Trophic interactions of meso- and macrozooplankton and fish in the Iceland Sea as evaluated by fatty acid and stable isotope analysis

A trophic study was carried out in August of 2007 and 2008 on the pelagic ecosystem in the Subarctic Iceland Sea. Carbon and nitrogen stable isotopes and fatty acid biomarkers were used to study trophic linkages and the trophic ecology of the most important pelagic species in this ecosystem, with emphasis on capelin (Mallotus villosus). According to 15N enrichment results, there are 3–4 trophic levels in this ecosystem excluding organisms of the microbial loop and birds and mammals. The primarily herbivorous copepod Calanus hyperboreus occupies the lowest trophic level of the animal species studied, and adult capelin and blue whiting (Micromesistius poutassou) occupy the highest level. Calanus spp. proved to be an important dietary component of most of the species studied, the euphausiid species Thysanoessa inermis and T. longicaudata being exceptions. The chaetognath Eukrohnia hamata is a pure carnivore, feeding heavily on Calanus spp., whereas most of the other zooplankton species studied practice an omnivorous–carnivorous feeding mode. The amphipod species Themisto libellula is important in the diet of adult capelin. Adult capelin and blue whiting share the same feeding habits and could therefore be competing for food.