Abundance and seasonal trophodynamics (specific growth rate, daily production, and grazing mortality) of the major picophytoplankton components, Synechococcus cyanobacteria (Syn) and picoeukaryotes (Pico-E), were studied at three stations in Sevastopol Bay and adjacent coastal waters (the Black Sea) in 2014 by flow cytometry and the dilution method. Pico-E abundance was shown to increase along the nutrient and pollution gradient from the coastal waters outside the bay (annual average of 7.3 +/- 5.4 x 10(3) cells mL(-1)) to the eastern corner of the bay (28.7 +/- 11.4 x 10(3) cells mL(-1)), while no relation was found between the water pollution status and Syn abundance (9.9 +/- 8.7 x 10(3) cells mL(-1); at all the stations, n = 27). Matter flows through the communities (daily production for Syn and Pico-E 0(-1)6.6 and 0(-1)9.3 A mu g C L-1 day(-1), respectively; grazing mortality for Syn and PicoE 0-3.6 and 0-21.2 A mu g C L-1 day(-1), respectively) were comparable to or even exceeded their biomass stocks (>0.05-6.8 and 0.9-26.5 A mu g C L-1 for Syn and PicoE, respectively), indicating high biomass turnover rates. The highest flow-to-stock ratio (up to 6 for Syn) and a significant imbalance between daily production (P) and grazing mortality (G) were observed in the most polluted and eutrophic waters of the bay in spring (Pico-E: P/G1). Black River inflow to the bay was hypothesized to be among the mechanisms maintaining this pronounced and long-term imbalance in the open system without any negative consequences for the picophytoplankton assemblages.