Description: During a winter expedition to the western Barents Sea in March 2003, benthic amphipods of the species Anonyx sarsi were observed directly below pack ice. Only males and juveniles [16.0-37.0 mm long, 16.2-120.8 mg dry mass (DM)] were collected. Guts contained macroalgal fibres, fish eggs and flesh from large carrion. Amphipods had very low levels of total lipids (2.7-17.2% DM). Analysis of lipid biomarkers showed that some of the specimens had preyed on pelagic copepods. Individual respiration rates ranged over 0.4-1.7 ml O2/day (mean: 1.2 ml, SD: 0.5 ml). Individual ammonia excretion rates varied between 7.8 µg and 49.3 µg N/day (mean: 30.7 µg, SD: 15.2 µg). The atomic O:N ratio ranged over 35 to 71 (mean: 55, SD: 14), indicating lipid-dominated metabolism. Mass-specific respiration ranged over 9.8-16.6 ml O2/day/g DM (mean: 13.1 ml, SD: 2.2 ml). The metabolic rates of A. sarsi were twice as high as those of the truly sympagic amphipod Gammarus wilkitzkii, which is better adapted to the under-ice habitat by its energy-saving attached lifestyle. It is concluded that males and juveniles of A. sarsi were actively searching for food in the water column and at the ice underside, but that the nutritional status of the amphipods in late Arctic winter was generally very poor.

Description: Stable isotopes of carbon and nitrogen (δ13C, δ15N) were analyzed using a mass spectrometer with IAEA-VPDB (IAEA-C1) and atmospheric air (IAEA-N1) as standards, respectively. Two different approaches were used to determine a δ15N baseline for the food web of the northern Humboldt Current System. i) Particulate organic matter (POM) samples representing trophic position 1 and ii) Filter-feeding salps (Iasis cylindrica) as primary consumers representing trophic position 2. In general, low isotope ratios at the surface occurred in Thaliacea (I. cylindrica and Pyrosoma sp.). δ15N values of copepods inhabiting waters 〈50 m depth ranged from 3.9 ± 0.2‰ in Nannocalanus minor to 6.7 ± 0.6‰ in Calanus chilensis. Deeper-living copepods (〉200 m depth) within the oxygen minimum zone had about 4 to 8‰ higher δ15N values (~11-15‰) than surface living ones. For the diel vertical migrating euphausiids, mean δ15N and δ13C' values were in a similar range as for copepods from the upper water layers. The squat lobster Pleuroncodes monodon had extraordinarily high δ13C'. Among the key fish species, the eastern Pacific bonito Sarda chilensis had a slightly higher δ15N value compared to the anchovy Engraulis ringens. In some species a strong increase of the δ15N ratio of up to ~5‰ was determined from North (8.5°S) to South (16°S). These pronounced differences in d15N values highlight the importance of adequate baseline values to avoid misinterpretations of a species' regional feeding behaviour. In order to compensate for the reginal shift, using a filter-feeding salp as reference for herbivorous consumers seemed advantageous. In fact, calculated trophic positions for C. chilensis and other species did not differ between regions, when shifts in the baseline were taken into account.