A Preliminary Assessment of Phytoplankton Community Characteristics and Environmental Associations in an Extremely High-Altitude River–Lake Confluence Zone

Phytoplankton are sensitive indicators of environmental change in high-altitude aquatic ecosystems, but their community responses across river–lake confluence zones remain insufficiently understood. We investigated phytoplankton community characteristics and environmental associations in the Siling Co river–lake continuum. This endorheic system lies above 4500 m, where river inflows, lake-water mixing, and evaporative concentration generate marked hydrochemical gradients. Genus-level cell density and estimated biomass were assessed with 22 environmental variables from 14 sampling events at 10 lake and river sites. We recorded 54 phytoplankton genera, with Bacillariophyta dominant. Lake and River samples overlapped in composition, whereas sampling period accounted for more community variation than water-body type. Major-ion and salinity-related gradients were associated with cell-density community dissimilarity after accounting for water-body type. Cell-density and biomass composition also showed partly distinct associations with pH and SiO2, and with Cl−, respectively. These observations indicate that phytoplankton communities in the Siling Co river–lake confluence zone are more closely associated with sampling-period variation and hydrochemical gradients than with Lake/River habitat classification. These findings provide an initial basis for integrated biological and hydrochemical monitoring.