Seasonal Dynamics of Phytoplankton with Relation to Physicochemical Water Parameters above and below the Hydroelectric Plant on the Pasleka River (North-East Poland)

The study of seasonal dynamics of net phytoplankton was carried out above and below the hydroelectric plant (HP) on the Pasleka River. Correlations were found between the abundance of diatoms and conductivity, O-2, PO4; the abundance of cyanobacteria and water temperature and conductivity, and the abundance of chlorophytes and O-2 and N-tot. Multiple correlations revealed the most significant factor for diatom development is PO4, for cyanobacteria is water temperature, and for chlorophytes is N-tot. A lower abundance and biomass of phytoplankton, and less species diversity and nutrient concentrations were registered below the HP than above it. The seasonal dynamics of phytoplankton abundance provided similar results. The maximum biomass of the assemblage was found earlier below the HP, in April, than above it, in May. However, no differences were recorded in the dominant species within phytoplankton abundance. The phytoplankton biomass was dominated by the Melosira varians and Spirogyra sp., which are typical taxa of plant periphyton and possibly emanate from the concrete elements of the dam. This HP changed the local hydrological and environmental conditions, affecting available nutrients, and thereby phytoplankton development in the vegetative season.