Plankton distributions related to hydrography and circulation dynamics on a narrow continental shelf off Northern Norway

Plankton distribution, hydrography and circulation dynamics were investigated in a shelf area off Northern Norway in June 2000 and June 2001. CTD and Optical Plankton Counter data were obtained from the upper 200 m by towing an undulating underwater vehicle across the continental shelf and shelf slope. In both years the front between Atlantic Water and Norwegian Coastal Water was sharp in the region west of the shelf break. In June 2000, convergence zones with lower salinities and higher temperatures than the ambient water were detected in the upper 30 m of the water column over the shelf with a corresponding increase in concentrations of phytoplankton and zooplankton. An interpretation of the field data combined with numerical 3D model simulations indicated that the observed patchiness of phytoplankton and zooplankton in these areas possibly originated from plumes of plankton-rich fjord water trapped in eddies over the banks. Such convergence structures were not observed in June 2001. The differing levels of freshwater discharge and the differing wind conditions before and during the two cruise periods can explain the observed differences in hydrography over the shelf, and are likely to have effect on the plankton distribution.