Changes in the Oxygen Regime in the Deep Part of the Black Sea in 1980–2019

Purpose. Assessing changes in the oxygen regime of the deep-water part of the Black Sea under the influence of climatic and anthropogenic factors over the past 40 years is the goal of the work. Methods and Results. For the analysis, the expedition data (2015–2019) and those from the Databank of the Marine Hydrophysical Institute, Russian Academy of Sciences, (1980–2013) were used. The data for the deep-water part of the Black Sea (the depths exceeding 200 m) were selected from the array. The data on hydrophysical and hydrochemical parameters were subjected to expert assessment, and those including random errors in the values were excluded from the array. Then the average profiles of temperature, salinity, oxygen concentration, oxygen saturation, and nitrates concentration were plotted in the density scale (σt). Averaging was carried out by the inverse distance method followed by additional smoothing by the low-frequency filtering method. Conclusions. Against the background of the tendency of temperature increase in the upper layers of the water column and decrease of winter convective mixing intensity, the oxygen supply diminishes in all the layers of the Black Sea aerobic zone. Having been analyzed, of the data for the 1980–2019 period showed that at the present stage, a "regime shift" resulting from the joint effect of climate changes and anthropogenic load took place in the ecosystem of the deep-water part of the Black Sea. Decrease in the oxygen saturation of water throughout the entire thickness of the aerobic zone shows that the process of the Black Sea basin eutrophication constitutes a significant factor affecting oxygen dynamics in the Black Sea waters.