Carnivorous feeding and respiration of the Arctic under-ice amphipod Gammarus wilkitzkii
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The dominant Arctic under-ice amphipod Gammarus wilkitzkii consumes a wide range of food items. The carnivorous feeding activity and energy budget of this large species were studied using three different approaches. Maximum potential ingestion rates Imax estimated from an allometric function taken from the literature and based on body mass were 2.1±0.4% of body carbon day–1. Based on respiration measurements, the specific ingestion rates required to meet metabolic demands were lower (1.4±0.4% of body carbon day–1). Feeding experiments, in which co-occurring pelagic calanoid (Calanus hyperboreus) or sympagic harpacticoid (Halectinosoma sp.) copepods were offered as prey, yielded actual ingestion rates of 8.0±5.6% of body carbon day–1 and 0.1±0.1% of body carbon day–1, respectively. These results indicate that predatory feeding on pelagic copepods may constitute an important food source for G. wilkitzkii. Abundances of G. wilkitzkii at the ice underside (median: 1.6 ind. m–2), Calanus spp. in the upper metre below the ice (2.6 ind. m–3), and Halectinosoma sp. in the lowermost 2–3 cm of the ice (393.5 ind. m–2) were determined from several multi-year pack-ice floes in the northern Greenland Sea and Fram Strait. Potential predation impact of G. wilkitzkii was estimated by combining information on ingestion rates with population densities. It was very high on Calanus spp. in the under-ice water layer (61.5% of the under-ice standing stock day–1), but comparatively low on Halectinosoma sp. in the bottom of the ice (3.8% of standing stock day–1). The observation of G. wilkitzkii preying on pelagic copepods in the under-ice water layer represents a hitherto unknown but obviously significant process and a new direction in the cryo-pelagic coupling in the Arctic marine ecosystem.