Hydrological seasonality shapes antibiotic resistome assembly and dissemination risk in a reclaimed-water-fed urban river.

Reclaimed-water-fed urban rivers are increasingly recognized as potential hotspots for antibiotic resistance gene (ARG) dissemination; however, the combined effects of hydrological variability and habitat heterogeneity on resistome dynamics remain poorly understood. Here, paired water and sediment samples were collected from 10 sites along the Qinghe River during non-flood and flood periods and analyzed using metagenomic sequencing. By integrating ARG host identification, mobile genetic element (MGE) profiling, spatial ecological analyses, and community assembly modeling, we characterized ARG composition, host associations, spatial organization, and ecological drivers. Water harbored a more diverse resistome than sediment, while flood-period water exhibited the highest ARG abundance and diversity, including pronounced enrichment of β-lactam resistance genes. A total of 415 ARG-hosting species were identified, including 41 potential human pathogenic bacterial (HPB) species, among which Acinetobacter spp. were dominant. The abundance and diversity of HPB increased markedly during the flood period. MetaCompare analysis and ARG-MGE co-occurrence patterns further indicated that flood-period water exhibited the highest community-level ARG transmission potential (risk score = 19.32), with MGEs showing stronger associations with pathogenic hosts, suggesting elevated dissemination potential. Neutral community modeling indicated that stochastic dispersal and ecological drift were the dominant assembly processes (R2 > 0.85), whereas partial least squares path modeling identified physicochemical conditions as the strongest deterministic drivers of ARG variation (path coefficient = 0.751, P < 0.001). Collectively, these findings identify flood-period water as the critical compartment where ARG enrichment, pathogenic host accumulation, and dissemination potential converge, providing an ecological framework for seasonally targeted antimicrobial resistance surveillance and reclaimed water management under increasing hydrological variability.