Hypolimnetic oxygen depletion by sediment-based reduced substances in a reservoir formerly affected by acid mine drainage

Hypolimnion oxygen depletion (HOD) in lakes and reservoirs can degrade water quality and prevent recovery from eutrophication owing to the release of reduced metals and nutrients from sediments. However, oxygen consumption processes in the hypolimnia of reservoirs resulting from deep-water anoxia are not well understood. Here, we present the results from a mesotrophic reservoir, which had previously suffered from acid mine drainage, to demonstrate the process of hypolimnetic hypoxia. Sampling was conducted monthly from April 2021 to March 2022, and the sediment oxygen demand (SOD) and flux of reduced substances, including CH4, S2-, NH4+, and Mn2+ from the sediment core samples were analyzed intensively. Based on long-term monitoring data, we found that HOD in the reservoir began in early April and ended in early November. This led to a hypolimnetic hypoxic state in the lacustrine zone of the reservoir for eight months. The areal hypolimnetic oxidation demand (AHOD) and SOD were 0.21–1.29 gO2 m−2 d-1 and 0.06–0.84 gO2 m−2 d-1, respectively. This resulted in 30.45%–65.07% HOD due to oxygen depletion at the sediment–water interface. The highest fraction of SOD to AHOD occurred in the lacustrine zone samples of the reservoir. The flux of reduced substances in the sediment revealed that CH4 oxidation at the sediment–water interface dominated oxygen depletion in the lacustrine zone of the reservoir; additionally, S2- oxidation played an important secondary role in exacerbating hypolimnetic oxygen depletion in the mesotrophic reservoir. Sediments were a source of reduced substances. Organic matter content and SO42- and NO3- concentrations in the pore water were the most critical drivers of spatial variation in CH4 and S2- fluxes from the sediments. Temperature, dissolved oxygen concentration and saturation, and oxidation–reduction potential controlled temporal variations in CH4 and S2- concentrations in the overlying water. Compared with previous reports on the flux of reduced substances from sediments in lakes and reservoirs, HOD in the studies reservoir was not only affected by nutrient conditions but was also related to the sulfur cycle. Our findings indicate a spatial various of HOD mechanism in the reservoir and emphasized the importance of the sulfate reduction process in the reservoir formerly affected by acid mine drainage.