From fringe to basin: unravelling the survival strategies of Calanus hyperboreus and C. glacialis in the Arctic Ocean

ABSTRACT: The large calanoids Calanus hyperboreus and C. glacialis dominate the zooplankton biomass in the central Arctic Ocean (CAO), but the absence of early life stages has raised speculation whether they complete their life cycle there, or whether they represent expatriates advected from adjacent regions. Our study, conducted across 2 transects of the CAO during fall 2011, focused on the distribution, stage composition, dry weight, individual lipid content, and egg production of these species. Although reproductive activity and early developmental stages were observed only on the fringes of the deep basins, late-stage copepodite and adult female abundances remained steady across the study area for C. glacialis and increased away from the shelves for C. hyperboreus. We found no decline in lipid content or dry weight in adult C. glacialis away from productive regions and only a minor reduction in adult C. hyperboreus. However, the lipid content and dry weight in C5 copepodites significantly decreased away from the shelf break, particularly in C. hyperboreus. This suggests that although early life stages struggle to survive in the resource-limited conditions of the deep CAO and even subadults remain vulnerable to starvation, adults have the resilience to survive long enough to be eventually transported by ocean currents to more favourable regions for reproduction. As such, we suggest that both species of Calanus are neither ‘residents’ nor ‘expatriates’ in the Arctic basins, but rather ontogenetic migrants that take advantage of different habitats within the Arctic Ocean to maximise their survival and reproductive success.