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

Abstract

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.

Seasonal variability in abundance, respiration and lipid composition of Arctic under-ice amphipods

“…It has been shown that abundances of this species are low or moderate in spring, reach their maximum in summer and decrease strongly towards autumn (Lønne & Gulliksen 1991a, Werner & Gradinger 2002. These seasonal dynamics are significantly related to ice-algal biomass and reflect the predominantly herbivorous feeding mode of this species (Werner 1997). It has been speculated that A. glacialis leaves the under-ice habitat to overwinter and reproduce in deeper waters (Melnikov & Kulikov 1980), but significant evidence for this hypothesis is still lacking.…”

“…Apherusa glacialis is a herbivorous species which feeds extensively on ice algae, predominantly diatoms, at the underside of the sea ice (Werner 1997, Scott et al 1999). In our summer and autumn data, this is reflected in very high concentrations of the 16:1(n-7) fatty acid, which is a typical component of diatoms (Graeve et al 1994, Falk-Petersen et al 1998, and low values for the isomer ratio 18:1(n-9)/ 18:1(n-7), which has been proposed as a relative index of carnivory (Graeve et al 1997.…”

“…Gammarus wilkitzkii is generally considered as omnivorous to carnivorous (Werner 1997, Poltermann 2001. Studies on its fatty acid composition in the Barents Sea (Scott et al 1999), as well as feeding experiments with individuals from the Greenland Sea revealed that calanid copepods represent a major dietary component of G. wilkitzkii during summer.…”

“…they occur in the under-ice habitat during all seasons and spend their entire life-cycle there (Lønne & Gulliksen 1991a. These amphipods feed on ice-bound and under-ice flora and fauna, as well as on ice-bound detritus (Werner 1997, Poltermann 2001. Biomarkers like fatty acids have been used to identify different feeding types among these species (Scott et al 1999).…”

“…Metabolic rates, e.g. ingestion and respiration, of under-ice amphipods are generally low, due to their energy-saving lifestyle attached to the underside of the pack ice (Aarset 1991, Werner 1997.…”