Upward Nitrate Flux and Downward Particulate Organic Carbon (POC) Flux along a Gradient of Stratification and Turbulent Mixing in an Arctic Shelf Sea (Barents Sea)

Arctic shelf seas currently face a decline in sea ice cover, which may strengthen the stratification due to sea ice melt and associated surface water freshening, but declining sea ice also exposes previously ice-covered regions to wind mixing. We used the Barents Sea (BS) as a model area to examine the upward nitrate flux and the downward flux of particulate organic carbon (POC) along a gradient of ice cover, stratification and turbulence. Drift ice and a moderate halocline stratification apparently hindered wind-induced deep-mixing in the northern, arctic influenced BS. The upward nitrate flux was negligible (flux into mixing layer: 0.004 mmol nitrate m-2 d-1), and, despite an ice edge diatom peak bloom in surface waters, only a moderate downward POC flux was observed (40-200 m: 150-250 mg POC m-2 d-1). The Atlantic influenced southern BS was ice-free and weakly stratified and thus more prone to wind mixing. We observed here a strong upward nitrate flux (flux into mixing layer: 5.395 mmol nitrate m-2 d-1) and a strong downward POC flux (40-120 m: 260-600 mg POC m-2 d-1) during a deep-mixed post bloom situation with high zooplankton abundance. In future high latitude shelf seas, the downward POC flux during spring and summer may decline if the nitrate replenishment and primary production weakens due to halocline strengthening by ice melt. However, the downward POC flux may also increase in regions, where the absence of (summer) sea ice coincides with strong wind events, weak stratification and a shallow nitracline. Pulsed nitrate fluxes into the surface layers may there stimulate primary production during a summer post bloom situation and cause an enhanced downward POC flux, either by active biomass down-mixing by hydrographical processes or by re-packaging of the produced biomass into fast-sinking particles, such as e.g., zooplankton fecal pellets.