High-resolution imaging reveals hidden phosphorus mobilization during the winter–spring transition in transboundary Lakes

The biogeochemical dynamics of phosphorus (P) beneath ice cover, particularly at the sediment–water interface (SWI), remain poorly understood due to the lack of high-resolution in-situ characterization, limiting our ability to predict nutrient cycling in cold-region ecosystems. Despite low temperatures and limited hydrodynamic disturbance, ice cover promoted substantial accumulation of labile P and Fe(II) at the SWI, with winter diffusive P fluxes reaching 0.46–0.50 mg m−2 d−1, comparable to or exceeding those reported for many eutrophic lakes during ice-free periods. Ice cover weakened nutrient exchange and hydrological connectivity between Lake Xingkai and Lake Xiaoxingkai, whereas spring thaw restored basin-scale connectivity and promoted nutrient redistribution. Turbidity was strongly associated with labile P and Fe(II) dynamics beneath ice, suggesting that particle-related processes may be linked to winter nutrient mobilization. These results reveal strong P–Fe coupling across the SWI during ice-covered and ice-breakup periods and demonstrate that winter represents a critical, yet underappreciated, phase of internal P loading in cold-region agricultural wetland–lake systems. Management of eutrophication in the Lake Xingkai/Khanka Basin should explicitly consider winter redox processes, seasonal hydrological reconnection, and maintenance of P limitation under ice-covered conditions.