Abstract
Archival data on the concentration of dissolved oxygen and the temperature of the upper layer of the Black Sea over the past 100 years are analyzed. It is found that in both the northwestern and open parts of the sea in the period from 1923 to 1932, the lowest concentrations of dissolved oxygen in the past 100 years were observed. They were equal to about 260 \(\mu\) M, which was significantly (by 14–33%) lower than the concentration of oxygen in the upper layer of different parts of the Black Sea observed during the period of its decline 50 years later. It is shown that the decrease in oxygen levels in the 1920s–early 1930s was primarily due to the anomalous heating of the surface water layer during this period. This is its fundamental distinction from the decline in dissolved oxygen that falls on the last quarter of the 20th century and the beginning of the 21st century. The main reason for occurrence of the secondary minimum of oxygen was eutrophication of water, which started in the northwestern part of the sea and then spread to the inner parts of the water area.
