Effect of Water Quality on Phytoplankton Structure in Deep Freshwater Lakes over 10 Years: A Case Study of Fuxian Lake and Xingyun Lake

Understanding the dynamics of water quality parameters and their relationship with algal biomass is crucial for the sustainable management of plateau lake ecosystems. This study presents a decade-long (2009-2018) comparative analysis of two typical plateau lakes on the Yunnan-Guizhou Plateau: the oligo-mesotrophic Fuxian Lake and the eutrophic Xingyun Lake. We investigated the spatiotemporal variations in key water quality parametersincluding nitrogen species, phosphorus, dissolved oxygen (DO), and chemical oxygen demand (COD)and their correlations with phytoplankton community structure and biomass. Our findings revealed stark contrasts between the two systems. Xingyun Lake exhibited significantly elevated nutrient levels with total nitrogen (TN) and total phosphorus (TP) consistently exceeding Class III water quality standards, indicative of eutrophication. In contrast, Fuxian Lake maintained good water quality with low nutrient concentrations and high DO levels. Phytoplankton community structure also differed markedly; Fuxian Lake supported a diverse assemblage dominated by Cryptophyceae, Chlorophyceae, and Bacillariophyceae, whereas Xingyun Lake was predominated by cyanobacteria, posing a high risk of harmful blooms. Correlation analysis further demonstrated that algal biomass in Xingyun Lake was more sensitive and strongly correlated with water quality parameters, particularly with NO₃-N in autumn and with temperature, COD, and nitrogen species in summer. In contrast, the relationships in Fuxian Lake were weaker and more seasonally variable. This study underscores the distinct drivers of algal growth in lakes of differing trophic states and provides vital data for identifying pollution sources. Our results are expected to inform targeted strategies for reducing algal bloom risk and advancing the sustainable management of plateau lake systems.