Dynamic reconstruction of sediment oxygen demand and quantitative elucidation of nutrient release mechanisms in a shallow eutrophic system

Shallow lake sediment oxygen demand and nutrient release are key processes driving bottom-water hypoxia and internal loading in lake ecosystems. To quantitatively characterise the dynamic nature of sediment oxygen consumption and to further elucidate the kinetic mechanisms underlying multi-elemental nutrient fluxes, this study integrated sediment incubation experiments with in-situ high-frequency monitoring data. The dynamic variations in sediment oxygen demand in the center of the shallow lake Ulansuhai were estimated, and the release characteristics of nitrogen, phosphorus, silicon and carbon fluxes were systematically analysed. The results showed that: (1) oxygen consumption by the sediment in the lake centre is mainly driven by autochthonous organic matter, and its actual flux is subject to multiple interactive controls, including offsetting by algal oxygen production, competition from organic oxygen consumption and anoxia feedback; (2) the release of NH₃-N and DIC at the sediment–water interface follows first-order kinetics driven by concentration gradients, whereas that of SRP and DSi is dominated by zero-order kinetics controlled by surface adsorption/desorption processes; (3) internal release caused the nutrient structure of the overlying water to evolve nonlinearly over time and tend to decouple, and exerted a dual driving effect on carbon limitation and nitrogen–phosphorus imbalance in the water column. This study revealed the differential impacts of physical forcing and biochemical consumption on sediment oxygen demand and the release mechanisms of internal pollution, and provides a new technical reference for the mining of monitoring data from similar lakes and the assessment of internal fluxes.