Climate change promotes harmful algal blooms in China's lakes and reservoirs despite significant nutrient control efforts

The increasing frequency and magnitude of harmful algal blooms (HABs) threatens the integrity of aquatic ecosystem functioning and human health worldwide. Nutrient reduction strategies have been widely used to mitigate HABs, but their efficiency in light of on-going changes in climate remains unclear. Here, we assembled an 18-year (2005-2022) national water quality dataset for 97 lakes across China. We examined the dynamics of HABs and their response to nutrient reduction under historical climate change trends using a combination of statistical and process-based modeling. The results revealed an increase in HABs despite a widespread decline in ambient nutrient levels, with 80.5 % of lakes experiencing a decline in phosphorus but 61.8 % displaying an increase in Chlorophyll a concentrations. We attributed this counterintuitive trend to climatic warming, which can hinder the mitigation of HABs until the ambient nutrients reach sufficiently low levels. The extent of HAB promotion by warming varied spatially, with a distinctly greater proliferation in China's lower-latitude lakes (<35 degrees N), primarily due to the prevailing warmer temperatures. Notwithstanding the persistence of HABs in China's lakes, national-scale modeling suggests that nutrient loading control remains valuable in protecting our water resources, as the HAB risk would have been 32.6 % higher due to climate change. The anticipated future nutrient reduction efforts in China are expected to alleviate higher latitude lakes from frequent HAB occurrences, but lower latitude lakes will still face considerable HAB risks. Our national-scale assessment demonstrates a variant efficiency of nutrient reduction in offsetting HAB risks amid rapid climate change, and highlights the need of adaptively enhancing our mitigation strategies in response to the ever-changing ecological conditions.