Hydrological Conditions and Dominant Phytoplankton Species in the Middle and Upper Reaches of the Yarlung Zangbo River, Tibetan Plateau

To investigate the structure of phytoplankton communities and the ecological niches of dominant species in the middle and upper reaches of the Yarlung Zangbo River, we collected samples at 14 sites in April (spring) and September (autumn) 2023. A total of 198 phytoplankton species were identified belonging to 6 classes, 13 orders, 24 families, and 53 genera. The community structure was dominated by diatoms, green algae, and cyanobacteria. In April, 163 species of phytoplankton from four phyla were identified, with abundance ranging from 2.94 × 105 to 2.32 × 106 cells/m3 and an average of 1.28 × 106 cells/m3. In September, the abundance of phytoplankton ranged from 1.52 × 105 to 1.58 × 106 cells/m3, with an average value of 6.76 × 105 cells/m3. Sixteen species were classified as dominant (Y > 0.02), among which four showed dominance in both sampling periods, with their dominance level and niche width differing with the season. Water temperature increased with decreasing altitude. At <3500 m in September, Ankistrodesmus falcatus and Oocystis borgei became dominant. Cymbella cistula, Amphora ovalis, and Navicula cryptocephala occupy broad ecological niches and can represent indicator species for water quality. Water temperature, pH, and salinity were identified as primary factors influencing the ecological niche differentiation of dominant phytoplankton species. The interspecific niche overlap was higher in September than in April and greater at >4500 m compared to other elevation ranges (>4500 m; 4000–4500 m; 3500–<4000 m; <3500 m). The effect of elevation on the community structure was greater than that of season. This is the first study to characterize the association of ecological niches of phytoplankton in the upper reaches of the Yarlung Zangbo River with physicochemical environmental parameters. This provides baseline information for the conservation of biodiversity and management of aquatic ecosystems in the rivers of the Tibetan Plateau.