Abstract: In this study, phytoplankton community structure were surveyed in eight sampling stations in both the main stream and the tributaries (Maiqu, Jinhe, Angqu, Requ) in the Tibet reaches of the Lancang River from 2017 to 2019, aiming to explore the relationship among phytoplankton and environmental factors. The results showed that there were 119 species of phytoplankton, belonging to 6 phyla and 49 genera. Bacillariophyta (87 species) were the dominant phytoplankton, accounting for 73.1% of the total species. Phytoplankton species diversity was higher in the tributaries than that in the main stream. The ranges of phytoplankton density and biomass were 1.11×10⁴-1.78×10⁷ cells·L^-1 and 0.01-6.44 mg·L^-1, with mean values of 1.57×10⁶ cells·L^-1 and 0.88 mg·L^-1, respectively. The average density and biomass of phytoplankton in tributaries except Jinhe were higher than that in the main stream (8.71×10⁵ cells·L^-1, 0.50 mg·L^-1). The average density and biomass of phytoplankton in spring (2.73×10⁶ cells·L^-1, 1.51 mg·L^-1) were higher than in autumn (4.18×10⁵ cells·L^-1, 0.24 mg·L^-1). The mean values of the Shannon diversity index and Pielou evenness index of the phytoplankton community in the Tibet reaches of the Lancang River were 2.83 and 0.74, respectively. The results of the community similarity analysis in different times showed substantial spatial and temporal variations of phytoplankton community. Results of Pearson correlation analysis and redundancy analysis showed that CODcr, pH, dissolved oxygen and watertemperature were the key factors affecting phytoplankton in the Tibet reaches of Lancang River. We recommend regular monitoring of phytoplankton in order to provide a reference for water ecological protection and management in the Tibet reaches of Lancang River.

Key words: Lancang River, phytoplankton, spatial-temporal dynamic, environmental factor, redundancy analysis.