Diapycnal transport of oxygen in the north-western Black Sea

The Black Sea is characterised by a persistent, strong stratification, which separates fresher surface water from more saline deep water. This stratification dampens vertical mixing and limits the ventilation of the deep water, thereby contributing to the oxygen deficiency underneath the pycnocline. However, plankton primary production regularly creates oxygen supersaturation at the pycnocline and thus improves the availability of oxygen in the pycnocline.We have measured vertical profiles of salinity, temperature, and oxygen in the north-western shelf of the Black Sea in late summer. The spatial variance of water density enabled us to estimate the vertical turbulent diffusivity. The spatial variance of oxygen concentration enabled us to employ a 1D reaction- transport model to estimate pelagic rates of oxygen production and consumption, and vertical fluxes of oxygen across the pycnocline. These model-based estimates are complemented by direct measurements of pelagic respiration rates and primary production rates. The combined results enable us to quantify oxygen fluxes across the density gradient and to elucidate the significance of deep primary production for the ventilation of the deep water.