Seasonal Variation in the Assembly Mechanisms and Co-Occurrence Patterns of Bacterial Communities in Water and Sediments of Reservoirs in Arid and Semi-Arid Regions.

Reservoir water and sediments are interconnected but ecologically distinct microbial habitats. Their seasonal bacterial dynamics and assembly processes remain poorly understood in arid and semi-arid regions. We investigated bacterial communities in the water and surface sediments of eight reservoirs in Guyuan, Ningxia, in April, July, and October 2025 using 16S rRNA gene sequencing. Water and sediment communities differed markedly in composition. Sediments had substantially higher α-diversity than water: mean Chao1 and Shannon indices were 3063.12 and 9.98 in sediments, compared with 748.38 and 6.25 in water. Seasonal turnover was stronger in water. Water communities differed significantly between April and July (R2 = 0.1139, p = 0.001) and between April and October (R2 = 0.0968, p = 0.013), whereas sediment communities did not differ between April and July (R2 = 0.0581, p = 0.877). Water communities were also more closely associated with seasonal changes in temperature, pH, and nutrients. βNTI-based null models indicated that water communities were mainly associated with stochastic processes sensu lato, whereas sediments showed a stronger deterministic signal, particularly heterogeneous selection. Sediment co-occurrence networks were more complex; in July, they contained 10,496 edges, compared with 7005 edges in water. Network complexity peaked in July in both habitats. These findings show that sediments act as relatively diverse and stable microbial reservoirs, whereas water communities respond more strongly to seasonal environmental variation. They clarify how habitat and season jointly structure bacterial communities in arid and semi-arid reservoirs.