Spatiotemporal changes in chlorophyll-a concentration in China's lakes and its driving factors.

This study provides a comprehensive analysis of the temporal and spatial variations in chlorophyll-a (Chl-a) concentrations across 524 lakes in China from 2007 to 2016, leveraging extensive water quality monitoring data to identify the key drivers of these changes. Our results indicate a general decline in Chl-a concentrations, suggesting a mitigation of eutrophication. However, significant regional and seasonal disparities persist, reflecting varying ecological conditions and anthropogenic influences. Elevated Chl-a concentrations were predominantly observed in regions with intensive agricultural and urban areas, such as the middle and lower reaches of the Yangtze River, the Pearl River Delta, and the North China Plain. In contrast, lower concentrations were recorded in areas with extensive forest and grassland cover, such as the northeast and southwest. Multiple linear regression analysis revealed that ammonia nitrogen (NH₃-N) and chemical oxygen demand (COD_Mn) were the primary factors influencing Chl-a levels during the growing season, while water temperature (W-T), COD_Mn, and pH had greater impacts in the non-growing season. Categorized according to the key factors influencing Chl-a concentration, 52 lakes were divided into 7 categories, and future Chl-a concentrations were projected using data from the China National Environmental Monitoring Centre and the China Environmental Status Bulletin. Specifically, we estimate a 3.060% annual decrease in COD_Mn and a 2.748% annual decrease in NH₃-N, total nitrogen (TN), and total phosphorus (TP), alongside a 0.350% annual increase in water temperature. These projections suggest that while most lakes are expected to see reductions in Chl-a concentrations, some COD_Mn-sensitive and water temperature-sensitive lakes may still experience increases, underscoring the complexity of interactions between multiple environmental factors. To address these dynamics, we recommend targeted management strategies, including reducing nutrient and organic matter inputs, enhancing ecological protection measures, and closely monitoring temperature fluctuations to mitigate the risk of algal blooms and maintain lake health.