Meiyu intensity modulates cyanobacterial bloom dynamics in sub-deep reservoirs via synergistic hydrological and nutrient regulation

Meiyu is a persistent rainy weather phenomenon in East Asia caused by a subtropical monsoon front. Under global climate change, the intensity and extremity of Meiyu have increased markedly, posing severe challenges to the water safety and management of regional reservoirs. However, the impacts of Meiyu intensity on cyanobacterial blooms in regional sub-deep reservoirs remain poorly understood. Here, we used Qishu Reservoir, a typical sub-deep reservoir in Yi County, Huangshan City, Anhui Province, China, as the study area. Based on monitoring data from two contrasting Meiyu years (normal, 2023; wet, 2024) and using multivariate statistics and partial least squares path modeling (PLS-PM), we revealed how Meiyu intensity regulates summer reservoir hydrodynamics, thermal stratification, nutrient dynamics, and phytoplankton communities. Results showed that frequent and intense rainfall during the wet Meiyu generated strong hydrological pulses and markedly shortened hydraulic retention time. TN was primarily driven by rainfall-induced non-point source inputs and increased with Meiyu intensity. In contrast, TP was markedly diluted during the wet Meiyu due to limited sources and strong hydrological pulses, making TP the key regulator of interannual differences in phytoplankton density. Thermal stratification stability was the core physical control of Microcystis-dominated cyanobacterial blooms. Hydrometeorological factors regulated phytoplankton dynamics mainly via indirect pathways. During the wet Meiyu year, these factors naturally suppressed cyanobacterial blooms via the synergistic effects of weakened thermal stratification and reduced TP. We propose a hierarchical bloom control strategy based on Meiyu intensity, providing a scientific basis for managing similar reservoirs in the East Asian subtropical monsoon region.