Abstract: The functional group composition of phytoplankton can reveal community succession pattern and indicate the quality of water environment. The Yarlung Zangbo River is an important water system in China. The phytoplankton community structure and water environmental factors in the middle and upper reaches of the Yarlung Zangbo River were investigated in August (summer) and November (autumn) of 2019 and in May (spring) of 2020 to explore the seasonal variations of phytoplankton functional groups and their indicative roles on water environment. The results showed that: (1) Water in the middle and upper reaches of Yarlung Zangbo River was alkaline, with significant differences in water temperature, velocity and turbidity among the three seasons. (2) A total of 121 genera of phytoplankton were identified, belonging to 8 phyla, 11 classes, 24 orders, 44 families. The phytoplankton community structure was diatomic-green algae-cyanobacteria type as a whole. The phytoplankton community could be divided into 28 functional groups with 16 dominant functional groups. The composition of phytoplankton functional groups varied obviously with seasons. (3) The results of redundancy analysis and variance partitioning analysis showed that water temperature, pH, and dissolved oxygen content were the main environmental factors influencing the distribution of dominant functional groups. (4) The comprehensive evaluation based on the Shannon index (0.56-3.09, with an average of 2.20) and the ecological status index (1.92-4.91, with an average of 3.63) in three seasons showed that water quality of the middle and upper reaches of the Yarlung Zangbo River was at the level of “good”. This study revealed the characteristics of phytoplankton functional groups and their indicative roles on water environment, and provided a scientific basis for aquatic ecological research in the middle and upper reaches of the Yarlung Zangbo River.

Key words: the middle and upper reaches of Yarlung Zangbo River, phytoplankton functional group, seasonal succession, state of water environment