Trophic interactions and energy flow within the pelagic ecosystem in the Iceland Sea

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A trophic study was carried out in August 2007 on the pelagic ecosystem in the sub-arctic Iceland Sea, north of Iceland, using carbon and nitrogen stable isotopes and fatty acid biomarkers. The aim was to study trophic linkages and positions of the most important pelagic species in this ecosystem with special emphasis on the trophic ecology of capelin. According to 15N enrichment it is concluded that there are around 4 trophic levels in this pelagic ecosystem excluding bird and mammals, where the primarily herbivorous copepod Calanus finmarchicus occupies the lowest trophic level of the studied species and adults of capelin (Mallotus villosus) and blue whiting (Micromesistius poutassou) the highest. Calanus spp. proved to be important diet component (high amount of Calanus fatty acid trophic markers in the neutral lipid fraction), of most of the studied species. However the euphausiid species Thysanoessa inermis and T. longicaudata are exceptions as Calanus spp. are of minor importance in their diet. The chaetognath, Eukrohnia hamata, is a pure carnivore, feeding almost exclusively on Calanus spp., while most of the others zooplankton species studied practice omnivorous-carnivorous feeding mode. The capelin fatty acid profiles, indicate a shift in diet after metamorphosis when the capelin gets more mobile. The importance of Calanus spp. or 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. Multivariate statistical methods were performed on fatty acid compositional data making it possible to detect relationships and patterns in the data. This study is a part of an ecological study in the Iceland Sea, with field work lasting from 2006-2008.