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.