On Changes in the Hydrophysical and Hydrochemical Structure of the Upper 200-Meter Layer of the Black Sea in the Last Decade and a Half

According to the long-term monitoring of the northeastern part of the Black Sea at the Gelendzhik research site of the Shirshov Institute of Oceanology, Russian Academy of Sciences, a salinization and warming trend of the Black Sea surface waters has been observed in the last 15 years. As a result, the salinity of the surface water layer has increased by approximately 0.5 PSU, and its temperature has increased by 0.5°C over these years. Alongside with that, an increase in water density in the upper 300-m layer has been recorded as a rise in isopycnals and a decrease in their depth. In the period of 2014–2024, the redox interface depth did not change and amounted to 156 ± 9 m in the observation area, while the potential density at which sulfide appeared changed from 16.15 ± 0.02 to 16.25 ± 0.02 kg/m3. It follows that the generally accepted relation of the appearance and disappearance of hydrochemical parameters to certain water density values should not be considered as absolute and constant over time. Under the revealed salinization and warming of the active layer of the Black Sea waters in the last 15 years, the oxygen and sulfide distribution in terms of depth can be considered as a more stable parameter, less susceptible to climate change, at least during the observation period of 2014–2024 described in this paper.