Phytoplankton community structure and its response to environmental factors in the cascade hydropower stations of the lower Jinsha River

The construction of cascade hydropower stations on the lower Jinsha River has significantly altered natural hydrological regimes and ecological processes, posing potential impacts on aquatic ecosystems. Understanding the spatiotemporal dynamics of phytoplankton communities and their responses to environmental changes is crucial for the ecological management of reservoirs and the protection of water environments. This study investigated four cascade reservoirs in the lower Jinsha River during June (summer) and December (winter) 2023. Pronounced thermal and chemical stratification observed in June became homogenized in December, with concentrations of TN, NO3-N, DTN, and TP significantly higher in December (p < 0.05) and accumulating downstream. Phytoplankton density generally decreased with increasing depth, and community structure differed significantly among the reservoirs (PERMANOVA, p < 0.05 except for WDD, p = 0.052). The density of Cyanophyta increased significantly from June to December (p < 0.05), whereas that of Bacillariophyta and Chlorophyta decreased. Mantel tests indicated significant correlations between phytoplankton communities and temperature, dissolved oxygen, and nitrate nitrogen (p < 0.05). Hierarchical partitioning revealed seasonal shifts in key driving factors: temperature (32.58%) and dissolved oxygen (20.13%) dominated in June, whereas nitrate nitrogen (21.86%) and suspended solids (15.22%) were the key factors in December. The elevated Cyanophyta density in December was associated with enhanced nutrient release from sediments, weakened dilution effects, and increased terrestrial inputs. This study reveals that phytoplankton community dynamics in subtropical cascade reservoirs are co-regulated by thermal stratification, nutrient availability, and seasonal changes. Future reservoir management strategies should prioritize monitoring thermal stratification and surface bloom risks in June, while focusing on controlling endogenous nutrient loading and optimizing water mixing processes in December. These findings provide important guidance for water quality management and ecological conservation strategies in regulated river systems.