Effective nutrient management reverses widespread deoxygenation of freshwater ecosystems

Dissolved oxygen (DO) concentrations regulate diverse aquatic ecosystem processes from organismal respiration to biogeochemical cycles, yet DO levels have declined markedly in flowing (−0.04 mg L−1 decade−1) and standing waters (−0.12 mg L−1 decade−1) of Europe and North America during recent decades. While atmospheric warming partially explains this deoxygenation, DO depletions have also intensified due to land-use change (farming, urbanization, etc.), opening the possibility that improved land management can prevent widespread oxygen loss from freshwaters. Here, we analyzed monthly DO concentration, percent oxygen saturation (DO%sat), 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 (~10×1012 Chinese Yuan year⁻1) to control nutrient pollution correspond with changes in freshwater oxygen levels. Despite surface water warming of +1.2°C decade−1, concentrations of DO increased markedly in rivers (+0.93 mg L−1 decade−1) and lakes (+0.38 mg L−1 decade−1), as did DO%sat (+10.7% decade−1 and +4.5% decade−1, respectively), resulting in a rapid and substantial decrease in the incidence of hypoxia and anoxia. Long-term increases in DO 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.