Grazing of Arctic under-ice amphipods on sea-ice algae

Feeding of the Arctic under-ice amphipods Apherusa glacialis, Onisimus spp. and Gammarus wilkitzkii on sea-ice algae was investigated in laboratory experiments. The specific ingestion rates of algal biomass declined with increasing size of the species (A. glacialis, 51; Onisimus spp., 11; G. wilkitzkii, 2 ng chl a equival. mg(-1) dry mass d(-1)). 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 Onisimus 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(-1) in the Laptev Sea, 2.6 % d(-1) in the Greenland Sea), indicating that food is not a limiting factor in this habitat.