Freshwater ecosystems worldwide have been suffering from declining oxygen levels—a trend known as deoxygenation—that threatens biodiversity, fisheries, and ecosystem stability. However, a new study published on June 26 in Nature Geoscience offers hope: targeted nutrient management via wastewater control can reverse this trajectory, even in the face of rapid climate warming.
Led by Professor ZHOU Yongqiang from the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences, an international research team analyzed 18 years of monthly data (2005–2022) from 972 rivers and 354 lake sites across China. Their findings challenge the prevailing narrative that aquatic deoxygenation is an unstoppable consequence of human development and global warming.
Although surface waters warmed by 1.2 °C per decade, a rate higher than the global average, the researchers found that dissolved oxygen (DO) concentrations increased across China’s inland waters during the 18 years of the study—a phenomenon that is not typically associated with rising water temperatures.
Specifically, DO levels rose by an average of 0.93 mg/L per decade in rivers and 0.38 mg/L per decade in lakes. This recovery led to a dramatic reduction in the incidence of hypoxia (low oxygen) and anoxia (no oxygen), with recorded hypoxic events in rivers falling from 170 occurrences in 2005–2010 to just 25 in 2017-2022.
The primary driver of this remarkable recovery, the team found, was not related to increased photosynthesis from algae, but rather a reduction in organic pollution.
Using variance partitioning and machine learning algorithms (XGBoost), the team found that decreases in biochemical oxygen demand (BOD), ammonium, and chemical oxygen demand (COD) were the best predictors of rising DO levels. In contrast, changes in phytoplankton abundance (measured as chlorophyll-a) showed no consistent relationship with DO trends, ruling out algal-driven oxygen supersaturation as a cause for the recovery.
China’s investments in environmental restoration, which surged from 1 trillion to 10 trillion RMB (approximately US$148 billion to US$1.48 trillion) annually between 2000 and 2022, expanded wastewater treatment coverage from 34.3% to 98.1% of the population. This resulted in nationwide declines in BOD, COD, and nutrients like nitrogen and phosphorus, the study suggested.
“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,” he added.
The study also highlights where recovery is strongest—small headwater streams and the warm-temperate zones of central China. However, challenges remain in regions dominated by agricultural nonpoint-source pollution.
According to the study, the rapid flushing rates of many Chinese freshwaters likely contributed to the swift response to management, as legacy pollutants stored in sediments were less of a factor than in deeper, stratified lakes.
“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.”
Nature Geoscience
Widespread deoxygenation of freshwater ecosystems regularly reversed by nutrient management
26-Jun-2026