Seasonal Dynamics of Phytoplankton Community Structure in Dongxi Reservoir, Wuyishan City and Relationship with Water Environmental Factors

The phytoplankton community in reservoirs is closely related to water ecological health, and the dynamic changes in community structure and its influencing factors are important for reservoir management. Therefore, this study focuses on Dongxi Reservoir in Wuyishan City, where seasonal dynamics and water environment monitoring of phytoplankton communities were conducted. Statistical methods such as variance analysis and ordination analysis were used to study the seasonal dynamics characteristics of phytoplankton and its key influencing factors. The results showed that 180 species of phytoplankton were identified, belonging to 7 phyla, 13 classes, 41 orders, 57 families, and 104 genera. The main dominant species was Synedra acus. There were significant changes in the species composition of algae among different seasons, with green algae and diatoms dominating in spring and autumn; green algae were dominant in summer, and diatoms were dominant in winter. The community density showed significant seasonal differences, with the cell density and biomass being highest in autumn, reaching 280.65 × 104 cells/L and 3.69 mg/L, respectively; while was lowest in summer, with values of 60.27 × 106 cells/L and 0.89 mg/L, respectively. The Shannon-Wiener diversity index and Pielou evenness index were highest in spring, with values ranging from 4.52 to 5.40 and 0.91 to 0.97, respectively. The Margalef richness index was highest in autumn, with values ranging from 2.54 to 5.36. The results of redundancy analysis (RDA) showed that electrical conductivity, ammonia nitrogen concentration, total phosphorus concentration, and pH were the main environmental factors influencing the phytoplankton community structure in Dongxi Reservoir, with pH having the greatest impact in spring. Electrical conductivity and nitrogen-phosphorus concentrations are directly related to human activities and have become the core factors driving the changes in the phytoplankton community of Dongxi Reservoir. Therefore, scientifically controlling the intensity of human activities in the upstream watershed and reducing the nutrient input into the reservoir is key to the current protection and restoration of the water ecological environment in Dongxi Reservoir.