Hydrology drives phytoplankton succession and productivity coupling in plateau hydropower reservoirs with implications for ecosystem health management

This study investigates the role of water level fluctuations (WLF) in shaping phytoplankton community structure and gross primary productivity (GPP) in Hongfeng Reservoir, a karst reservoir in Southwest China, through long-term monitoring (2016-2023) across seven sampling sites. Results revealed substantial seasonal and interannual variations in water levels (maximum fluctuation range: 9.78 m) and nutrient concentrations, with total nitrogen (TN) and phosphorus (TP) peaking during wet seasons. Chlorophyta dominated species diversity (42-48%), while Cyanophyta exhibited the highest relative abundance. GPP fluctuated markedly between the dry (586.09 mg C/m²/d) and wet seasons (3018.66 mg C/m²/d), driven by phytoplankton abundance and community stability. The Bray-Curtis (BC) dissimilarity index declined over time, showing a negative correlation with WLF (r = -0.03) and a significant negative impact on GPP. WLF indirectly enhanced GPP by altering community dynamics. Findings highlight WLF as a critical regulator of phytoplankton structure and productivity, emphasizing the importance of hydrological management in mitigating algal blooms and balancing ecological stability. This study provides actionable insights for optimizing water level regulation to sustain reservoir ecosystem health in karst regions.