Abstract <p>Two spring events of abnormally low dissolved oxygen concentrations in surface waters of the northwestern Black Sea were detected over a hundred-year period in [26]. The presence of the first extreme in the 1920s and 1930s was explained by an abnormal increase in temperature, and the second extreme in the last decades of the 20th century was explained by eutrophication. This work shows that the decrease in dissolved oxygen concentrations in the upper 10-m layer coinciding in time with the warming in the first third of the 20th century was caused not only by abnormally warm climatic conditions, but also by extremely low river runoff into the northwestern Black Sea in that period. Reduced discharge of fresh river waters saturated with oxygen (especially pronounced during the period of increased runoff in spring) was one of the reasons of an increase in the salinity of the surface sea layer and a sharp decrease in the concentration of dissolved oxygen. Direct measurements have shown that water transparency on the northwestern shelf of the Black Sea increased in the first third of the 20th century, confirming the weakening of the influx of turbid river waters during that period.</p>