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Mankovsky V.I., Vladimirov V.L., Afonin E.I., Mishonov A.V., Solov'ev M.V., Anninskiy B.E., Georgieva L.V., Yunev O.A.
Long-term variability of the Black Sea water transparency and factors determined its strong decrease in the late 1980s early 1990s. - 1996, 32pp. MHI & IBSS Preprint, Sevastopol. (In Russian).


The 72-years long data series (1923-1995) of the Black Sea water transparency had been analyzed using the results of the Secchi disk depth measurements (Zd). Some other hydrooptical (vertical profiles and spectral distribution of the attenuation coefficient, water color, radiance index) and biochemical variables (chlorophyll "a", pheophytin, primary production, particulate and dissolved matter, biomass of microzooplankton, jelly-fishes biomass; biomass, quantity and types of phytoplankton) were Included in the analysis.

Transparency in the deep central part of the sea (mean annual values) during this period changed more than three times, from 21.3 to 6.2 meters. The most pronounced variability of Zd had been found from 1986 till 1995, when the values of Zd decreased from 16m in 1986 to 6.2m in 1992, and increased again to 17.7m in 1995.

The principal mechanisms responsible for these changes were determined using modeling, statistical processing, and analysis of the data stored in the multivariable biooptical database.

There are three main reasons responsible for the strong water transparency decrease: the 11-year cycle of solar activity, significant eutrophication occurs both in the near-shore and deep areas of the Black Sea, and the influence of the biological invader Mnemiopsis Leidyi on the ecosystem.

The enhanced bloom of Coccolitophores was observed in the Black Sea during the drastic transparency decrease from 1986 to 1992. Their content in the Black Sea was more than 1.5 - 2 orders of magnitude larger than in previous years and reached 2-3 billions per cubic meter. A significant increase of this type of plankton has changed the structure of the plankton community. Now nanoplankton, which contributes immensely to the light scattering, accounts for 90% of the phytoplankton content.

Phytoplankton structure was strongly impacted by the biological invader Mnemiopsis, which appeared in the Black Sea in the early 1980s, and whose biomass amounted to one billion tons in 1989. As a result, the amount of herbivorous microzooplankton, which is a part of the Mnemiopsis food, has decreased by several times, whereas the phytoplankton content conversely, increased. The great amount of dissolved organic matter released by Mnemiopsis also facilitated the increase of some types of phytoplankton and bacteria.

Periodical oscillations in the long-term variability of the Black Sea water transparency have been found to occur, which correlate with the 11-year cycle of solar activity. The time lag between extremums of the transparency and solar activity is two-three years.

Model calculations of the spectral attenuation coefficient Secchi disk depth and spectral radiance index were performed. They have confirmed that the current phytoplankton composition and concentration can explain the observed changes of water transparency.