Recent State of Phytoplankton Community in the Varna Bay

Institute of Fisheries and Aquaculture, BG - 9000 Varna, Bulgaria


PETROVA, D., V. VELIKOVA and D. GERDJIKOV, 2006. Recent state of phytoplankton community in the Varna Bay. Bulg. J. Agric. Sci., 12: 247-260

Varna Bay is one of the hot spots along the Bulgarian Black Sea coastal zone. Its ecological state is strongly influenced by the connection with the Varna lake. Along the lake coast are located many sources of pollution such as rivers, ports, chemical industry.
In the Black Sea, as a semi-enclosed basin with a large river discharge, the phytoplankton community manifests naturally predetermined high level of productivity and blooms have never been something extraordinary, especially in coastal waters. However, during the period of progressive eutrophication (1970s, 1980s) the microalgae in the Black Sea was too repeatedly referred to an "eutrophic plankton culture" and "marine seaweeds". In the 1990s signs of relaxation in the frequency and magnitude of phytoplankton blooms were reported.
Results of 26 cruises in this period of the studies are reported. The horizontal and vertical distrubution of phytoplankton, species composition, seasonal dynamics of density and biom-ass of microalgaes are discussed in this paper. A good half carried out within the Project "Nutrient management in the Danube basin and its impact on the Black Sea" (DANUBS-EVK1-CT-2000-00051).
Since 1995 decreasing tendency of phytoplankton biomass was observed in the Bulgarian Black Sea. In 1998 - 2003 this tendency was confirmed and the level of algal biomass was lower even the comparison with previous period 1991 - 1997. These periods was characterized by moderate diversity of phytoplankton with predomination of a few Bacillariophyceae and Dinophyceae species - Skeletonema costatum, Cerataulina pelagica, Chaetoceros socialis, Emiliania huxleyi, etc. The maximal algal biomasses was created in spring and summer.

Key words: phytoplankton, density, biomass, annual dynamics, Varna Bay