Nutrient and Organic Matter Driven Reorganization of Phytoplankton Communities Downstream of Wastewater Discharge in the Styr River (Ukraine)

The study investigated changes in the phytoplankton community of the Styr River in the area affected by wastewater discharge from a nuclear power plant. The aim was to assess the effects of nutrient loading associated with wastewater discharge on phytoplankton species composition, abundance, biomass, and community structure. Samples were collected in May - August 2024 at two river sites located upstream and downstream of the wastewater discharge point. Water samples were analyzed for dissolved organic carbon (DOC), total nitrogen (TN), and total phosphorus (TP). UV–Vis optical indices indicated a downstream shift toward more aromatic and higher-molecular-weight DOM characteristics, consistent with changes in the composition of the dissolved organic matter pool. Phytoplankton was assessed in terms of abundance, biomass, mean cell biomass, and taxonomic structure. Correlation analysis and principal component analysis (PCA) were used to examine relationships between hydrochemical and biological variables. Downstream of the discharge point, DOC, TN, and TP concentrations increased by 19.4%, 56.5%, and 72.7%, respectively, compared with the reference site. Against this background, phytoplankton abundance decreased from 3.0 × 10⁶ to 0.5 × 10⁶ cells dm⁻³, while biomass decreased from 0.70 to 0.15 mg dm⁻³. At the same time, mean cell biomass increased, indicating a shift in the size structure of the community. Bacillariophyta dominated upstream, accounting for 79% of total abundance and 85% of total biomass. Downstream, Chlorophyta and Euglenophyta became predominant, whereas the contribution of diatoms declined markedly. The saprobity index increased from 1.96 to 2.61, indicating increased organic loading. Correlation analysis showed negative relationships between nutrient concentrations and Bacillariophyta, but positive relationships with Chlorophyta and Euglenophyta. PCA confirmed clear spatial separation of the phytoplankton communities along the trophic gradient. Overall, increased organic matter and nutrient concentrations in the discharge-affected area were accompanied by a restructuring of the phytoplankton community, characterized by a reduced contribution of diatoms and an increased contribution of taxa tolerant of organic pollution.