BEIJING: Climate warming is causing a widespread and sustained decline in oxygen levels in rivers around the world, putting freshwater ecosystems and aquatic life at risk, according to a new study.
The study, published in Science Advances, found that nearly 80 percent of more than 21,000 river reaches analysed globally showed signs of deoxygenation between 1985 and 2023. Researchers said the trend could increase the risk of low-oxygen events that threaten fish and other freshwater species.
The research was conducted by a team led by scientists from the Nanjing Institute of Geography and Limnology under the Chinese Academy of Sciences. The team used nearly 40 years of data and advanced machine-learning methods to track changes in river oxygen levels.
According to the study, rivers are losing oxygen at an average rate of 0.045 milligrams per liter per decade.
Tropical rivers face higher risk
The study found that tropical rivers, located between 20 degrees south and 20 degrees north, are being affected more severely than rivers in higher-latitude regions.
Researchers said this finding differs from earlier expectations that high-latitude rivers would face the greatest impact because of stronger warming in those areas.
Tropical rivers already tend to have lower oxygen levels and are losing oxygen more quickly, making them more vulnerable to extreme low-oxygen conditions. Such events can harm or kill fish and other aquatic organisms.
Warmer water holds less oxygen
Researchers found that climate-driven decline in oxygen solubility was the main cause of river deoxygenation, accounting for nearly 63 percent of the oxygen loss. Warmer water holds less dissolved oxygen than cooler water.
Changes in ecosystem metabolism, linked to temperature, light and water flow, accounted for another 12 percent of the decline.
The study also found that heatwaves contributed significantly to the trend. Extreme heat events were responsible for nearly 23 percent of global river deoxygenation and increased the rate of oxygen loss by 0.01 milligrams per liter per decade compared with normal conditions.
Water flow also influenced oxygen trends. Very low-flow conditions reduced the deoxygenation rate by about 19 percent compared with normal conditions, while high-flow conditions reduced it by 7 percent.
Researchers urge action
The researchers warned that declining oxygen levels could threaten river biodiversity as aquatic organisms struggle to survive in oxygen-depleted waters.
They urged policymakers to prioritise strategies to address deoxygenation, especially in tropical regions where rivers are most vulnerable.
Shi Kun, a researcher at NIGLAS, said the study provides an important baseline for understanding and reducing oxygen loss in rivers worldwide.
“As climate warming continues, protecting the breath of our rivers may become one of the greatest environmental challenges of our time,” he said.
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