Mixing and stratification dynamics shape bacterial community composition, assembly patterns, and co-occurrence...

Bacterial communities in cascade reservoirs are receiving increasing attention owing to their roles in biogeochemical cycling and water quality regulation. However, there is a need to identify how the temporal, spatial, and vertical dimensions jointly structure water-column bacterial communities in Han River cascade reservoirs, determine whether these dimensions are coupled, and elucidate the physical mechanisms underlying the observed variability. In this study, PERMANOVA indicated that temporal variation (R² = 0.224, p < 0.001) explained more of the bacterial community variation than spatial differences among the reservoirs (R² = 0.144, p < 0.001), whereas depth had no overall main effect (R² = 0.016, p = 0.710). Spatial differentiation among the reservoirs was strongest during the early stratification period. Despite the non-significant main effect of depth, the compositional dissimilarity of the surface-bottom bacterial community increased with stratification intensity (most clearly during the fully stratified period). β-diversity was dominated by species turnover rather than nestedness. Bacterial community assembly was largely stochastic, whereas deterministic influences strengthened transiently during the early stratification period. Co-occurrence networks became progressively more modular from the mixing to full stratification period, with positive associations increasingly prevailing. Partial least squares path modeling suggested that water temperature and electrical conductivity were associated with bacterial community indices via keystone taxa. This study characterizes the water column bacterial community variation across temporal, spatial, and vertical dimensions in Han River cascade reservoirs and provides important ecological insights for water quality management and monitoring.