Biogenic carbon and nitrogen export in a deep-convection region: simulations in the Labrador Sea

The Labrador Sea is a major sink of anthropogenic CO2 due to deep-water formation in winter. To investigate the relative importance of different forms of export flux, we used a physical-biogeochemical model to simulate the vertical fluxes of particulate and dissolved biogenic carbon as a function of winter convection, food web dynamics and zooplankton vertical migration. The C:N ratio of these export fluxes was simulated based on trophic dynamics and bacterial activity. The model was run using winter convection and seasonal mixed layer evolution extracted from multi-year physical data collected in the central Labrador Sea. Comparisons between model output and data from the Labrador Sea and other systems indicate that the model provides a realistic picture of carbon and nitrogen pools and fluxes. Our results suggest that on an annual basis, dissolved organic carbon (DOC) export by deep, vertical convection is greater than that of the sinking flux of POC. Furthermore, the C:N ratio of exported dissolved organic matter (DOM) is higher than that of the particle sinking flux, resulting in 23% more carbon exported than would be estimated if predictions were made from the Redfield ratio (e.g., 11.4 vs. 7.0 for DOM and particulate organic matter, respectively, at the bottom of the euphotic zone and 17.2 vs. 9.3 at 1000 m depth). The active export of carbon by the respiration and mortality of migrating zooplankton amounts to 19% of sinking flux annually, but only 6% of total carbon export because of the high rates of DOC export in deep-water formation regions. Our model simulations indicate that non-Redfield ratio DOC export characterizes the function of the biological pump in deep-water formation regions.