Recovery in Dissolved Oxygen Levels in Chinese Freshwater Ecosystems in the Past Three Decades

Deoxygenation is a serious problem in oceanic waters, but few studies have documented dissolved oxygen (DO) changes in freshwater systems. In this study, the changes in DO at the national scale were analyzed in the past 30 years. The results showed that the proportion of sites with DO greater than 5 mg L–1 increased from 29.1% in 2002 to 87.45% in 2020, and the proportion of sites with DO less than 2 mg L–1 decreased from 44% in 2001 to 0.2% in 2020. There was a change point in the DO levels in 2007. DO levels can recover benefit from reduction in the pollution load. Meanwhile, further improvement in DO is challenged by global warming and inefficiency of pollutant collection and treatment. There is no doubt that the Chinese freshwater system has basically crossed the first stage of pollution control and treatment, laying a foundation for entering the stage of ecological restoration.