Spatiotemporal Dynamics of Planktonic Bacterial Communities and Their Assembly Mechanisms in a Newly Created Riverine Reservoir

The distribution patterns and assembly mechanisms of planktonic bacteria are fundamental to understanding reservoir material cycling and water quality. However, systematic studies on these topics—particularly regarding abundant versus rare subcommunities—remain limited for newly formed riverine reservoirs. A seasonal survey was conducted in Guandaoqiao Reservoir from autumn 2023 to summer 2024, employing high-throughput sequencing and multivariate statistical analyses. Results showed that abundant and rare subcommunities differed significantly in composition (p < 0.05), sharing Pseudomonadota and Actinomycetota as dominant phyla, and hgcI_clade and CL500-29_marine_group as dominant genera. Community structure varied seasonally (p < 0.05) but not spatially, with rare subcommunities exhibiting higher diversity (p < 0.05). Planctomycetota , SAR324_clade , and Chlorobiota increased significantly within the reservoir (p < 0.05). pH, temperature, conductivity, and phosphorus were key environmental drivers. Assembly of abundant subcommunities was consistently governed by stochastic processes (dispersal limitation and drift) year-round, whereas rare subcommunities shifted from deterministic homogeneous selection in spring–summer to stochastic processes in autumn–winter. The co-occurrence network showed significant seasonal differentiation in size, complexity, and modularity (p < 0.05), with positive interspecific associations dominant across all seasons. Keystone species underwent seasonal turnover and were mainly contributed by rare taxa. Reservoir impoundment significantly reduced network robustness (p < 0.05), indicating weakened community stability. Our findings reveal seasonal shifts in assembly mechanisms and network dynamics of planktonic bacteria in Guandaoqiao Reservoir, providing a microbial perspective for ecological assessment of newly formed riverine reservoirs.