Mechanisms and management of rainfall-triggered phytoplankton bloom in a deep stratified reservoir: key drivers and reactivation thresholds.

Water-lifting aerators (WLAs) developed by our teams are typically employed to improve water quality via artificial mixing. However, the WLA deactivation following rainfall frequently results in phytoplankton blooms in the reservoirs. The mechanisms by which rainfall events trigger blooms and the sedimentation characteristics of suspended solid (SS) and total phosphorus (TP), associated with WLA reactivation, remain unclear. Therefore, this study explored the key drivers of rainfall-triggered phytoplankton proliferation and established WLA reactivation thresholds for different intensities events using field monitoring and indoor sedimentation experiments during the summer months (June-August) of a reference year (2017) and operational years (2022-2024) in a deep stratified reservoir. The results indicated that rainfall inhibited phytoplankton blooms by decreasing water temperature (WT), light availability (Zeu/Zmix), and unfavorable meteorological factors during in-storm (Rain 1-3), while simultaneously triggering blooms via increasing nutrient concentrations (e.g., P) and rapidly restoring WT and meteorological conditions post-storm (Rain 4-7). Artificial mixing achieved more than 95 % reduction in cell density relative to the reference year by decreasing WT, Zeu/Zmix and nutrient concentrations and increasing mixing depth (Zmix). Random forest, Partial Least Squares Path Model (PLS-PM), and the Mantel test based on Spearman rank correlation revealed that phytoplankton dynamics were significantly correlated with WT, nutrients, and relative water column stability (RWCS). Additionally, the static sedimentation behaviors of TP and SS fitted well with the pseudo-second-order kinetic equation, with a maximum correlation coefficient of 0.99. WLA reactivation thresholds were determined to be 72, 89, and 164 h at maximum discharges of 65, 165, and 680 m3/s based on the calculated TP concentration (T0.05), respectively. Increasingly frequent extreme rainfall events, driven by climate change, may heighten the risks of phytoplankton blooms. This study provides practical engineering guidance for phytoplankton bloom management in deep stratified reservoirs.