Effect of water trophic status on bio-optical properties and productive characteristics of phytoplankton of the Black Sea coastal waters near Sevastopol

ABSTRACT Spectral bio-optical properties and phytoplankton photosynthetic parameters were measured in Black Sea coastal waters near Sevastopol in May–July 2020–2021. Chlorophyll a concentration (Chl-a) differed by more than one order of magnitude between stations (0.20–5.6 mg/m3) due to the effect of the Chyornaya river runoff. Light absorption coefficients by all optically active components as well as total non-water absorption coefficients co-varied with Chl-a, which reflected a link between trophic status and water optical properties. An increase in total non-water absorption resulted in a shift of the maximum of downwelling irradiance to the longer wavelength (from 511 nm to 567 nm). Phytoplankton photosynthetic parameters were depth-dependent variables due to phytoplankton acclimations to light intensity without dependence on water trophic status. Gross primary production was estimated by the full spectral approach. Primary production (1.3–31 mgC/m3/h) co-varied with Chl-a. Chlorophyll a specific photosynthesis rate (4.3–8.0 mgC/mgChl/h) varied by about twice at the sea surface but no dependence on Chl-a was revealed.