Contrasting hydrological states are associated with sediment bacterial community variation and micropollutant biodegradation potential in a river-connected floodplain lake

Hydrological contraction is a defining feature of river-connected floodplain lakes, but its association with sediment bacterial communities and contaminant biodegradation potential remains incompletely understood. During one observed wet-to-dry transition in Dongting Lake, China, we combined sediment physicochemical and enzyme measurements, pre-incubation bacterial community analyses, and standardized microcosm assays of 17α-ethinylestradiol biodegradation potential (EBP₃₀) in sediments. As mean local water coverage decreased from 76.4% in the expanded state to 35.2% in the contracted state, dry-period sediments generally exhibited higher TOC, Fe(III), potential dehydrogenase activity, and field estrogen concentrations. Wet-period sediments showed higher bacterial richness, diversity, and community-level niche breadth and lower compositional dissimilarity and dispersion, whereas dry-period communities exhibited greater relative contributions of dispersal limitation and environmental selection, together with a denser, less modular, and more positively cohesive correlation network. Despite reduced alpha diversity, EBP₃₀ increased from 0.76 ± 0.06 μg g^-1 during the wet period to 0.93 ± 0.18 μg g^-1 during the dry period, with the largest wet-to-dry increase in East Dongting Lake. Partial least-squares path modeling (PLS-PM) and site-blocked cross-validation indicated that EBP₃₀ was more closely associated with bacterial compositional indicators, dry-period-associated taxa, and sample-level network cohesion than with taxonomic richness alone. A sensitivity PLS-PM using site-level water coverage retained similar explanatory power and identified a supported negative total association between water coverage and EBP₃₀. These findings identify hydrological state as an important correlate of sediment microbial functional potential and emerging-contaminant fate in dynamic floodplain lakes.