China’s cleaner wastewater tied to rising oxygen in rivers and lakes
A Nature Geoscience study links expanded wastewater treatment to oxygen gains across hundreds of Chinese freshwater sites despite warming waters.
By Tom Brennan · Health & Medicine Correspondent
3 min read
China’s rivers and lakes gained dissolved oxygen over 18 years even as their surface waters warmed, according to a new study in Nature Geoscience. The finding matters because falling oxygen levels can damage freshwater ecosystems, fisheries and biodiversity, and the study points to wastewater control as a way to reverse that trend.
The research was led by Professor Zhou Yongqiang of the Nanjing Institute of Geography and Limnology at the Chinese Academy of Sciences, according to the academy. The international team examined monthly records from 2005 through 2022 at 972 river sites and 354 lake sites across China.
The researchers found that surface waters warmed by 1.2 degrees Celsius per decade during the study period, a pace they described as above the global average. Despite that warming, dissolved oxygen rose by an average of 0.93 milligrams per liter per decade in rivers and 0.38 milligrams per liter per decade in lakes, according to the study.
Low-oxygen events also became less common. The study reported that hypoxic events in rivers fell from 170 occurrences in 2005-2010 to 25 in 2017-2022.
Pollution controls linked to oxygen gains
The team attributed the recovery mainly to reduced organic pollution rather than to increased oxygen production from algae. Using variance partitioning and XGBoost machine-learning models, the researchers found that falling biochemical oxygen demand, ammonium and chemical oxygen demand best predicted rising dissolved oxygen levels.
The study said changes in phytoplankton, measured by chlorophyll-a, did not show a consistent relationship with oxygen trends. That finding led the authors to reject algal-driven oxygen supersaturation as the main explanation for the oxygen rebound.
China’s environmental restoration spending rose from 1 trillion RMB to 10 trillion RMB annually between 2000 and 2022, roughly US$148 billion to US$1.48 trillion, according to figures cited by the study. Over the same period, wastewater treatment coverage expanded from 34.3% to 98.1% of the population, the researchers reported.
Those changes were associated with nationwide declines in biochemical oxygen demand, chemical oxygen demand and nutrients including nitrogen and phosphorus, according to the study. Zhou said the models showed that cutting oxygen demand through pollution control more than offset the oxygen losses expected from warmer water.
Zhou also said provincial investment in sewer infrastructure and the amount of wastewater treated were strongly correlated with the scale of dissolved oxygen recovery.
Recovery varied by place
The study found the strongest improvements in small headwater streams and in warm-temperate areas of central China. The researchers also reported continuing difficulties in areas where agricultural nonpoint-source pollution is dominant.
According to the authors, many Chinese freshwater systems respond quickly to management because they flush rapidly. That means older pollutants stored in sediments may play a smaller role than they do in deeper, stratified lakes.
Zhou said the results offer support for freshwater restoration efforts beyond China. Effective water-quality management can raise oxygen levels and reduce the risk of deoxygenation while climate warming continues, he said.
The paper, titled “Widespread deoxygenation of freshwater ecosystems regularly reversed by nutrient management,” was published in Nature Geoscience. Its DOI is 10.1038/s41561-026-02024-y.
This story draws on original reporting from Phys.org.