The species composition, abundance, and biomass of micro- (>15 mum) and nano- (<15 mu m) phytoplankton were studied along the southern Black Sea during June-July 1996 and March-April and September 1998. A total of 150 species were identified, similar to 50% of them being dinoflagellates. The average total phytoplankton abundance changed from 77 x 10(3) Cells l(-1) in spring to 110 x 10(3) cells l(-1) in autumn and biomass from 250 mu g l(-1) in summer to 1370 mu g l(-1) in spring. Based on the extensive sampling grid from June-July 1996, phytoplankton seemed to have a rather homogeneous biomass distribution in the southern Black Sea. In all periods, the coccolithophorid Emiliania huxleyi was the most abundant species, its contribution to the total abundance ranging from 73% in autumn to 43% in spring. However, in terms of biomass, diatoms made up the bulk of phytoplankton in spring (97%, majority being Proboscia alata) and autumn (73%, majority being Pseudosolenia calcar-avis), and dinoflagellates in summer (74%, Gymnodinium sp.). There was a remarkable similarity in the dominant species between the western and eastern regions of the southern Black Sea, indicating transport of phytoplankton within the basin. (C) 2003 Elsevier Science B.V. All rights reserved.