The phytoplankton population of the southwestern Black Sea in May 2001 was studied by taxonomic analysis using microscopic examination and by pigment analyses using high-performance liquid chromatography (HPLC). Pigment data, which identified phytoplankton assemblages dominated by dinoflagellates, diatoms and coccolithophores in May 2001, were compared to phytoplankton cell counts and biomass. There were significant (p < 0.002-0.01, r=0.56-0.67) relationships between the taxon-specific pigment concentrations and the taxon-specific cell numbers during this sampling period. The ratios of chlorophyll-a to the dominant accessory pigments calculated by multiple linear regressions were 1.2 (chlorophyll-a: peridinin) in dinoflagellates, 1.8 (chlorophyll-a: fucoxanthin) in diatoms, and 2.66 (chlorophyll-a: 19'-hex on oyloxyfucoxanthin) in coccolithophores. HPLC-determined chlorophyll-a biomass correlated well with the sum of the group-specific pigment biomass (p < 0.001, r(2) = 0.95). The phytoplankton assemblage as revealed by the microscopic and HPLC analyses was thus made up of common Black Sea groups showing that HPLC pigment analysis can be used to quantify phytoplankton assemblages in the Black Sea based on simple ratios. (c) 2006 Elsevier Ltd. All rights reserved.