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

Abstract

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.