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Bio-optical data, obtained during six cruises in the Black Sea carried out during periods of seasonal stratification in years between 1996 and 2016, have been used to parametrize phytoplankton light absorption () in the deep chlorophyll maximum (DCM) layer located near the bottom of euphotic zone. Relationships between and the sum of chlorophyll-a and phaeopigment concentrations (Chl-a) differed from those for the summertime upper mixed layer (UML). Notably, chlorophyll a specific absorption coefficients () were lower in the DCM and more comparable with values typical for winter phytoplankton in the Black Sea. The spectral shapes in the DCM differed markedly from those in winter and in the summer UML, due to a shoulder at ~490 nm and a local maximum at ~550 nm corresponding to the absorption bands of phycourobilin and phycoerythrobilin. Light absorbing properties of phytoplankton in the DCM (amplitude and spectral shape of ) reflected physiological acclimation to local conditions on the cellular level and population shifts leading to changes in the biomass-dominant species, with Synechococcus spp. domination in the DCM. The parameterization of phytoplankton absorption in the DCM will enable refined spectral models of the downwelling radiance and primary production in the Black Sea. [ABSTRACT FROM AUTHOR]

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 × 10 cells mL) to the eastern corner of the bay (28.7 ± 11.4 × 10 cells mL), while no relation was found between the water pollution status and Syn abundance (9.9 ± 8.7 × 10 cells mL; at all the stations, n = 27). Matter flows through the communities (daily production for Syn and Pico-E 06.6 and 09.3 µg C L day, respectively; grazing mortality for Syn and PicoE 0-3.6 and 0-21.2 µg C L day, respectively) were comparable to or even exceeded their biomass stocks (>0.05-6.8 and 0.9-26.5 µg C L 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. [ABSTRACT FROM AUTHOR]