Oxygen levels are dropping in rivers across the US and central Europe

Rivers in the US and central Europe are rapidly losing oxygen due to rising temperatures, which is putting fish at risk.

Li Li at Pennsylvania State University and her colleagues have reconstructed daily oxygen and temperature levels for 796 rivers in the US and central Europe, such as Austria and Hungary, between 1981 and 2019, using several data sources.

Assessing oxygen levels accurately is difficult because there is a lack of high-quality daily data on rivers, says Li. So, to produce a comprehensive daily dataset, she and her colleagues combined a wide range of data sources for hundreds of rivers, including water temperature and oxygen level, weather information for the river locations and data about surrounding land.

The researchers then used a machine-learning model to integrate this data and produce estimated daily oxygen and temperature levels for the 796 rivers. They statistically validated the estimates against the 25 per cent of existing data they didn’t use. Li says this was achievable for oxygen and water temperature levels because these variables are quite dependent on the local temperature.

The researchers found that 87 per cent of the rivers had been getting warmer over the past four decades and that 70 per cent of them had been losing oxygen over this period. “For any liquid, if it’s warmer, its capacity for holding gases is smaller,” she says.

While urban rivers warmed the fastest, oxygen loss happened quicker in rivers through agricultural areas. “My guess is that agricultural rivers have more nutrients that consume oxygen,” says Li. Nutrients can run off into rivers from fertilisers that are sprayed onto farmland, she says.

This loss of oxygen puts fish at risk, says Li. “When oxygen levels drop to a certain level, they essentially suffocate and cannot breathe,” she says. Healthy rivers typically have a dissolved oxygen level above 8 milligrams per litre. Rivers below 3mg per litre are considered hypoxic and pose a particularly acute danger to fish, says Li.

The researchers then used their model to make projections about river temperature and oxygen levels until 2100. They found that future deoxygenation rates in rivers were 1.6 times higher than historical conditions if temperatures rise by 2.7°C by the end of the century.

Li says that Brooker Creek in Florida, for example, has about 204 hypoxia days a year and the model projects that this number will rise by 5.7 days per decade.

Topics:

• climate change/
• Save Britain’s Rivers