Mesoscale distribution and advection of overwintering Calanus finmarchicus off the shelf of northern Norway

Data collected on a cruise in January 2008, using a laser optical plankton counter, conductivity-temperature-depth sensors, and a lowered acoustic Doppler current profiler, was used to study the mesoscale distribution and advection of overwintering Calanus finmarchicus in its deep water winter habitat off the shelf of northern Norway. The overwintering animals were generally located immediately below the Atlantic Water (AW) in Arctic Intermediate Water (AIW), in the 600-1200 m depth range. The depth of the interface between AW and AIW varied considerably in the area and this was clearly reflected in the C. finmarchicus distribution. Maximum abundance varied from about 80 ind m−3 to more than 200 ind m−3 at the different stations. Current measurements showed the richness of mesoscazle eddies, with speeds exceeding 70 cm s−1 at the surface and rapidly decreasing with depth. In the main overwintering layer the eddy features were also clearly seen, but with speeds generally below 20 cm s−1. C. finmarchicus were found in the whole survey area, but they were not homogeneously distributed. Advection of the copepods resulted in relatively high local rates of change in overwintering C. finmarchicus abundance with mean value of 8% per day in the area. It is clear that mesoscale physical processes greatly contribute to the variability in the abundance of overwintering C. finmarchicus off the shelf of northern Norway. The collected data are also a valuable addition to the generally sparse datasets on the C. finmarchicus winter distribution and the role of the Lofoten basin in the large scale system is also discussed.