Phytoplankton dynamics and red tides (1987-1997) in the Bulgarian Black Sea

Time series of phytoplankton density, biomass and species composition in th e Bulgarian Black Sea (specifically Varna Bay) ate analysed. The main patte rns of year-to-year fluctuations in the phytoplankton community controlled by natural and man-induced factors are discussed. Many indications lead to the conclusion that phytoplankton blooms have not become less frequent in c omparison with the period of progressive eutrophication (1975-85), and ther e is a trend towards an increase in the number of blooming species involved . The community maintains the capacity to produce a lot of biomass. Areas o f high productivity occur mainly in coastal waters. In open sea areas the l evel of algal biomass is higher than in the 80s. irrespective of low silica te concentrations, in general the 90s are the period of diatom predominatio n. (C) 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All ri ghts reserved.