundance and biomass of ice amphipods decreased along a latitudinal gradient from north to south across the Marginal Ice Zone. Their distribution was also related to the under-ice topography with regard to mesoscale structures (edge, flat area, dome and ridge). Overall, the abundance and biomass on ridges were much higher in comparison to other mesoscale structures, although edges also showed high abundance, but low biomass. The large G. wilkitzkii was consistently abundant on ridges. The small A. glacialis was predominately associated with edges, but also showed high numbers in dome-shaped areas. The Onisimus species were present in low numbers at all structures, and their biomass contributed <10% on any one structure. The reasons for different distribution patterns of the dominant amphipod species under Arctic sea ice are probably related to different requirements of the species, especially for food, shelter and physiological conditions. Accepted: 27 November 1999     

Faecal pellet production by Arctic under-ice amphipods – transfer of organic matter through the ice/water interface

The underside of Arctic sea ice is inhabited by several autochthonous amphipod species (Apherusa glacialis, Onisimus spp., Gammarus wilkitzkii). The amphipods graze on ice-bound organic matter, such as ice algae, detritus and ice fauna, and release faecal pellets into the underlying water column, thus forming a direct link between the sea ice and the pelagic ecosystems. Experiments on faecal pellet production rates showed species-specific differences, which were related to size of the animals. The smallest species, A. glacialis, produced the highest mean number of pellets (15.4 pellets ind.^-1 d^-1), followed by Onisimus spp. (2.7 pellets ind.^-1 d^-1) and the largest species, G. wilkitzkii (1.1 pellets ind.^-1 d^-1). Relative carbon content of the pellets was very similar in all species (21.2–22.6% dry mass). Juvenile amphipods (Onisimus spp., G. wilkitzkii) produced more pellets with less POC than adults. Based on field determinations of the POC concentration in the lowermost 2 cm of the sea ice (mean: 36.4 mg C m^-2) and mean amphipod abundances (A. glacialis: 33.8 ind. m^-2, Onisimus spp.: 0.5 ind. m^-2, G. wilkitzkii: 9.4 ind. m^-2) in the Greenland Sea in summer 1994, the amount of POC transferred from the ice to the water by faecal pellet production was estimated (0.7 mg C m^-2 d^-1 or almost 2% of ice-bound carbon). Since this process probably takes place in all ice-covered Arctic regions as well as during all seasons, grazing and pellet production by under-ice amphipods contributes significantly to matter flux across the ice/water interface.     

Seasonal variations in biomass, abundance and composition of zooplankton in the subarctic waters north of Iceland

The seasonal variations in biomass, abundance and species composition of zooplankton in relation to hydrography and chlorophyll a were studied in the subarctic waters north of Iceland. The sampling was carried out at approximately monthly intervals from February 1993 to February 1994 at eight stations arranged along a transect extending from 66°16′N–18°50′W to 68°00′N–18°50′W. The mean temperature at 50 m depth showed a clear seasonal pattern, with lowest water temperatures in February (∼1.1°C) and the highest in July (∼5.4°C). The spring growth of the phytoplankton began in late March and culminated during mid-April (∼7.0 mg Chl a m⁻³). Both the biomass and the abundance of total zooplankton were low during the winter and peaked once during the summer in late May (∼4 g m⁻² and ∼38,000 individuals m⁻²). A total of 42 species and taxonomic groups were identified in the samples. Eight taxa contributed ∼90% of the total zooplankton number. Of these Calanus finmarchicus was by far the most abundant species (∼60% of the total zooplankton). Less important groups were ophiuroid larvae (∼9%), Pseudocalanus spp. (∼8%), Metridia longa (∼4%), C. hyperboreus (∼3%), Acartia longiremis (∼2%), chaetognaths (∼2%) and euphausiid larvae (∼2%). The dominant copepods showed two main patterns in seasonal abundance: C. finmarchicus, C. hyperboreus and C. glacialis had one annual peak in numbers in late May, while Pseudocalanus spp., M. longa and A. longiremis showed two maxima during the summer (July) and autumn (October/November). Ophiuroid larvae and chaetognaths (mainly Sagitta elegans) peaked during the middle of July, while the number of euphausiid eggs and larvae was greatest from May to July. The succession in population structure of C. finmarchicus indicated its main spawning to be in April and May, coincident with the phytoplankton spring bloom. A minor spawning was also observed sometime between August and October. However, the offspring from this second spawning contributed only insignificantly to the overwintering stock of C. finmarchicus. Received: 12 September 1997 / Accepted: 1 March 1998     

Life histories of the copepods Pseudocalanus minutus, P. acuspes (Calanoida) and Oithona similis (Cyclopoida) in the Arctic Kongsfjorden (Svalbard)

The year-round variation in abundance and stage-specific (vertical) distribution of Pseudocalanus minutus and Oithona similis was studied in the Arctic Kongsfjorden, Svalbard. Maxima of vertically integrated abundance were found in November with 111,297 ind m⁻² for P. minutus and 704,633 ind m⁻² for O. similis. Minimum abundances comprised 1,088 ind m⁻² and 4,483 ind m⁻² in June for P. minutus and O. similis, respectively. The congener P. acuspes only occurred in low numbers (15–213 ind m⁻²), and successful reproduction was debatable. Reproduction of P. minutus took place in May/June, and stage distribution revealed a 1-year life cycle with copepodids CIII, CIV, and CV as the overwintering stages. Oithona similis exhibited two main reproductive peaks in June and August/September, respectively. Moreover, it reproduced more or less continuously throughout the whole year with all stages occurring during the entire sampling period, suggesting two generations per year. Both species migrated towards greater depth in November, but O. similis preferred to stay longer in the upper 100 m as compared to Pseudocalanus. The reproduction of the two species in Kongsfjorden seemed to be linked to phytoplankton dynamics.     

Seasonal home ranges and fidelity to breeding sites among ringed seals

Population structure and patterns of habitat use among ringed seals (Phoca hispida) are poorly known, in part because seasonal movements have not been adequately documented. We monitored the movements of 98 ringed seals in the Beaufort and Chukchi seas between 1990 and 2006 using three forms of telemetry. In the winter—spring period (when the seals were occupying shorefast ice), we used radio and ultra-sonic tags to track movements above and below the ice, respectively. We used satellite-linked transmitters in summer and fall (when the seals ranged away from their winter sites) to track at-sea movements. In the shorefast ice habitat, the home ranges of 27 adult males ranged from <1 to 13.9 km² (median = 0.628) while the home ranges of 28 adult females ranged from <1 to 27.9 km² (median = 0.652). The 3-dimensional volumes used by 9 seals tracked acoustically under the ice averaged 0.07 (SD = 0.04) km³ for subadults and adult males and 0.13 (SD = 0.04) km³ for adult females. Three of the radio-tracked seals and 9 tracked by satellite ranged up to 1,800 km from their winter/spring home ranges in summer but returned to the same small (1–2 km²) sites during the ice-bound months in the following year. The restricted movements of ringed seals during the ice-bound season—including the breeding season—limits their foraging activities for most of the year and may minimize gene flow within the species.     

Copepods in summer platelet ice in the eastern Weddell Sea,Antarctica

Copepods in platelet-ice layers underlying fast ice and in the water column below were studied at Drescher Inlet, eastern Weddell Sea in February 1998. Three copepod species were found: Drescheriella glacialis and Paralabidocera antarctica occurred in platelet-ice layers, while Stephos longipes was only present in the water column. The distribution of all species varied considerably between station and depth. D. glacialis dominated the platelet-ice community and occurred at all five platelet-ice sampling sites, except one, with numbers of up to 26 ind. l^–1. In contrast, P. antarctica was only found in low numbers (up to 2 ind. l^–1) at one site. The total copepod abundance in the platelet ice was not associated with algal biomass, although it was strongly correlated with high ammonium concentrations (up to 9 M) in the interstitial water between the platelets. This is the first indirect evidence to support the hypothesis that zooplankton excretion can partly account for the high ammonium values often found in platelet-ice layers.     

Protist assemblages in winter sea ice: setting the stage for the spring ice algal bloom

This study documents, for the first time, the abundance and species composition of protist assemblages in Arctic sea ice during the dark winter period. Lack of knowledge of sea-ice assemblages during the dark period has left questions about the retention and survival of protist species that initiate the ice algal bloom. Sea-ice and surface water samples were collected between December 27, 2007 and January 31, 2008 within the Cape Bathurst flaw lead, Canadian Beaufort Sea. Samples were analyzed for protist identification and counts, chlorophyll (chl) a, and total particulate carbon and nitrogen concentrations. Sea-ice chl a concentrations (max. 0.27 μg l⁻¹) and total protist abundances (max. 4 × 10³ cells l⁻¹) were very low, indicating minimal retention of protists in the ice during winter. The diversity of winter ice protists (134 taxa) was comparable to spring ice assemblages. Pennate diatoms dominated the winter protist assemblage numerically (averaging 77% of total protist abundances), with Nitzschia frigida being the most abundant species. Only 56 taxa were identified in surface waters, where dinoflagellates were the dominant group. Our results indicate that differences in the timing of ice formation may have a greater impact on the abundance than structure of protist assemblages present in winter sea ice and at the onset of the spring ice algal bloom.     

Meso- and bathypelagic distribution and abundance of chaetognaths in the Atlantic sector of the Southern Ocean

We conducted multinet sampling during winter and summer in the Southern Ocean (Atlantic sector) to investigate the effect of water mass, season and water depth on abundance and species composition of meso- and bathypelagic chaetognaths. Eukrohnia hamata (mean 115 ind. 1,000 m⁻³) and Sagitta marri (mean 51 ind. 1,000 m⁻³) were dominant, complemented by E. bathypelagica (mean 19 ind. 1,000 m⁻³) and E. bathyantarctica (mean 19 ind. 1,000 m⁻³) below 1,000 m. A further six species were identified, among them the rare bathypelagic species Heterokrohnia fragilis and the subtropical Eukrohnia macroneura that is new to the Antarctic. Water depth and season were the principal determinants of abundance and species composition patterns, indicating vertical seasonal migration and vertical segregation of species. The life cycles of E. hamata and S. marri were studied additionally. Their maturity stages were vertically segregated and prolonged reproductive periods are suggested for both species.     

Biogeographic responses of the copepod Calanus glacialis to a changing Arctic marine environment

Dramatic changes have occurred in the Arctic Ocean over the past few decades, especially in terms of sea ice loss and ocean warming. Those environmental changes may modify the planktonic ecosystem with changes from lower to upper trophic levels. This study aimed to understand how the biogeographic distribution of a crucial endemic copepod species, Calanus glacialis, may respond to both abiotic (ocean temperature) and biotic (phytoplankton prey) drivers. A copepod individual‐based model coupled to an ice‐ocean‐biogeochemical model was utilized to simulate temperature‐ and food‐dependent life cycle development of C. glacialis annually from 1980 to 2014. Over the 35‐year study period, the northern boundaries of modeled diapausing C. glacialis expanded poleward and the annual success rates of C. glacialis individuals attaining diapause in a circumpolar transition zone increased substantially. Those patterns could be explained by a lengthening growth season (during which time food is ample) and shortening critical development time (the period from the first feeding stage N3 to the diapausing stage C4). The biogeographic changes were further linked to large‐scale oceanic processes, particularly diminishing sea ice cover, upper ocean warming, and increasing and prolonging food availability, which could have potential consequences to the entire Arctic shelf/slope marine ecosystems.     

Distribution of <Emphasis Type="Italic">Calanus</Emphasis> populations in a glaciated fjord in the Arctic (Hornsund,Spitsbergen)—the interplay between biological and physical factors

The distribution and diversity of copepods of the genus Calanus were investigated in Hornsund Fjord (on the southwest coast of Spitsbergen) in summer 2001. The Bhattacharya method was used to sort individuals by species based on their prosome length. The established prosome length boundary values for the Calanus copepodid stages coincided with those defined for the Calanus species from Kongsfjorden (on the northwest coast of Spitsbergen). The predominant species in the main and inner fjord basins was Calanus glacialis, whereas Calanus finmarchicus was the prevailing species outside Hornsund. Younger copepodid stages (CI–CIII) of both species concentrated in the surface water layers (0–50∼70 m), while older copepodids (CIV–CVI females) that were ready for wintering stayed in deep layers (50∼70 m to bottom). Calanus hyperboreus was present in low numbers, predominantly as CIV, and in Hornsund deep water layers. The distribution and diversity of Calanus species complied with the notion that the marine fauna in Hornsund is of a more Arctic character than in Kongsfjorden, a fjord 260 km to the north on the west coast of Spitsbergen.     

Linking ice structure and microscale variability of algal biomass in Arctic first-year sea ice using an in situ photographic technique

Microscale photographs were taken of the ice bottom to examine linkages of algal chlorophyll a (chl a) biomass distribution with bottom ice features in thick Arctic first-year sea ice during a spring field program which took place from May 5 to 21, 2003. The photographic technique developed in this paper has resulted in the first in situ observations of microscale variability in bottom ice algae distribution in Arctic first-year sea ice in relation to ice morphology. Observations of brine channel diameter (1.65–2.68 mm) and number density (5.33–10.35 per 100 cm²) showed that the number of these channels at the bottom of thick first-year sea ice may be greater than previously measured on extracted ice samples. A variogram analysis showed that over areas of low chl a biomass (≤20.7 mg chl a m⁻²), patchiness in bottom ice chl a biomass was at the scale of brine layer spacing and small brine channels (∼1–3 mm). Over areas of high chl a biomass (≥34.6 mg chl a m⁻²), patchiness in biomass was related to the spacing of larger brine channels on the ice bottom (∼10–26 mm). Brine layers and channels are thought to provide microscale maxima of light, nutrient replenishment and space availability which would explain the small scale patchiness over areas of low algal biomass. However, ice melt and erosion near brine channels may play a more important role in areas with high algal biomass and low snow cover.     

Living conditions, abundance and biomass of under-ice fauna in the Storfjord area (western Barents Sea, Arctic) in late winter (March 2003)

The under-ice habitat and fauna were studied during a typical winter situation at three stations in the western Barents Sea. Dense pack ice (7–10/10) prevailed and ice thickness ranged over <0.1–1.6 m covered by <0.1–0.6 m of snow. Air temperatures ranged between –1.8 and –27.5°C. The ice undersides were level, white and smooth. Temperature and salinity profiles in the under-ice water (0–5 m depth) were not stratified (T=–1.9 to –2.0°C and S=34.2–34.7). Concentrations of inorganic nutrients were high and concentrations of algal pigments were very low (0.02 g chlorophyll a l^–1), indicating the state of biological winter. Contents of particulate organic carbon and nitrogen ranged over 84.2–241.3 and 5.3–16.4 g l^–1, respectively, the C/N ratio over 11.2–15.5 pointing to the dominance of detritus in the under-ice water. Abundances of amphipods at the ice underside were lower than in other seasons: 0–1.8 ind. m^–2 for Apherusa glacialis, 0–0.7 ind. m^–2 for Onisimus spp., and 0–0.8 ind. m^–2 for Gammarus wilkitzkii. A total of 22 metazoan taxa were found in the under-ice water, with copepods as the most diverse and numerous group. Total abundances ranged over 181–2,487 ind. m^–3 (biomass: 70–2,439 g C m^–3), showing lower values than in spring, summer and autumn. The dominant species was the calanoid copepod Pseudocalanus minutus (34–1,485 ind. m^–3), contributing 19–65% to total abundances, followed by copepod nauplii (85–548 ind. m^–3) and the cyclopoid copepod Oithona similis (44–262 ind. m^–3). Sympagic (ice-associated) organisms occurred only rarely in the under-ice water layer.