http://english.cas.cn/newsroom/research-news/202606/t20260611_1161691.shtml

https://www.nature.com/articles/s41561-026-02024-y

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 CAS Nanjing Institute of Geography and Limnology, an international research team analyzed 18 years of monthly data (2005–2022) from 972 rivers and 354 lake sites across China.

Although surface waters warmed by 1.2 °C per decade, a rate higher than the global average, dissolved oxygen (DO) concentrations increased across China’s inland waters during the 18 years of the study. DO levels rose by an average of 0.93 mg/L per decade in rivers and 0.38 mg/L per decade in lakes. Recorded hypoxic events in rivers fell from 170 occurrences in 2005–2010 to 25 in 2017-2022.

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. Instead,

• China’s annual investments in environmental restorationbetween 2000 and 2022 surged from 1 trillion to 10 trillion RMB (approximately US$148 billion to US$1.48 trillion).
• Expanded wastewater treatment coverage rose from 34.3% to 98.1% of the population.

The correlations between provincial investment in sewer infrastructure, the volume of wastewater treated, and the magnitude of DO recovery were exceptionally strong, in Jiangsu.

Further highlights of the study:

• in small headwater streams and the warm-temperate zones of central China, recovery was strongest.
• Challenges remain in regions dominated by agricultural nonpoint-source pollution
• 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.

According to ZHOU, effective water quality management can improve oxygen levels, protecting aquatic life and reducing the risk of deoxygenation while the climate continues to warm.