Long-term variability of phytoplankton communities in open Black Sea in relation to environmental changes

Data on total biomass and taxonomic structure of phytoplankton communities, collected in the Black Sea during the period 1978-1992 were analysed. Comparisons were restricted to the biological summer-autumn season, when the ecosystem is in most stable state. Changes in phytoplankton communities were investigated in the entire basin as well as in five regions of the Black Sea: central cyclonic gyres, central convergence zone, western and eastern parts of Rim Current. The range of interannual oscillations was comparable with seasonal changes. The mean biomass of the total phytoplankton in the water column varied by about 5 times, from 4 to 21 g m(-2), in the period. The general trend of biomass increase from 1978 to 1992 was observed in the 5 studied regions as well as in the entire basin. The highest phytoplankton biomass was observed in the western part of Rim Current. An increase in large phytoplankton groups was evident in the 1992-1995 period, during which an increase of diatoms and a decrease of dinoflagellates was also observed. The analysis of changes in zooplankton communities occurring after the Mnemiopsis intrusion have shown that grazing control was not the main factor determining total phytoplankton abundance in deep waters of the Black Sea. Changes in grazing control could have defined the changes in size and taxonomic structure of the phytoplankton community. On the other hand, interannual oscillations of average air temperature in winter appears inversely correlated to mean phytoplankton biomass in summer, suggesting that bulk of tile phytoplankton in the summer-autumn season is linked to intensification of general hydrodynamic activity in the Black Sea driven by climatic variations. A review of earlier data on phytoplankton, collected during the 1948-1976 period is also consistent with this hypothesis.