Phytoplankton production in the North Water Polynya: size-fractions and carbon fluxes, April to July 1998

Abstract

In order to understand the mechanisms responsible for the high productivity and bio- geochemical cycling of carbon in the North Water Polynya (NOW), we determined physical proper- ties and nutrient concentrations of the upper water column, and phytoplankton production, during spring/summer (April to July) 1998. Phytoplankton production of total organic carbon (PTOC) was par- titioned into production of dissolved and particulate organic carbon (PDOC and PPOC, respectively), the latter being further partitioned into production of large and small phytoplankton (PL and PS, respectively) using 5 µm as threshold. The highest PTOC was 6 g C m -2 d -1 at peak bloom. The fraction of PDOC in PTOC was lower in periods of high PTOC than those of low PTOC. Averaged over the whole polynya for the sampling period, PDOC and PPOC accounted for 34 and 66% of the fixed carbon, respec- tively, and 81 and 19% of PPOC were in the PL and PS fractions, respectively. Variations in the inte- grated assimilation numbers of large and small phytoplankton were mostly explained by nutrients and irradiance. Even though PPOC was dominated by large phytoplankton, the sinking rates of the phytoplankton cells were relatively low (0 to 0.7 m d -1 ), hence low export of PPOC to depth (17%), and relatively high potential transfer to large pelagic organisms through the herbivorous food web. This explains why the NOW is a major feeding and spawning area for fish, mammals and birds.