Fluxes and budget of sulphide and ammonia in the Black Sea anoxic layer

Using data on average distribution, and applying profiles of the vertical velocity and turbulent diffusivity derived from a stationary one-dimensional model of ventilation of the Black Sea, we calculate and discuss the fluxes and production/consumption rates of ammonia and sulphide in the anoxic layer of the Black Sea in the 1960s and 1990s. The results reveal that sulphide, in terms of its flux and production/consumption rate, is not equivalent to ammonia. The existence of a lateral flux of oxygen, as we believe, associated with the Bosphorus downflow is the basic reason making the difference. Both ammonia and sulphide are of biogenic origin, but a significant part of sulphide is oxidised within the upper 150–250 m layer of the anoxic zone, while ammonia, possessing a higher red-ox potential, is oxidised in the vicinity of the onset of sulphide. The calculated sulphide consumption rate, which is derived from the observed vertical profiles, varies insignificantly for the 1960s and 1990s. The rate of sulphide production derived from the published data on the rate of primary production, sulphate reduction, ammonia oxidation and flux of sinking POM was close to the calculated consumption rate of sulphide in the 1960s, but considerably exceeded it in the 1990s. This suggests that sulphide inventory might increase by the 1990s. The rate of possible changes in the inventory of sulphide is less than 1% per year for the layer above 1000 m, suggesting a slow pace of temporal change for sulphide.