Impacts of rainfall runoff on algal community in a humid region reservoir riverine zone

The expansion of algal blooms in reservoirs, driven by climate change and human activities, poses intensifying threats to water supply safety. Although altered rainfall runoff patterns act as critical climate multipliers, the response process of the algal community to different rainfall runoff intensities remains poorly understood, particularly in stratified reservoirs. To clarify the pathways and mechanisms by which altered hydrological processes drive algal community, we conducted high-frequency (every 3 d) monitoring program of hydrology, nutrients, and algae through manual sampling combined with data from autonomous buoys from 2018 to 2019 in Xin’anjiang Reservoir, a subtropical reservoir in the humid region of China. The results indicate that the impact of rainfall runoff on algal community is closely associated with its intensity. When the inflow exceeds 300~500 m³/s, it significantly inhibits algal growth in the riverine zone, with suppression lasting up to 6 days. Compared to nutrient factors, photothermal environmental conditions, especially after rainfall during the flood season, play a more important role in driving algal community changes. Path analysis and correlation analysis further reveal that rainfall runoff not only directly influences algal community dynamics through dilution and pulse effects induced by high inflow, but also indirectly affects algal changes by altering photothermal conditions and nutrient concentrations. Our study quantifies the algal community dynamics and their relationship between environmental factors during variable rainfall runoff processes, improving our knowledge of the driving mechanisms of algal bloom to safeguarding drinking water safety in reservoirs under the context of future climate change.