Abstract: In order to assess the ecological health of Gucheng Lake-an important regulating lake in the lower reaches of the Yangtze River, and to enhance the management and protection of Gucheng Lake, four seasons of sampling surveys were carried out in Gucheng Lake in 2019. The phytoplankton diversity index and evenness index methods were used to evaluate the water quality status of Gucheng Lake. The non-parametric test for multiple paired samples was used to explore the variability of the phytoplankton community at each sampling site at the seasonal scale, and redundancy analysis (RDA) was used to investigate the relationship between phytoplankton community characteristics and environmental factors in four seasons. The results show that the Shannon-Wiener diversity index of phytoplankton in Gucheng Lake ranged from 1.87 to 2.40, with a global average of 2.05. The Pielou evenness index ranged from 0.46 to 0.92, with a global average of 0.69, and the water was in a light-β-moderate pollution state. The cyanobacteria are the main phytoplankton species in Gucheng Lake, followed by green algae and diatoms, with the seasonal mean phytoplankton abundance varying from 5.63×10⁵-1.67×10⁸ L^-1. The dominant species are Chlorophyta and Diatom in spring, Cyanophyta in summer, Diatom and Cryptoalgae in autumn and Chlorophyta in winter. The dominant species in the annual waters are Anabaena circinalis., Anabaena sp., Oscillatoria sp., Phormidium sp., Pseudanabaena sp. and Spirulina sp.; RDA analysis shows that the main environmental drivers of phytoplankton community structure in Gucheng Lake are TN in spring, TP in summer, EC and COD_Mn in autumn, and TP and WD in winter.

Key words: Gucheng Lake, characteristics of phytoplankton species, seasonal patterns, environmental factors, redundancy analysis