Trophic interactions among zooplankton and fish species within the pelagic ecosystem of the Iceland Sea

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Trophic relationships of important pelagic zooplankton and fish species were investigated in August 2007 and 2008 in the Iceland Sea, north of Iceland. Carbon and nitrogen stable isotopes and fatty acid biomarkers were used to study trophic linkages and trophic ecology of the most important pelagic species in this ecosystem. It is concluded that there are 3–4 trophic levels in this pelagic ecosystem, excluding birds and mammals. The primarily herbivorous copepod Calanus hyperboreus occupies the lowest trophic level of the animal species studied but adults of capelin (Mallotus villosus) and blue whiting (Micromesistius poutassou) the highest. Calanus spp. proved to be an important diet component of most of the studied species. The euphausiid species Thysanoessa inermis and T. longicaudata, however, are exceptions as Calanus spp. are of minor importance in their diet. The chaetognath Eukrohnia hamata is a pure carnivore, feeding heavily on Calanus spp., while most of the other zooplankton species studied practise omnivorous–carnivorous feeding mode. Young euphausiids are an important food component for capelin larvae, and the amphipod species Themisto libellula is important in the diet of adult capelin. The importance of Calanus spp. or a Calanus‐derived diet increases with the size of capelin. Adults of capelin and blue whiting share the same feeding habits and could therefore be competing for food.