Phytoplankton blooms in the Western part of the Black Sea in the 1990s

Violeta Velikova
Institute of Fishery and Aquaculture Varna, National Center for Agrarian Sciences, 4, Primorski blvd, P.O. Box 72, Varna- 9000, Bulgaria

For the Black Sea ecosystem, the phenomenon of progressive eutrophication has been evident since the early 1970s. It has significantly influenced the level of phytoplankton density, biomass, species composition, and seasonal and inter-annual variability of the algal community. In the period from1972-90, the frequent and prolonged phytoplankton blooms made the Black Sea a famous reference point for water quality deterioration. The phytoplankton community of the Western Black Sea shifted from a highly diverse system to a virtually eutrophic phytoplankton culture. The substantial increase in loadings of inorganic nitrogen and phosphorus and decrease in silicon loading , compared to the1960s, induced numerous monospecific blooms, non-traditional summer maxima in biomass, and an increase in number of prominent, non-diatom outbursts. Since the early 1990s, the persistent reports about the irreversibly degrading Black Sea have started to disappear. A large data set, reflecting the temporal and spatial variability of microalgae in the Western part of the Black Sea in the 1990s, allows us to compare the present-day functioning of the phytoplankton community with previous periods, and to conclude that there are some positive signs of easement. The phytoplankton community maintains the capacity to produce large biomass, but with decreasing tendency. There was>an increase in Si:P and Si:N molar ratios and concurrent increase in the diversity of mass species (mainly diatoms), but only a few of them generated exceptional outbursts with a density of more than 10 million cells per litre. The observed positive tendencies in eutrophication alleviation are related to the economic status of the adjoining countries and consequent reduction of eutrophication.