Imbalance of internal nutrient retention in a typical eutrophic plateau lake associated with ecological water replenishment and its implications

Study region The Dianchi Lake, Kunming, Yunnan Province, China. Study focus Ecological water replenishment (EWR) is widely applied to improve lake hydrology and water quality, yet its effects on internal nutrient cycling in eutrophic lakes remain unclear. This study investigated sediment nutrient retention changes and mechanisms in Dianchi Lake after a decade of EWR, aiming to elucidate the decoupled responses of sedimentary nitrogen (N) and phosphorus (P) cycling to hydrological regulation and lake restoration. New hydrological insights for the region After EWR, hydraulic residence time (HRT) decreased by ~50%, while total N (TN) and total P (TP) concentrations declined, and dissolved oxygen (DO) increased in water column. Enhanced hydrodynamics and oxygenation accelerated sediment organic N mineralization, increasing ammonium (NH₄⁺-N) potential diffusive flux by 62%. In contrast, sediments shifted from a P source to a sink, with soluble reactive phosphorus (SRP) fluxes turning negative. Meanwhile, sediment dissolved organic matter (DOM) became more humified, transitioning from protein-like to humic-like dominance. These physicochemical changes were further reinforced by ecosystem recovery. Consequently, the water-column TN/TP ratio increased significantly, while sediment TN/TP and potentially mineralizable N/P ratios declined by 48.01% and 58.28%, respectively. These results reveal a decoupling of sediment N and P cycles under hydrological regulation, characterized by enhanced N release but preferential P retention, highlighting the need for coordinated N and P management strategies for sustainable lake restoration.