The Response to Hydrological Regime Change of Nitrogen Transformation Processes at the Sediment-Water Interface of Seasonal Floodplain Lakes: Insights From the Yangtze River-Poyang Lake System

Poyang Lake, the largest freshwater lake in China and a globally significant wetland, is intricately connected to the hydrological dynamics of the Yangtze River via a complex river-lake exchange system. This system generates to unique seasonal fluctuations, forming a distinctive seasonal lake system, which influences hydrological and hydrodynamic processes across floodplains. Recent years have witnessed significant alterations in the hydrological patterns of the Yangtze River, notably in water levels, thereby impacting the nutrient dynamics such as nitrogen transformation at the sediment-water interface of Poyang Lake. This study establishes a coupled model integrating hydrodynamics and nitrogen transformation to elucidate the impacts of the hydrological regime of Yangtze River on nitrogen transformation in Poyang Lake after the operation of Three Gorges Dam. Findings reveal spatiotemporal variations in both hydraulics and nitrogen transformation within the seasonal lake system. Notably, the recharge rate between surface water and groundwater experiences a substantial shift, surpassing 60%. Furthermore, the nitrification rate at the sediment-water interface escalates by 28.5%, and the denitrification rate increases by 21.3% owing to pronounced alterations in the hydrological regime. However, this intensified transformation does not translate to enhanced efficiency, as the nitrogen transformation efficiency declines to 72.3% of its original rate. This research provides a theoretical framework for understanding the ecological and environmental impacts of human interventions on Poyang Lake and highlights the implications for managing other floodplains and seasonal lakes globally, such as lakes on floodplains of Amazon River and Mekong River, which face similar challenges in hydrological dynamics and ecosystem health.