Analysis of changes in phytoplankton and water quality in the upper reaches of the Yongding River

Sampling was conducted in April (spring), July (summer), and October (autumn) 2019 to perform a qualitative and quantitative study on phytoplankton in the Guanting Reservoir and the upper reaches of the Yongding River. During the survey, a total of 226 species (including varieties and variants) of eight phytoplankton phyla were identified, and the dominant species were in the diatom clade, accounting for 42.03% of the total species of phytoplankton . Phytoplankton abundance and biomass showed significant seasonal changes. The highest phytoplankton abundance (26.10 × 10⁶ ind·L^–1) was recorded in summer, whereas the lowest phytoplankton abundance (13.64 × 10⁶ ind·L^–1) was recorded in spring. The highest phytoplankton biomass (53.06 mg·L^–1) was recorded in summer, and the lowest phytoplankton biomass (11.37 mg·L^–1) was recorded in autumn. The results of the evaluation of the water quality combined with the evaluation of biological characteristics of phytoplankton indicate d that the water quality of the basin was relatively good in summer and autumn, but poor in spring, and the overall water quality of the 17 sampling sites was considered to be moderately polluted. The results of redundancy analysis (RDA) indicate that total nitrogen (TN) and nitrate nitrogen (NO₃-N) played an important role in promoting the growth of dominant species of phytoplankton. We believe that the upstream region of the Yongding River in the Guanting Reservoir is the point source and non-point source of pollution in the reservoir basin, and in the Yanghe River basin and Qinghe River basin. Efforts should be made to reduce the N concentration in the water to control the degree of eutrophication of the water in the Guanting Reservoir, and protect the health of the reservoir ecosystem.