Seasonal dynamics and environmental associations of benthic algae in the upper Red River Basin

Benthic algae are important primary producers and sensitive bioindicators in river ecosystems, yet their dry–wet seasonal dynamics remain poorly understood in subtropical plateau rivers. We investigated benthic algal communities and environmental variables at 45 sites in the upper Red River Basin during the dry and wet seasons of 2021. BIO-ENV analysis and redundancy analysis were used to identify environmental variables associated with seasonal community variation. Benthic algal communities showed a clear dry–wet contrast, with lower richness and total density but higher evenness during the wet season. Total density declined by approximately 51.6%, and dominance shifted from Navicula in the dry season to Pseudanabaena in the wet season. Nutrient concentrations increased markedly during the wet season, and N:P ratios pointed to a potential shift from phosphorus limitation in the dry season to nitrogen limitation in the wet season. Community–environment associations also differed seasonally: dry-season communities were linked to a broader set of nutrient, habitat, and biotic variables, whereas wet-season communities were more closely tied to nutrient and water-chemistry variables. These findings provide empirical evidence for dry–wet seasonal restructuring of benthic algal communities and emphasize the role of hydrology–nutrient interactions in subtropical monsoon-driven rivers.