Feeding of the Arctic under-ice amphipods Apherusa glacialis. Onisimus spp. and Gammarus willutzkii on sea-ice algae was investigated in laboratory experiments. The specific ingestion rates of algal biornass declined with increasing size of the species ( A . glacialis, 51; Onisimus spp., 11; G , wilkjtzkli, 2 n g chl a equival. mg-' dry mass d-l). Juveniles of Onisimus spp. and G. wilkitzkii had specific rates that were 1 to 2 orders of magnitude higher than those of the respective adults. Calculations of the carbon budget and observations of amphipods behaviour indicate that A. glacialis is predominantely herbivorous, whereas Onjsimus spp, are herbivorous/detritivorous, and G. wilkitzkii is herbivorous/detritivorous/carnivorous. The grazing impact on the ice-algal standing stock at the ice underside in summer, estimated for 2 different areas in the Arctic, was low (1.1 % d-' in the Laptev Sea, 2.6% d-' in the Greenland Sea), indicating that food is not a limiting factor in this habitat.

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The Arctic under-ice amphipods Apherusa glacialis, Onisimus glacialis, O. nanseni andGammarus wilkitzkii are autochthonous below the pack ice, i.e. they occur in this habitat during all seasons. In this study, seasonal dynamics in abundance were only evident in A. glacialis. Respiration rates did not differ significantly between summer and winter in any species, indicating that the under-ice habitat is a comparatively stable environment in terms of seasonal variations. The assessment of total lipids and fatty acid biomarkers revealed differences in feeding behaviour and overwintering strategy among the amphipod species. A. glacialis is strictly herbivorous, feeding on ice-algae at the underside of the ice during the productive season. The species overwinters in an active state, fuelled at least partly by internal lipid reserves stored during summer. O. glacialis switches from a predominantly herbivorous diet in summer to metazoan prey, including planktonic copepods, in winter, so that lipid catabolism is less important to winter survival. The congener O. nanseni relies on an opportunistic omnivorous diet throughout the year, with a higher proportion of metazoan prey during summer compared to O. glacialis. G. wilkitzkii is able to exploit a wide spectrum of food sources, from predominantly ice algae in summer to more ice fauna and planktonic copepods in winter. This species partly meets its energy requirements by using internal lipid reserves during winter.

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