Inter-annual and decadal variability of Calanus finmarchicus and C. hyperboreus in Subarctic waters north of Iceland 1990-2020

This study compares inter-annual dynamics of the two biomass dominant copepods in the Iceland Sea, the North Atlantic species Calanus finmarchicus and the Arctic C. hyperboreus, in an era of relatively low temperatures in the beginning of the study period (1990-1995) and higher temperatures thereafter. Samples were collected annually in May along a cross-shore transect of eight stations. The long-term variability in abundance of the two Calanus species was analysed in relation to hydrography, nutrients and phytoplankton dynamics, and large-scale climatic changes in the North Atlantic Ocean. Sea surface temperatures showed an increasing trend from the beginning to the end of the time series for all stations. From the end of the 1990s, C. finmarchicusshowed an increasing trend at the outermost station, whereas C. hyperboreus generally showed a decreasing trend along the whole transect from the mid-2000s. For both species, temperature was the single most important environmental factor explaining variance in abundance, however with opposite effects for the Arctic (C. hyperboreus) and the Atlantic (C. finmarchicus) species. It is hypothesized that warming north of Iceland may lead to increase of C. finmarchicus abundance through increased recruitment and advection, and decreased C. hyperboreus abundance. The change in species composition may in turn influence upper trophic levels, e.g. the capelin, the region’s main planktivorous fish.