Overwintering habitat of Calanus finmarchicus in the North Atlantic inferred from autonomous profiling floats

Autonomous instrumented floats deployed by the international Argo programme and its predecessors were used to produce inferred temperature–salinity–depth records for overwintering plankton in the North Atlantic, with special reference to Calanus finmarchicus (the biomass-dominant mesozooplankton in the region). In addition, a model for the density of a plankter was used to estimate changes in vertical position over time. Results from 456 individual winter float records show that environmental conditions were most stable in the Nordic Seas, less so in the subarctic basins of the Labrador Sea, Irminger Sea, and Iceland Basin, and most variable in regions to the south. Model results show that neutral buoyancy at the beginning of the overwintering period is insufficient to guarantee that individuals will stay at depth for the entire overwintering period, which suggests some kind of active buoyancy control mechanism is at work.