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[Objective] The present paper aimed to know the seasonal and spatial variations of environmental factors and phytoplankton communities, and then analyze the correlations of them in Guandi reservoir. [Method] Form October 2018 to November 2019, the physicochemical property and nutrients of the reservoir were measured in different seasons. The species of algae in the water also were identified, and its numbers were counted in per unite water. With SPSS 22. 0 and Canoco 4. 5 software, the correlations of environmental factors and phyto-plankton communities were analyzed. [Result] Annual mean values of water temperature (WT), dissolved oxygen (DO), pH, Transparency (SD), Hardness of Groundwater (HG), Conductivity (CT) and chemical oxygen demand (COD) were 14. 9 "C, 10. 8 mg/L,8. 51,238 cm,4. 0 mmoVL,243. 4 μ,sf cm and 5. 1 mg.IL In winter and spring, the water had high total nitrogen (TN) content, but extremely low total phosphorus (TP) content. In summer and autumn, it had low TN content and high TP. Except sampling site B in winter and spring, Shannon-Wiener Index, Pielou' s Evenness Index and Simpson's diversity index was high. The identification showed that there was a total of 84 algae species which belonged to 6 phylum. There were 26 algae species which had high annual dominance. The numbers of species of Chlorophyta and Bacillariophyta were the largest and second. The numbers of phylum and species in autumn were significantly lower. The algae' s density increased gradually from upstream to downstream, and the annual mean value of Guandi reservoir was 139. 8 x 104 cell/L [Conclusion] The water temperature was low, but the DO was high, and the water was weakly alkaline. It had low nutrient contents, and belonged to oligotrophic water. According to ' Environmental quality standards for surface water' (GB 3838- 2002), the water quality was between grade I and Ill. The high diversity indexes indicated that the water quality was good. In winter and spring, the algae of Bacillariophyta, Chlorophyta and Cyanophyta had advantage. In summer and autumn, the algae of Bacillariophyta and Chlorophyta had advantage. So the algae of Bacillariophyta and Chlorophyta were the main on the whole in the Guandi reservoir. HG, WT, COD and TP, especially WT, were the main environmental factors which caused the variations of phytoplankton communities. By comparison, TP in winter and spring was one of the main limiting environmental factors of algae growth, and it was TN in summer and autumn. [ABSTRACT FROM AUTHOR]

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