Trophic relationships and the role of Calanus in the oceanic ecosystems south and north of Iceland

Abstract

The waters south and north of Iceland vary greatly both oceanographically and biologically with the rather stable and warm Atlantic waters south and west of Iceland and the more variable and cold Arctic and sub-Arctic waters, north and east of Iceland. The aim of this study was to increase the knowledge on the role of Calanus copepods and trophic relations of the key components of the oceanic ecosystems south-west (over the Reykjanes Ridge) and north (in the Iceland Sea) of Iceland. The trophic relationships and energy transfer to higher trophic levels were estimated by using fatty acid trophic markers, by comparing fatty acid profiles among species and by applying stable isotopes of carbon and nitrogen. The inter-annual variability in abundance and community composition of zooplankton on the shelves south and north of Iceland in relation to environmental condition in spring were also observed. The energy rich Calanus species are key links between primary producers and higher trophic levels in the Icelandic waters. The Calanus species dominate the zooplankton biomass around Iceland and their markers (20:1n9 and 22:1n11) are found in high amount in animals at higher trophic levels. Calanus finmarchicus plays important role as a forage species in the Atlantic water south-west of Iceland while its high importance is replaced by the larger lipid rich C. hyperboreus in the Arctic and sub-Arctic waters north of Iceland. Even though Calanus based food-web is the main driver in both areas, there exist a pathway where Calanus species are of less importance and the energy is channeled via euphausiids to higher trophic levels. Around four trophic levels were observed in the two oceanic ecosystems where adult fish occupied the highest trophic levels. Over the Reykjanes Ridge vertically migrating mesopelagic fish, in pronounced deep-scattering layers, are actively bringing energy to deeper layers by feeding on C. finmarchicus in the upper layers. In the Iceland Sea comparable deep-scattering layers were not observed. Epipelagic schools of capelin (Mallotus villosus) are important component of the pelagic ecosystem in the Iceland Sea. After feeding intensively in the Iceland Sea, they migrate and thus transfer the energy to adjacent ecosystems. The ecosystem around Iceland is extremely sensitive to climate variations. Increase in sea temperature has been related to shift in distribution and feeding migrations of capelin in the Iceland Sea with the result of diet switch of older capelin. The importance of the Arctic amphipod Themisto libellula in the diet of adult capelin increased in their new distribution area in the colder western part of the Iceland Sea. A shift in distribution of species may thus influence the whole ecosystem structure and food web interactions in larger area. This study presents novel important knowledge on the food web structures and carbon flow in Icelandic waters.