ABSTRACT: We examined the species composition and concentration of planktonic algae as well as irradiance and near-surface temperature and salinity 3 times per week for 1 yr in a turbulent marine environment with continuously high nutrient concentrations. Seasonalvariations in temperature and salinity were small, and diatoms composed the largest percentage of phytoplankton (>=5 µm) cell numbers throughout the year. We observed at least 96 diatom species, of which some were present throughout most of the year whileothers were observed for periods as short as 1 to 2 wk. Diatom species diversity reached minimum and maximum values during winter and summer months, respectively, and was positively correlated to temperature and daylength and negatively correlated tosalinity. Winter and spring blooms occurred and were numerically dominated by small and large cells of Skeletonema costatum, respectively, and Pseudo-nitzschia delicatissima, resulting in minimum seasonal values of species evenness, whichpeaked during summer months. The first 4 axes of a canonical correspondence analysis (CCA) explained 76% of the variance in the diatom data; the measured environmental variables explained 90% of this variation, of which temperature had a dominant role. Wediscuss possible environmental optima of 44 diatom species based on results of CCA and make suggestions regarding altered measures of diversity.