When researchers first documented oxygen levels in rivers and lakes across China in 2005, the outlook was grim. Rising temperatures were expected to drain life-giving oxygen from freshwater ecosystems worldwide—a process called deoxygenation that threatens fish, insects, and the entire aquatic food web. Eighteen years later, the data tell a different story. Despite warming waters that rose 1.2 degrees Celsius per decade—faster than the global average—dissolved oxygen levels in China's inland waters climbed steadily upward. The number of hypoxic events recorded in rivers plummeted from 170 between 2005 and 2010 to just 25 between 2017 and 2022.
The findings, published in Nature Geoscience by Professor Zhou Yongqiang of the Nanjing Institute of Geography and Limnology, offer what the research team calls "clear optimism for global restoration efforts." Using 18 years of monthly monitoring data from 972 rivers and 354 lakes, the international team found that oxygen levels rose by an average of 0.93 milligrams per liter per decade in rivers and 0.38 milligrams per liter per decade in lakes. The recovery was not driven by algal blooms—as some had speculated—but by something far more actionable: cuts to organic pollution.
The mechanism behind the rebound lies in China's massive investment in wastewater infrastructure. Between 2000 and 2022, the country ramped up environmental spending from 1 trillion to 10 trillion RMB annually—roughly $148 billion to $1.48 trillion. Wastewater treatment coverage expanded from 34.3 percent of the population to 98.1 percent. This resulted in nationwide declines in biochemical oxygen demand, chemical oxygen demand, and nutrient pollutants like nitrogen and phosphorus. The team used machine learning algorithms to confirm that reductions in these pollution markers, not changes in phytoplankton abundance, best predicted rising oxygen levels.
"While water temperature remains a strong predictor of oxygen solubility, our models show that reducing oxygen demand through pollution control has more than offset the oxygen loss expected from warming," Zhou said. "The correlations between provincial investment in sewer infrastructure, the volume of wastewater treated, and the magnitude of DO recovery are exceptionally strong."
Recovery was not uniform across every waterway. The swiftest improvements appeared in small headwater streams and warm-temperate zones of central China, where the rapid flushing of water likely helped pollutants dissipate quickly from sediments. Agricultural regions still struggle with nonpoint-source pollution from fertilizers and manure, which remains harder to capture than industrial and municipal wastewater. But the overall trend is unmistakable, the researchers say.
"These results provide clear optimism for global restoration efforts," Zhou said. "Effective water quality management can improve oxygen levels, protecting aquatic life and reducing the risk of deoxygenation while the climate continues to warm."
The study challenges a prevailing narrative that aquatic deoxygenation is an inevitable consequence of development and warming. Instead, it suggests that when governments act decisively to cut pollution, freshwater ecosystems can recover—even against formidable headwinds.