Steroid estrogen biodegradation across hydrological seasons is shaped by distinct substrate and bacterioplankton network controls in a river-connected floodplain lake.

Emerging contaminant attenuation in freshwater lakes may depend on metabolically available organic substrates and the organization of indigenous microbial communities, yet their relative contributions across hydrological seasons remain unclear. Using estrone as a representative steroid estrogen, we investigated its biodegradation potential (EBP) across contrasting wet and dry periods in Dongting Lake, China's second-largest freshwater lake. Dissolved organic matter (DOM) optical properties, biodegradable dissolved organic carbon (BDOC%), bacterioplankton community assembly, and co-occurrence network attributes were characterized to evaluate the associations of substrate conditions and community organization with estrone biodegradation. Wet-period conditions were characterized by stronger humified and terrestrial-associated DOM signatures and lower BDOC%, whereas dry-period conditions featured more autochthonous or microbially processed DOM with greater biodegradability. Higher BDOC% coincided with a more biodegradable DOM background and higher carbon-normalized EBP (C-EBP), which increased from 23.51 ± 5.35 ng mg-1 C during the wet period to 34.05 ± 6.98 ng mg-1 C during the dry period. However, the concurrent increase in biomass-normalized EBP (B-EBP) was not associated with BDOC%, but was linked to dry-period bacterioplankton community reorganization characterized by DOM-utilizing bacterial groups, stronger heterogeneous selection, and more connected, cohesive, and robust association networks. Piecewise structural equation modeling further identified positive associations of DOM biodegradability and network stability with C-EBP, whereas B-EBP was primarily associated with network stability. These findings indicated that seasonal variation in steroid estrogen biodegradation reflected distinct but interconnected associations of substrate availability and bacterioplankton organization, providing an integrative ecological framework for evaluating emerging contaminant degradation capacity in river-connected floodplain lakes.