Seasonal Effect of a Small Hydropower Plant on Benthic Biofilm Microbial Community Structure and Predicted Functional Potential.

Benthic biofilm microbial communities play a critical role in riverine primary production, nutrient cycling, and organic matter decomposition, yet their responses to hydrological alterations due to small hydropower plants (SHPs) remain poorly understood, particularly with respect to seasonal dynamics. This study conducted a one-year monitoring survey across four river sections (upstream, reservoir, dewatered reach, and downstream) in the Oujiang River Basin, Zhejiang Province, China, covering all four seasons. The results showed that the reservoir section exhibited distinct bacterial composition, while the dewatered reach harbored the highest diversity, broader niche breadth, and was governed by stochastic assembly processes. Seasonally, deterministic processes dominated in summer, whereas stochastic processes prevailed in winter. Predicted functional profiling suggested that the SHP may exert a regulatory role in carbon and nutrient metabolism, with pronounced functional differences observed in summer. This study indicates that SHPs reshape microbial assembly and predicted function by altering hydrological regimes. As an observational case study based on a single representative facility, these findings should be interpreted as hypothesis-generating rather than conclusive; however, they provide key ecological insights and a methodological reference for future impact assessments of similar diversion-type SHPs in subtropical river systems.