Long-term changes in phytoplankton and macrophyte communities in an eutrophic shallow reservoir and prospects for its restoration

Long-term changes in hydrochemistry and community structure of phytoplankton and macrophytes were analyzed in the Sestroretskiy Razliv reservoir (northwestern Russia). The average content of total phosphorus (TP) in May–October increased from 73 μg P l⁻¹ in 1980 to 163 μg P l⁻¹ in 2000. A significant increase in average chlorophyll a content from 16.6 μg l⁻¹ in 1980 to 84.7 μg l⁻¹ in 2000 and a shift in phytoplankton composition to the dominance of cyanobacteria over diatoms indicated a change in the trophic status of the reservoir from meso-eutrophic to hypertrophic. In 2016 and 2018, average TP was 96 and 101 μg P l⁻¹, respectively. The average content of chlorophyll a was 43.6 μg l⁻¹ in 2016 and 66.6 μg l⁻¹ in warmer 2018, indicating persistent eutrophic conditions. Diatoms dominated both in 2016 and 2018, especially in 2016 characterized by unfavorable weather conditions. Cyanobacteria were more abundant in 2018 with higher summer temperatures. The decline of the total area covered by aquatic vegetation from 157 ha in 1980 to 76 ha in 2016 likely resulted from an increase in phytoplankton biomass and water turbidity. Based on the results of our observations, in addition to further reduction in nutrient loading, biomanipulation by introducing predatory fish as a restoration measure was proposed to improve the ecological status of the reservoir.