Distribution Patterns and Driving Factors of the Phytoplankton Community in the Middle Reaches of the Yarlung Zangbo River

The middle reaches of the Yarlung Zangbo River are a hot zone of the Alpine Water System and its ecological environment is diverse but vulnerable. We systematically examined and detailed the phytoplankton community’s composition, spatial-temporal dynamics, and driving factors in this area. A total of 224 samples from 28 sampling sites across wet and dry seasons were analyzed. The results showed that: (1) the community structure of the main stream is more complex and stable than that of the tributaries; (2) the number of species, average cell abundance, and average biomass in the main stream were higher than those in the tributaries; (3) TN, TUR, WT, and pH were the main influencing factors for the difference in the phytoplankton community’s structure between the main stream and the tributaries; (4) the phytoplankton community had a closer structure, higher connectivity, stronger resistance to environmental disturbance, and higher stability in the main stream, while those in the tributaries had higher interspecific synergy; and (5) the phytoplankton community’s assembly process in the main stream was mainly influenced by random processes and was mainly driven by dispersal limitation in the middle reaches of the Yarlung Zangbo River.