Ecosystem structure in the Iceland Sea and recent changes to the capelin (Mallotus villosus) population

The objective of the Iceland Sea Ecosystem Project was to analyse principal ecosystem patterns, including the life history of capelin. Ten surveys were conducted for this purpose between 2006 and 2008, and hydrographic data from 1970 to 2011 were analysed. Longterm temporal patterns indicate an increase in the temperature of surface and intermediate waters during recent years in the Iceland Sea. The phytoplankton consisted mainly of diatoms in spring, with flagellates and dinoflagellates appearing later in the year. Zooplankton and fish were dominated by a few species. In late summer, the pelagic foodweb ranged between trophic levels 2.4 and 3.6. Trends in nutrients and primary production showed a clear seasonal pattern with a spring bloom in late May, followed by a peak in secondary production in July/August. The spatial structure of the lower ecosystem levels in summer was stable across the 3 years, determined mainly by the structure of the water masses. The pelagic fish fauna consisted primarily of adult pelagic fish and transient numbers of larval species. The spatial pattern of capelin indicated northward displacement of 0-group capelin and westward displacement of older capelin in recent years. The annual biomass was estimated at 3.8 million tonnes of phytoplankton, similar to 21 million tonnes of zooplankton, and 1.0 million tonnes of fish.