ENSO impacts the scales of cyanobacterial bloom in Lake Taihu, China.

The long-term interaction between climate change and eutrophication has increased the frequency and uncertainty of cyanobacterial blooms in Lake Taihu. As a dominant interannual climate signal, the El Niño-Southern Oscillation (ENSO) plays an important role in global climate systems and ecosystems. While its effects on marine phytoplankton are well documented, the potential pathways linking ENSO to cyanobacterial blooms in freshwater lakes remain unclear. This study systematically examined the ENSO and bloom relationship in Lake Taihu using long-term datasets, including the Oceanic Niño Index (ONI), annual bloom area, chlorophyll a (Chl a), precipitation in the upper Taihu Lake Basin, water temperature and external nitrogen and phosphorus loads. The results revealed significant lagged responses of Chl a, precipitation and water temperature to ONI. El Niño and La Niña phases were associated with contrasting interannual bloom patterns. El Niño elevated water temperatures (R2 = 0.484*), whereas La Niña depressed them (R2 = 0.337***), potentially contributing to changes in cyanobacterial overwintering and spring resurgence. Although El Niño significantly increased precipitation (R2 = 0.312**), accompanied by higher external nitrogen (R2 = 0.305**) and phosphorus loads (R2 = 0.183*), La Niña showed opposite effect (R2 = 0.262***). However, no significant correlations were found between external loads and either annual average Chl a or cyanobacterial bloom area. These findings suggest that ENSO may affect cyanobacterial bloom in Lake Taihu more strongly through climate-mediated changes in water temperature than through external loads, potentially reflecting the role of internal nutrient cycling. This study highlights the importance of incorporating climate variability into cyanobacterial bloom risk assessment and ecological management strategies.