Pathways of carbon cycling in marine surface waters: the fate of small-sized phytoplankton in the Northeast Water Polynya

The fate of small-sized phytoplankton (<5 mu m) and pathways of carbon cycl ing in surface waters, i.e. recycling within or export out of the mixed lay er, were investigated in the Northeast Water (NEW) Polynya (77-81 degrees N ) from 23 May to 22 July 1993. The sampling covered a wide range of ice, hy drographic and nutrient conditions. Chlorophyll a concentrations, phytoplan kton production rates and zooplankton abundances were determined in the fie ld, and potential rates of grazing by protozoa, copepods and appendicularia ns were calculated from abundances, using assumptions from the literature. To our knowledge, this is the first published attempt to assess concurrentl y the grazing of these three plankton groups in the Arctic. The production rate of small-sized phytoplankton was significantly higher in ice-free comp ared with ice-covered areas, but the biomasses of small-sized phytoplankton and zooplankton were not. Potential recycling, downward export and horizon tal advection of phytoplankton were calculated by resolving carbon budgets for the mixed layer. A large fraction of the small-sized phytoplankton prod uced inside the polynya was advected horizontally to the ice-covered part o f the NEW, where these algae were necessary to sustain the heterotrophic co mmunity. We conclude that the fate of small-sized phytoplankton production was mostly recycling (>70%). Downward export would have occurred infrequent ly, as a result of intense grazing by appendicularians. Size-differential p athways of carbon cycling in planktonic food webs are discussed.