Abstract: We investigated the seasonal variation of phytoplankton community structure and its relationship with environmental factors in Xin’anjiang River Basin (Tunxi section) from 2017 to 2018. A total of 160 species (including variants) from 96 genera belonging to seven phyla were identified, among which, Chlorophyta (72 species from 42 genera), Bacillariophyta (49 species from 26 genera) and Cyanophyta (19 species from 15 genera) were dominant. Photosynthetically active radiation (PAR), UVA and UVB had the highest intensity in summer, followed by spring, autumn and winter. The pH, dissolved oxygen, and water temperature differed among seasons. The concentration of ammonia nitrogen (NH₃-N) concentration and turbidity were significantly higher in winter than that in other seasons. There was no significant difference in total phosphorus (TP) concentration and oxidation reduction potential among the four seasons. Chlorophyll a and carotenoid concentrations varied spatially and temporally, with higher concentrations in summer and winter than in spring and autumn. The Shannon index, Simpson index, Margalef index and Pielou index did not change across seasons and sampling sites. Water quality was between the criteria of type I and type III, with most of which being located in type I and type II. The pollution indicator species showed a mesosaprobic zone. Shannon index and Margalef index revealed a slight pollution or nonpollution status of the studied area. Water quality of Xin’anjiang River Basin (Tunxi section) was generally in good condition according to the comprehensive analyses of both chemical and biological indices. Redundancy analysis results showed that the number of phytoplankton species in each phylum  had positive relationship with NH₃-N and TP, where UVB had a strong negative relationship with Bacilariophyta. PAR, UVA, UVB and water temperature were the main factors regulating phytoplankton abundance, which had positive relationship with  abundance of Bacillariophyta, Dinophyta and Euglenophyta but negative relationship with  abundance of Chloropyta, Cyanobacteria, Chrysophyta and Cryptophyta.

Key words: soil respiration, experimental warming, adaptability.,