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Phytoplankton abundance and seasonality in the northeastern Adriatic Sea were directly related to the annual regime of the Po River discharge in the period 2002–2007. In 2005, 2006, and 2007, there was a shift in the timing of the annual Po River maximum discharge and of phytoplankton maxima from spring to autumn. In addition, the eastern coastal area was greatly influenced by oligotrophic karstic rivers and the Eastern Adriatic Current. Dominant phytoplankton taxa were defined on the basis of abundance and frequency of appearance. Such an approach may help in the comparison of phytoplankton communities within different environments. The dominant taxa were the diatoms Cerataulina pelagica, Chaetoceros socialis, Chaetoceros vixvisibilis and Pseudo-nitzschia spp., which appeared at maximum abundances >106 cells l-1 in more than 20% of samples. Among other phytoplankton, the most common was the coccolithophorid Emiliania huxleyi (maximum abundance of 2×105 cells l-1 in 50% of samples), and the dinoflagellate Prorocentrum minimum (maximum abundance of 3×105 cells l-1 in 12% of samples). The present state of the phytoplankton community may provide valuable information for evaluation of possible future environmental changes in the shallow Mediterranean ecosystem.