Recent changes and dynamics of cyanobacterial bloom intensities in Lake Taihu

Over recent decades, algal blooms in inland freshwaters have become increasingly prevalent worldwide, posing substantial risks to ecosystem services, local economies, and human health. Nutrients and meteorological conditions represent key factors influencing algal growth and bloom formation. Nutrients provide the essential foundation for algal proliferation, while excessive nutrient inputs and resultant eutrophication serve as the primary drivers of algal expansion and bloom development. Meteorological and hydrological factors—such as temperature, wind speed, underwater light availability, and water levels—constitute critical habitat conditions for cyanobacterial bloom outbreaks. These habitat factors are directly or indirectly linked to the geomorphological characteristics of lakes, including elevation, latitude, surface area, and water depth. Shallow lakes are particularly prone to eutrophication, with remediation efforts often yielding limited effectiveness. This vulnerability is largely attributable to the geographic characteristics of such lakes. Shallow lakes are characterized as well-mixed systems lacking stable thermal stratification in summer and are commonly designated as polymictic lakes.Lake Taihu, a large shallow lake situated on the alluvial plain of the Yangtze River, exhibits a mean depth of less than 2 mand a maximum depth of less than 3 m. Since the 1980s, Lake Taihu has progressively transitioned into a eutrophic state. Following the 2007 drinking water crisis, the lake underwent extensive pollution control and nutrient reduction measures. However, despite more than a decade of remediation efforts, Lake Taihu experienced its most severe cyanobacterial bloom on record in 2017, with bloom coverage peaking at two-thirds of the lake surface. High-intensity cyanobacterial blooms persisted until 2020. Thereafter, bloom intensity gradually declined. By 2024, cyanobacterial bloom severity had decreased markedly, and Lake Taihu’s water quality attained the national Grade III surface water quality standard. Nevertheless, the mechanisms driving fluctuations in algal bloom dynamics remain incompletely understood. Using a phytoplankton dynamic simulation model, we determined that climate variability during 2015–2020 was the primary driver of cyanobacterial bloom resurgence after intensive pollution control. Climate warming promotes cyanophyte dominance and facilitates cyanobacterial bloom formation. Warm winters induced earlier bloom phenology. Algal blooms further enhanced phosphorus release from sediments by altering dissolved oxygen and pH conditions, thereby promoting cyanobacterial growth and bloom development and establishing a positive feedback loop. This self-reinforcing cycle between cyanobacterial blooms and sediment phosphorus release sustained the high-intensity blooms observed from 2015 to 2020. The post-2021 bloom weakening was also linked to climate change, particularly the abrupt decline in photosynthetically active radiation during spring 2021, which disrupted the feedback between algal blooms and sediment phosphorus release. This effectively suppressed algal growth in the critical spring period and prolonged its effects on summer bloom intensity, thereby weakening the mutual reinforcement between cyanobacterial blooms and sediment phosphorus release. It is the algae-mediated phosphorus cycling mechanism within the water–sediment system of shallow lakes that amplifies or attenuates bloom intensity. Therefore, amid future climate warming, we recommend implementing stricter nutrient reduction measures alongside enhanced control of internal phosphorus loading from sediments, particularly in shallow eutrophic lakes prone to cyanobacterial blooms.