Long-term Transparency Variability of the Black Sea Surface Waters

Lee M. E., Latushkin A. A., Martynov O. V.

Changes in the transparency of the surface waters of the Black Sea are analyzed on the basis of long-term Secchi depth definitions. The analysis used literature data and the results of definitions during expeditions on research vessels and a stationary oceanographic platform. It is noted that after a catastrophic recession of the surface waters transparency in the early 1990s, a gradual increase in its values occurred, and the situation was returned to the one earlier observed in the 1960—1980. The decrease in transparency was due to the reorganization of the plankton community structure of the Black Sea waters owing to the arrival of large amounts of anthropogenic origin nutrients with the river runoff, as well as the appearance of the invader — ctenophore Mnemiopsis. The subsequent improvement of the situation was due to the crisis phenomena in the economic activity of the coastal countries, and the emergence of the second invasive ctenophore Beroe ovata, which reduced the impact of the ctenophore Mnemiopsis on the plankton community. It is shown that a complete restoration of the ecosystem of the sea did not occur, and as a consequence of this, relapses of abnormal phytoplankton bloom sometimes take place. However, the duration of the ecosystem restoration after relapse is now 3—4 months, unlike the several years in the early 1990s.

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