Lake–Floodplain Substance Transport Processes Under the Influence of Water Control Project: Insights From Poyang Lake, China

ABSTRACT Substance transport dynamics in floodplain lakes are fundamental to ecosystem stability, yet their spatiotemporal variability under changing hydrological regimes remains insufficiently understood. The construction of a water control project further complicates these dynamics and poses new challenges to ecosystem integrity. This study integrated a hydrodynamic model with a particle‐tracking approach to quantify the substance transport timescales and pathways in Poyang Lake, China. Employing e‐folding flushing time as a diagnostic metric, we simulated the transport dynamics under both natural and regulated conditions. Results suggest that (1) the coupled modelling framework reliably captures the transport dynamics of floodplain lakes, effectively representing the spatial and temporal variability of flushing time and transport trajectories; (2) Poyang Lake exhibits pronounced seasonal and spatial heterogeneity in flushing time, ranging from approximately 10 days in spring and winter to over 20 days in summer and autumn, with longer retention in embayment areas than in the main channel and floodplain and (3) while the Poyang Lake Water Control Project alleviates seasonal water shortages, it markedly prolongs flushing times during autumn and winter and reduces spatial heterogeneity, increasing the potential for substance accumulation in low‐exchange zones. These findings provide mechanistic insights into the hydrodynamic processes governing substance transport in floodplain lakes and offer a scientific foundation for optimising water resource management and mitigating pollutant accumulation under anthropogenic interventions.