Variations in the chemistry of the Black Sea on a time scale of decades (1960–1995)

We use data collected on 52 cruises from 1960 to 1995 to analyze the decadal variability in basic chemical properties (oxygen, sulfide, nitrate, phosphate and silicate) throughout the water column of the Black Sea. This analysis reveals some new features in the recent evolution of the chemical structure of the Black Sea: an increase in the inventory of sulfide and nutrients in the anoxic zone, a decrease in the inventory of oxygen in the upper layer, and variations in the thickness of the sub-oxic zone. These changes suggest that an increase in the flux of sinking particulate organic matter (POM) has perturbed the biogeochemistry of the Black Sea. The flux balance that controlled the distribution of nutrients before the early 1970s has been modified due to intensive eutrophication. That has led to an increase in the inventory of nutrients in the anoxic zone due to an increased flux of POM. This increased flux of POM increased the rate of sulfate reduction and created an imbalance in the sulfide budget. As a result, sulfide concentrations have increased in the anoxic zone over the past 20 to 25 years. Additional anthropogenic changes are reflected in the distributions of nitrate and oxygen. Climate related and anthropogenic changes in the sources and sinks of these compounds appear to be equally important for causing variations in their inventory. Unlike oxygen and nitrate, the budget of sulfide depends primarily on the flux of sinking POM and the intensity of the lateral flux of oxygen contained in the Bosporus plume. More than 50% of sulfide production appears to be oxidized by the lateral ventilation flux of oxygen. Thus, different processes govern the dynamics of the upper and lower boundaries of the sub-oxic zone. Variations in the upper boundary of suboxic zone depend on consumption of oxygen by the flux of sinking POM, while the lower boundary is controlled by the balance between the upward flux of sulfide within the anoxic zone and the lateral flux of oxygen associated with the Bosporus plume.