Reasons for the Long-Term Variability of the Dissolved Oxygen Concentration in the Upper Layer of the Black Sea

According to archival data for the period from 1923 to 2022, the long-term (interdecadal and interannual) variability of the dissolved oxygen concentration and temperature in the surface layer of the Black Sea in the spring hydrological season (from May to June) is analyzed. The lowest oxygen concentrations over the entire period of instrumental observations (approximately 100 years) were observed on the northwestern shelf and in the open part of the Black Sea for the period from 1923 to 1932. The negative anomalies of dissolved oxygen exceeded (in absolute value) the corresponding anomalies noted during the period of eutrophication of the Black Sea waters in the last decades of the 20th century. The main reason for the extreme anomalies discovered in the 1920s is the increase in the temperature of the surface layer of the Black Sea waters during this period.