Changes in the ecosystem structure and function of a cyanobacteria bloom-dominated, shallow lake after ten-year eutrophication management

Large and shallow lakes are widely influenced by multiple anthropogenic stressors, including eutrophication. Normally, diverse strategies of eutrophication management are needed to restore the lake ecosystems. Dianchi Lake, a subtropical plateau lake in Yunnan Province, SW China, has long experienced the effects of eutrophication and cyanobacterial blooms. To mitigate the eutrophication of Dianchi Lake, various efforts have been implemented since 2000, including reducing nutrient inputs, reestablishing aquatic macrophytes in lakeside zones, manipulating fishes and so on. However, little is known about the changes in its ecosystem structure and function after long-term efforts of eutrophication management in the lake. Therefore, this study on such changes was conducted by comparing the field data-based Ecopath models between 2009 and 2019, a decade marked by a rapid implementation of eutrophication management policies. Results show that both top-down and bottom-up processes have influenced the ecosystem structure and function. First, over this decade, nutrient reduction significantly reduced phytoplankton biomass by 49.4%. Nutrient recycling and path length in food web began to decrease, as indicated by the decrease of Finn's cycling index and Finn's mean path length. Secondly, fishing management strategy has greatly changed the composition of fish assemblage, which was dominated by the small zooplantivorous fishes with ecological niche overlapping with shrimps. In general, the stability of the ecosystem has been decreasing, due to the dramatic decrease in zooplankton biomass (83.67%) and hence a collapse of the microbial loop in the food web. Therefore, we strongly advocate the persistent efforts to mitigate the risk of recurrent cyanobacteria blooms in Dianchi Lake, necessitating stricter regulation of nutrient levels and implementing effective fish population management techniques.