Abstract: Phytoplankton is the most important primary producer in the river ecosystem. In order to reveal the temporal and spatial distribution characteristics of phytoplankton community in Ganjiang River and evaluate the nutritional status of Ganjiang River water body, the community structure of floating plants in the middle and lower reaches of Ganjiang River was investigated in January, April, July, and October of 2019, representing winter, spring, summer and autumn, respectively. The results show that there were 6 phyla and 53 genera of phytoplankton in the middle and lower reaches of Ganjiang River, including 25 genera of Chlorophyta, accounting for 47.17% of the total species; 16 genera of Diatoms, accounting for 30.19% of the total species; and 6 genera of cyanobacteria, accounting for 11.32% of the total species; Other phyla include 3 genera of gymnophyta, 2 genera of dinoflagellate and 1 genus of Cryptophyta. The phytoplankton density at each sampling point was 11.0×10⁴-468.7×10⁴ L^-1, with an average of 93.1×10⁴ L^-1. The biomass ranged from 0.15 to 4.67 mg·L^-1, and the average biomass was 1.16 mg·L^-1. There are significant differences in the spatial distribution of phytoplankton. The abundance and biomass of phytoplankton at each sampling point in the upper reaches of Wan'an Reservoir and Xiajiang reservoir were higher than those at other sampling points, and the sampling point GJ11 had the highest abundance. In terms of seasonal variation, phytoplankton biomass generally showed the trend of winter > autumn > spring > summer; It was the highest in winter, 2.23 mg·L^-1, and diatom is the dominant category; The biomass in summer was the lowest, 0.66 mg·L^-1, and Chlorophyta is the dominant category; Compared with the survey in the 1980s, the number of phytoplankton species (genera) has decreased in following years, and the richness and evenness are also lower than the early survey results. The evaluation results of physical and chemical indexes of water quality show that the water quality in the middle and lower reaches of Ganjiang River is Class V; The evaluation of Shannon Wiener Index, Pielou Evenness Index and Margalef Biodiversity Index shows that the water body is in the state of moderate to severe pollution.

Key words: Ganjiang River, phytoplankton, community structure, biological evaluation of water quality