Structure and seasonal trophodynamics of picophytoplankton in sevastopol bay and adjacent waters (the Black Sea)

Abstract

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 × 10³ cells mL^–1) to the eastern corner of the bay (28.7 ± 11.4 × 10³ cells mL^–1), while no relation was found between the water pollution status and Syn abundance (9.9 ± 8.7 × 10³ cells mL^–1; at all the stations, n = 27). Matter flows through the communities (daily production for Syn and Pico-E 0^–16.6 and 0^–19.3 µg C L^–1 day^–1, respectively; grazing mortality for Syn and PicoE 0–3.6 and 0–21.2 µg C L^–1 day^–1, respectively) were comparable to or even exceeded their biomass stocks (>0.05–6.8 and 0.9–26.5 µg 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/G < 1) and late summer (Syn: P/G > 1). 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.