Widespread deoxygenation of freshwater ecosystems regularly reversed by nutrient management

Abstract

Dissolved oxygen concentrations regulate diverse aquatic ecosystem processes from organismal respiration to biogeochemical cycles, yet concentrations have declined markedly in flowing and standing waters in Europe and North America during recent decades. While atmospheric warming partially explains this deoxygenation, land-use changes such as farming and urbanization have also intensified depletion of dissolved oxygen, which suggests that improved land management could prevent widespread oxygen loss from freshwaters. Here we analysed monthly dissolved oxygen concentration, percent oxygen saturation and coeval environmental variables in 972 river and 354 lake sites in China from January 2005 to December 2022 to assess how extensive restoration efforts to control nutrient pollution correspond with changes in freshwater oxygen levels. Despite surface water warming of +1.2 °C per decade, dissolved oxygen concentration and percent saturation increased markedly in rivers and lakes resulting in a rapid and substantial decrease in the incidence of hypoxia and anoxia. Long-term increases in dissolved oxygen were correlated negatively with biochemical oxygen demand, but not with phytoplankton abundance. We conclude that effective nutrient management at the sub-continental scale can reverse widespread freshwater deoxygenation, thereby improving fisheries, biodiversity and ecosystem health, while reducing the risk of exceeding critical planetary boundaries even in the face of global warming. Dissolved oxygen concentrations increased in rivers and lakes across China between 2005 and 2022 due to nutrient management and despite climate warming-related decreases, according to a study of dissolved oxygen and environmental variables.