Light limitation during a compound drought and heat event inhibited algal blooms in a nutrient-rich shallow lake

Extreme weather events caused by climate change have a significant impact on the lake ecosystems. While many researchers believe that heatwaves and droughts may enhance algal bloom, heatwaves and droughts reduced algal blooms in Lake Chaohu during the spring and summer of 2022. To investigate the mechanism behind this phenomenon, on-site sampling and online monitoring were conducted in Lake Chaohu from 2019 to 2022. The results showed that non-algal turbidity played a critical role in temporarily inhibiting algal blooms. The water level in spring and summer of the drought year was 1.2 m lower than that in conventional years, resulting in the area of shoals where resuspension could occur being nearly 4 times larger than in conventional years. Strong resuspension caused turbidity in spring and summer to be more than double that of conventional years, sharply reducing gross primary productivity by 39 %, which led to lower chlorophyll a concentration than in conventional years. These results indicate that drought does not necessarily exacerbate algal blooms, and changes in shoal area due to water level fluctuations are a key factor affecting algal blooms in shallow lakes prone to resuspension. Furthermore, these results suggest lake managers can control algal blooms by adjusting water level to increase turbidity during or before algal bloom seasons.