Abstract <p>The current study focuses on the spatial and time variation of photosynthetic activity in phytoplankton as part of climate and environmental monitoring. Photosynthetic activity was assessed by parameters of the active chlorophyll <i>a</i> fluorescence (the maximal quantum photochemical efficiency and the relative electron transport rate), using a WATER-PAM-II fluorimeter (Walz). The measurements were taken in the southeastern part of the Baltic Sea in 2022–2025. The photosynthetic activity during the warm season was maximal in April. However, during the cold season, the high production potential of phytoplankton was almost completely unrealized: the maximal quantum efficiency reached its highest values for the entire study period, while the relative electronic transport speed was minimal. The relative velocity of electronic transport during the warm season varied widely from year to year. The range of values for both parameters was the greatest in open-sea areas as compared to near-coastal ones, which indicated a diversity of hydrological conditions in open seas that influenced phytoplankton populations. During the spring season, the relative electron transport rate was significantly higher in the open sea as compared to near-coast areas.</p>