Variability of Phytoplankton and Bacterioplankton Functional Activity in Plumes of Rivers with Different Trophicity in the Northeastern Black Sea

The study considers the spatial variability of phytoplankton abundance and photosynthetic capacity, as well as bacterioplankton abundance and activity, in the plume of small rivers of the Black Sea with different hydrological characteristics, such as the Tuapse and Shakhe. The maximum quantum efficiency of phytoplankton photosystem II ( $${{{{F}_{{\text{v}}}}} \mathord{\left/ {\vphantom {{{{F}_{{\text{v}}}}} {{{F}_{{\text{m}}}}}}} \right. \kern-0em} {{{F}_{{\text{m}}}}}}$$ ), characterizing the maximum photosynthetic potential, was high in both plumes (0.516–0.669), while the activity in both cases was higher in the freshened mouth zones of these plumes. The maximum electron transport rate in the phytoplankton photosystem II (rETRmax), characterizing the maximum possible photosynthetic activity in the mouth zones, exceeded almost twice the values obtained in the seaward part of these plumes. In contrast to phytoplankton, bacteria metabolic activity was higher in the oligotrophic Shakhe River than in the eutrophic Tuapse River. The proportion of active bacteria in the total abundance was 5 and 1%, respectively. The maximum proportion of active bacteria in the desalinated and sea zones (4%) was at the stations following the mouth stations also characterized by the phytoplankton activity maximum.