Rapid restructuring of surface-water bacterial communities across the spring-to-early-summer transition in a...

Reservoirs are engineered freshwater ecosystems in which seasonal environmental variability interacts with the physical and physicochemical conditions created by water-conservancy infrastructure. However, bacterial succession during relatively short seasonal transitions remains insufficiently resolved in many subtropical reservoirs. Here, we investigated surface-water bacterial communities in Guanchang Reservoir, southeastern China, across eight sampling campaigns from 14 April to 23 June 2026. Thirty-two samples were collected from four fixed stations spanning the reservoir, including the reservoir-head area, an intermediate station, the water-intake zone, and the dam-front area. Bacterial communities were characterized using 16S rRNA gene amplicon sequencing and examined together with water temperature, pH, dissolved oxygen, ammonium nitrogen, total phosphorus, and total nitrogen. Bacterial richness and Shannon diversity differed significantly among sampling dates. Community composition exhibited pronounced temporal succession, with non-metric multidimensional scaling showing clear differentiation among sampling periods (stress = 0.098, R = 0.7654, P = 0.001). Pseudomonadota, Actinomycetota, Bacteroidota, Cyanobacteriota, Bacillota, Planctomycetota, and Verrucomicrobiota dominated the assemblages, but substantial turnover occurred at finer taxonomic levels. A particularly conspicuous transition occurred in early June, when Cyanobium PCC-6307 increased simultaneously across all four stations, whereas several taxa characteristic of earlier communities, including Clostridium, Turicibacter, Terrisporobacter, Polynucleobacter, and the CL500-29 marine group, declined. Canonical correspondence analysis linked bacterial turnover to changing environmental gradients, while genus-level correlations showed contrasting associations of spring- and early-summer-associated taxa with temperature, dissolved oxygen, and nutrient concentrations. These findings demonstrate rapid and coordinated bacterial succession during the spring-to-early-summer transition in a regulated reservoir and highlight the ecological importance of resolving transient microbial states within water-conservancy ecosystems.