Analysis of contributing factors to dissolved oxygen depletion in the anoxic zone and their respective contributions based on stratified hypolimnetic oxygen depletion

Hypolimnetic anoxia is often observed in large reservoirs during thermal stratification. In the hypolimnion layer, dissolved oxygen (DO) is continuously consumed by biological and chemical reactions, resulting in a stable anoxic zone at the bottom of the reservoir. The areal hypolimnetic oxygen depletion (AHOD) has been a widely used indicator for hypolimnetic DO depletion, but this indicator could not reflect the DO depletion process at the bottom of deep reservoirs with deposits of pollutant sediments. With Daheiting Reservoir as the study case, the concept of stratified areal hypolimnetic oxygen depletion (S-AHOD) was proposed, and an S-AHOD calculation model was built, which could accurately describe the DO depletion process in the anoxic zone at the bottom of Daheiting Reservoir, identify the DO depletion rate at different water depths in the anoxic zone and quantify the contribution of sediment oxygen demand to the overall oxygen depletion. It was found that sediment oxygen demand contributed considerably (41.4%) to the overall hypolimnetic DO depletion in the reservoir, DO depletion rate varied considerably at different water depths, which increased from the surface to the bottom of the reservoir and reached the maximum at the bottom layer. Finally, based on results of existing works, an AHOD calculation equation applicable to lakes and reservoirs with considerable deposits of heavily pollutant sediments was put forward.