The phytoplankton community structure and abundance in the south-eastern Black Sea was measured from February to December 2009 using and comparing high performance liquid chromatography pigment and microscopy analyses. The phytoplankton community was characterized by diatoms, dinoflagellates and coccolithophores, as revealed by both techniques. Fucoxanthin, diadinoxanthin, peridinin and 19'-hexanoyloxyfucoxanthin were the main accessory pigments showing significant correlation with diatom-C r(2) = 0.56-0.71, P < 0.05), diatom-C (r(2) = 0.85-0.91, P < 0.001), dinoflagellate-C (r(2) = 0.39-0.88, P < 0.05) and coccolithophore-C (r(2) = 0.80-0.71, P < 0.05), respectively. Microscopy counts indicated a total of 89 species, 71% of which were dinoflagellates, 23% were diatoms and 6% other species (mainly coccolithophores). Pigment-CHEMTAX analysis also indicated the presence of pico- and nanoplankton. Phytoplankton carbon (phyto-C) concentrations were highest in the upper water column, whereas chlorophyll-a (Chl-a) showed a deep maximum. Average phyto-C was higher at the coastal station (291 +/- 66 mu g l(-1)) than at the offshore station (258 +/- 35 mu g l(-1)), not statistically different (P > 0.05). The coastal station also had higher Chl-a concentrations (0.52-3.83 mu g l(-1)) compared to the offshore station (0.63-2.55 mu g l(-1)), not significant (P > 0.05). Our results are consistent with other studies and indicate that the southern Black Sea is shifting towards mesotrophy with the increasing prevalence of dinoflagellates compared to diatoms.