Kathmandu. A new study shows that oxygen levels in rivers around the world are steadily decreasing due to climate change, posing a serious threat to freshwater.

The study was published in the journal Science Advances. The research was led by researchers from the Nanjing Institute of Geography and Limnology under the Chinese Academy of Sciences.

The research team analysed data from about 40 years from 1985 to 2023. The study used advanced computer-based analysis methods to measure oxygen levels in more than 21,000 river stretches around the world.

According to the study, the oxygen level in rivers is decreasing at an average rate of 0.045 milligrams per liter per decade. About 80 percent of the rivers studied were deficient in oxygen.

Rivers in the tropics have been particularly affected. While high-latitude areas were previously thought to be hardest-hit, the study showed that tropical rivers are already losing oxygen faster because they are already low on oxygen.

This can lead to very low oxygen levels, which can lead to the death of fish and other aquatic creatures.

According to the researchers, the main reason is the decrease in the capacity of oxygen in the water, which accounts for about 63 percent of the total oxygen loss. Hot water can hold less oxygen than cold water.

Changes in temperature, light and water flow processes contributed to an additional 12 per cent oxygen reduction, the study said.

Extreme heat waves have also played a major role in the depletion of oxygen in the rivers. The study found that about 23 per cent of global river oxygen shortages are linked to heat waves.

The study also found that too little or too much water flow could reduce the rate of oxygen loss to some extent. The rate of oxygen depletion was found to decrease by 19 per cent when the flow was less than normal, while the rate of oxygen depletion decreased by 7 per cent due to high flow.

Researchers warn that falling oxygen levels could have a serious impact on the river’s biodiversity. When oxygen is low, fish and other organisms struggle to survive.

They urged policymakers, especially in tropical countries, to adopt effective strategies.

Shi Kun, a researcher at the Nanjing Institute of Geography and Limnology, said: “This study provides an important basis for understanding and mitigating oxygen depletion in rivers around the world. ”