Preliminary Study on the Dissolved Oxygen Recovery Process in Freshwater Ecosystems under the Coupling Effect of Oxygen-Consuming Pollutants and Temperature

Widespread deoxygenation of the oceans has caused great concern, but few large studies of changes in dissolved oxygen (DO) concentrations in freshwater ecosystems and the factors driving such changes have been performed. Here, DO and oxygen-consuming pollutant (OCP) data for freshwater ecosystems in China from 2008 to 2018 were used to assess changes in DO concentrations and the mechanisms involved. DO concentrations in freshwater ecosystems in China recovered markedly in the study period. The DO concentrations have linearly increased (R2 = 0.95, p < 0.01) in that period. The chemical oxygen demand (CODMn) and NH3–N concentrations have significantly decreased [polynomial fitted curves (n = 2): R2 = 0.77, p < 0.01 for CODMn and R2 = 0.95, p < 0.01 for NH3–N]. The polynomial curves fitted to the CODMn, NH3–N, and DO data indicated that there are thresholds for oxygen-rich and deoxygenation processes dominated by OCPs in freshwater ecosystems. Decreasing OCP concentrations will not promote DO recovery if the CODMn and NH3–N concentrations are <4 and <0.4 mg L–1, respectively. The effect of temperature on the DO concentration in freshwater was obscured by inputs of large amounts of OCPs. The discovery of thresholds for the effect of OCPs led to the division of the DO recovery process into stages by increases in the temperature.