Freshwater ecosystems worldwide face pressure from pollution, shifting weather and higher temperatures. Many rivers and lakes have lost dissolved oxygen, stressing fish and other aquatic life. Warmer water holds less oxygen, a trend often linked to climate change. New findings from China show a different outcome. Long-term data indicate that better water quality can counter some effects of rising temperatures. The study in Nature Geoscience tracked inland waters across China for 18 years through 2022. Monthly readings came from hundreds of lakes and nearly a thousand river sites. Water temperatures rose about 1.2 degrees Celsius per decade. Normally this would lower oxygen levels, yet average concentrations increased. Rivers improved most, while lakes gained less. The change differed from patterns seen in much of the world. Researchers used statistical and machine learning methods to identify causes. Cleaner water proved the main factor. Organic pollutants declined sharply, cutting the oxygen used in breakdown processes. Measures of wastewater pollution such as biochemical oxygen demand, chemical oxygen demand and ammonium all fell. Wastewater treatment coverage grew from roughly one-third of the population early in the century to near full coverage by 2022. This reduced untreated sewage and nutrient releases. Provincial spending on sewer systems correlated with greater oxygen recovery. Low-oxygen events also became rarer, especially in smaller upstream rivers and central warm-temperate zones. Lakes recovered more slowly due to differences in water movement.
