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 cycling in surface waters, i.e. recycling within or export out of the mixed layer, 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, hydrographic and nutrient conditions. Chlorophyll a concentrations, phytoplankton production rates and zooplankton abundances were determined in the field, and potential rates of grazing by protozoa, copepods and appendicularians were calculated from abundances, using assumptions from the literature. To our knowledge, this is the first published attempt to assess concurrently the grazing of these three plankton groups in the Arctic. The production rate of small-sized phytoplankton was significantly higher in ice-free compared with ice-covered areas, but the biomasses of small-sized phytoplankton and zooplankton were not. Potential recycling, downward export and horizontal advection of phytoplankton were calculated by resolving carbon budgets for the mixed layer. A large fraction of the small-sized phytoplankton produced inside the polynya was advected horizontally to the ice-covered part of the NEW, where these algae were necessary to sustain the heterotrophic community. We conclude that the fate of small-sized phytoplankton production was mostly recycling (>70%). Downward export would have occurred infrequently, as a result of intense grazing by appendicularians. Size-differential pathways of carbon cycling in planktonic food webs are discussed.