Environmental Filtering Correlates More Strongly With Taxonomic and Functional Beta Diversity of Phytoplankton Assemblages Than Spatial Structuring in a High‐Altitude River–Lake System

ABSTRACT Understanding overall beta diversity ( β total ) and its components—species replacement ( β repl ) and richness difference ( β rich )—is essential for disentangling the mechanisms that structure aquatic communities. However, it remains poorly understood how taxonomic and functional beta diversity correlate with environmental and spatial processes in high‐altitude river–lake systems. This study quantified spatial and seasonal variation in phytoplankton beta diversity and identified correlations with environmental and spatial variables across the Qinghai Lake Basin, located on the northeastern Qinghai–Tibet Plateau. We integrated taxonomic and functional approaches to quantify beta diversity patterns using phytoplankton assemblages collected from inflowing tributaries (IT) and the main lake (ML) during summer and autumn. Beta diversity was partitioned into replacement and richness components, and its relationships with environmental and spatial effects were examined using variation partitioning analysis and partial Mantel tests. Taxonomic β total and β repl were marginally higher in tributaries than in the main lake, whereas functional beta diversity and its two components were significantly higher in tributaries. Both taxonomic and functional beta diversity remained stable between seasons in the main lake but increased markedly from summer to autumn in tributaries. Taxonomic beta diversity mainly comprised species replacement (56.65%–69.17%), while functional beta diversity was dominated by richness difference (62.96%–76.19%). Environmental filtering variables, particularly salinity and water temperature, explained a greater portion of community variation than spatial variables. These findings highlight that niche‐based environmental filtering and dispersal limitation jointly correlated with phytoplankton beta diversity in this high‐altitude river–lake system. Salinity emerged as a particularly influential factor, shaping phytoplankton assemblage variation through strong physiological filtering. The present study highlights the value of integrating both taxonomic and functional perspectives to better elucidate spatio‐temporal patterns of phytoplankton biodiversity in high‐altitude aquatic ecosystems.