Biogenic Gas (CH4, N2O, DMS) Emission to the Atmosphere from Near-shore and Shelf Waters of the North-western Black Sea

The marine emissions of biogenic gases (CH4, N2O, DMS) were determined on the north-western shelf of the Black Sea in July 1995. Methane (CH4) saturation ratios (SR) averaged 930% (173–10 500%) and were inversely proportional to salinity. The mean nitrous oxide (N2O) SR was 111% (96–149%), which is similar to the values found in other coastal water masses, and did not present any significant variability as a function of salinity. Dimethyl sulphide (DMS) concentrations ranged between 2·96 and 11·9 nM and chlorophyll a-normalized DMS concentrations were found to increase exponentially with salinity, illustrating the influence of plankton speciation. Coastal and shelf waters affected by riverine inputs, which represent 30% of the Black Sea surface area, contributed up to 53% of the CH4 flux during the summer period. Biogenic sulphur emissions, primarily as DMS, showed a strong gradient, increasing from open Black Sea waters to estuarine Danube waters. There was less variation in the flux densities of N2O for the different water masses. By extrapolating values obtained during summer 1995, upper limits for the emissions from the Black Sea to the atmosphere were estimated to be 6·7–11·7 Gmol yr 1 of CH4, 0·45–0·75 Gmol yr 1 of N2O and 0·30–0·80 Gmol yr 1 of DMS, using two different air–sea gas exchange models.