Habitat partitioning shapes divergent bacterial community assembly and carbon–nitrogen functional responses under cascade-dam fragmentation and seasonality

Cascade-dam systems impose repeated hydrological discontinuities that convert a river continuum into alternating reservoir–river segments, enhancing environmental heterogeneity and longitudinal gradients beyond the influence of a single dam. Seasonal shifts in temperature and hydrologic conditions also modulate connectivity along the cascade, yet how these drivers jointly shape bacterial community assembly remains poorly understood. This study investigated 10 cascade-dammed reaches of the Qijiang River, collecting paired sediment and water samples in summer and winter. Using an integrated approach of 16S rRNA gene amplicon sequencing, functional gene quantification, and potential process assays, we analyzed the impacts of spatial fragmentation and seasonality on bacterial community structure, assembly mechanisms, and carbon-nitrogen functions. The results revealed a habitat partitioning phenomenon characterized by two divergent assembly mechanisms. The sediment community, predominantly shaped by stable spatial gradients, followed a deterministic track where assembly was consistently dominated by selection. Conversely, the highly sensitive water community followed a season-responsive track, with its assembly shifting from mixed assembly with elevated stochasticity in summer to deterministic control in winter. This functional partitioning was also evident: sediment functions were more strongly associated with community structure, while the water community exhibited high functional redundancy, maintaining relatively stable functional potential despite marked seasonal compositional shifts. Together, these results provide a mechanistic explanation for why dam cascades matter by demonstrating that repeated, season-modulated fragmentation generates habitat-specific assembly pathways and may buffer functional stability in regulated river landscapes.