Abstract—
The seasonal dynamics of phytoplankton photosynthetic activity parameters, measured by the active chlorophyll а fluorescence method, was investigated for the first time. The parameters studied include the maximum quantum efficiency ( \({{F}_{{v}}}\) /Fm), the effective quantum yield in photosystem II (Y), the relative electron transport rate in photosystem II (rETR), and the maximum electron transport rate in photosystem II (rETRmax). Measurements were conducted in 2022‒2025 at a carbon supersite in the southeastern Baltic Sea. It was demonstrated that all parameters exhibit distinct seasonal patterns, reflecting different aspects of the efficiency of the phytoplankton photosynthetic apparatus. A high photosynthetic capacity of phytoplankton during the winter period was established. It was found for the first time that for the Baltic Sea, rETR and Y are the most informative measures for seasonal assessment of phytoplankton under natural conditions within climate and ecological monitoring frameworks.
