This study was carried out on the qualitative and quantitative characteristics of phytoplankton community structure and the environmental factors that affect its distribution and the changes in Izmit Bay, which is an eutrophic bay of Marmara Sea. The water samples were collected bimonthly between February 1999 and September 2000 from the 10 sampling sites (from surface, -5 m, -10 m, and -20 m). Phytoplankton densities were calculated as cells l(-1), biomass was estimated by biovolumes and chlorophyll a content. During the study period, a total of 77 taxa from five algal classes, Bacillariophyceae, Dinophyceae, Chrysophyceae, Haptophyceae and Dicthyochophyceae were determined. Species composition of phytoplankton was typical of eutrophic conditions and was frequently characterized by the presence of Dinophyceae. In general, the most important groups were Dinophyceae and Bacillariophyceae in species numbers and density. The bay was characterized by intensive dinoflagellate (mainly Prorocentrum spp.) dominated bloom in all sampling periods, while the diatoms (shifts were observed in dominant diatoms) contributed most to the settling phytoplankton biomass. Differences among sampling sites and depths were recorded in terms of phytoplankton density, biovolumes and chlorophyll a values. Correlation between chlorophyll a, cell count and biomass was significant. In general, the bay sustained a poor phytoplankton community with higher diversity relatively towards the outer than the interior zone in terms of Shannon-Weaver diversity index (H'). Some environmental parameters seem to play an important role in determining the phytoplankton community succession and then diversity, favoring or limiting the growth of the different groups of the phytoplankton. The Secchi disc depth, an indicator of primary production in the marine environment was comparatively low and limited vertical distribution of phytoplankton. The results indicate that primary production of the bay may be limited by nitrogen and silicate in some periods. (c) 2005 Elsevier Ltd. All rights reserved.