Cold water formed on the western continental shelf of the Black Sea in February 1990 was studied. The band of cold water follows the coast and interacts with coastal geometry, resulting in winter phytoplankton blooms particularly along the southwestern coast. Species composition and surface spatial distribution of net phytoplankton (> 55 mu m) showed an apparent species differentiation between the shelf and open ocean plankton. Offshore waters exhibit high diversity indices compared to shelf waters where the intense outburst induced by Chaetoceros sp. occurs. This species of diatom thrived successfully in the cold waters, reaching cell densities > 10(6) cells/l in the vicinity of Sakarya river and its eastward extension.