Winter mortality in Calanus populations in two northern Norwegian fjords from 1984 to 2016

Abstract

Marine copepods of the genus Calanus have been extensively studied due to their central position in Arctic and Sub-Arctic food webs. Yet, our insight into how mortality in Calanus populations varies in time and space remains limited and restricts our ability to develop mechanistic understanding of the population dynamics of these key species. Based on 33 years of zooplankton data, collected in two Sub-Arctic fjords in Northern Norway, both at the onset of (October) and toward the end of winter (February), we estimated interannual variability in wintertime mortality rates for overwintering populations of Calanus spp. (C. finmarchicus and C. glacialis) and C. hyperboreus. Available hydrographical and meteorological information indicated that the influence of advection on the fjord basins were limited and was mainly associated with extreme weather conditions (strong winds or current). Estimated mortality of Calanus spp. was higher in the fjord where potential net-caught predators were more abundant (6.7 vs. 12.0 × 10−3 day−1), while mortality in the larger C. hyperboreus varied less between years and fjords (ca 4.5 × 10−3 day−1). This corroborates the view that predation is a key factor regulating loss from high-latitude Calanus populations in winter, but specific relationships between predators and Calanus mortality could not be established. These findings provide an outline of the relative potential of biological and physical mechanisms behind variability in Calanus mortality during overwintering. This approach highlights the need of integrative studies, combining advection and trophodynamics, in the study of fjord ecology.