Flood regime and dam operation jointly determine cyanobacterial dynamics in deep oligotrophic-mesotrophic reservoirs

Extreme rainfall and flooding are intensifying globally, yet their interaction with reservoir operations on cyanobacterial dynamics remains unclear. This study compares two flood years (2020 and 2024) in Lake Qiandaohu, China, using 2021-2023 as a baseline, to disentangle how rainfall patterns and flood-control operations shape cyanobacteria succession. The 2020 event featured concentrated summer rainfall and rapid high-discharge releases (max_outflow = 7228 m³/s). Under this regime, the turbid inflow plunged to 20-40 m near the dam, and post-flood cyanobacterial responses were brief and confined to the reservoir inflow region. In contrast, the anomalously wet spring in 2024 (46% rainfall more than in 2020), combined with moderate staged discharge operations during the summer flood (max_outflow = 4430 m³/s), caused sustained water-level rise and allowed the turbidity plume to intrude into the euphotic zone (0-20 m) near the dam. This press disturbance shifted the dominant control from physical flushing to nutrient-hydrodynamic coupling. Persistent spring rainfall first promoted pre-flood dominance of the shade-tolerant Pseudoanabaena. The coupling between lagged total phosphorus accumulation and cyanobacterial responses, together with prolonged phosphorus retention in the euphotic zone, further promoted five months of post-flood dominance by nitrogen-fixing and vertically migrating Aphanizomenon. These findings extend pulse-press disturbance theory to reservoir ecosystems. Under climate change, intensified extreme rainfall may increase press-type disturbances and associated cyanobacterial blooms in deep oligotrophic-mesotrophic reservoirs. Adaptive reservoir management should therefore dynamically optimize outflow/inflow ratios and avoid sustained water-level rise that traps nutrients within the euphotic zone, thereby reducing the risk of long-tail post-flood cyanobacterial blooms.