ABSTRACT: In order to understand the mechanisms responsible for the high productivity and biogeochemical cycling of carbon in the North Water Polynya (NOW), we determined physical properties and nutrient concentrations of the upper water column, andphytoplankton production, during spring/summer (April to July) 1998. Phytoplankton production of total organic carbon (P_TOC) was partitioned into production of dissolved and particulate organic carbon (P_DOC andP_POC, respectively), the latter being further partitioned into production of large and small phytoplankton (P_L and P_S, respectively) using 5 µm as threshold. The highest P_TOC was 6 gCm^-2 d^-1 at peak bloom. The fraction of P_DOC in P_TOC was lower in periods of high P_TOC than those of low P_TOC. Averaged over the whole polynya for the samplingperiod, P_DOC and P_POC accounted for 34 and 66% of the fixed carbon, respectively, and 81 and 19% of P_POC were in the P_L and P_S fractions, respectively. Variations inthe integrated assimilation numbers of large and small phytoplankton were mostly explained by nutrients and irradiance. Even though P_POC was dominated by large phytoplankton, the sinking rates of the phytoplankton cells were relativelylow (0 to 0.7 m d^-1), hence low export of P_POC 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 areafor fish, mammals and birds.