Watercourses are warming rapidly and losing oxygen faster than the oceans

The study by the international research team with the participation of the University of Natural Resources and Life Sciences (BOKU) Vienna was published in the current issue of the renowned scientific journal Nature Climate Change.

"We know that climate warming has led to warming and oxygen depletion in the oceans, but we didn't expect this to happen in flowing, shallow waters. This is the first study to take a large-scale look at temperature changes and oxygen depletion in streams - and the results show significant impacts on water quality and the health of aquatic ecosystems," said corresponding author Li Li, Professor of Civil and Environmental Engineering at Pennsylvania State University in the US.

The international team of researchers found that 87% of 580 rivers in the United States and 216 rivers in Central Europe experienced warming and 70% oxygen loss between 1981 and 2019.

The study also revealed that urban rivers have warmed the fastest in the recent past, while agricultural rivers have warmed the slowest but deoxygenated the fastest. Using a specially trained machine learning model, it was predicted that the rates of change in terms of temperature increase and oxygen depletion will increase even further in the future.

In the next 70 years, certain fish species could become completely extinct, especially in the USA, due to prolonged periods of very low oxygen concentrations, which, according to Li, would threaten aquatic diversity in general.

BOKU's focus was on data acquisition and processing for the project area in Central Europe. The CMIP6 climate projections (Coupled Model Intercomparison Project, Phase 6) were used to estimate the trend in oxygen depletion in Central European rivers up to the year 2100.

"BOKU developed an efficient algorithm for extracting and aggregating the CMIP6 climate projections, as a very large volume of data (big data) had to be processed due to the size of the study area in combination with a large number of catchment areas," says Christoph Klingler from the Institute of Hydrology and Water Resources Management at the University of Natural Resources and Life Sciences, Vienna.

In contrast to the watercourses in the USA, there is hardly any danger of very low oxygen concentrations in the Austrian watercourses investigated (provided there are no other anthropogenic circumstances such as very high nutrient inputs or organic pollution), "nevertheless, in Austria in the current century, under the climate scenarios investigated, an almost nationwide rise in temperature and a decrease in oxygen concentrations in the watercourses is to be expected," emphasizes the BOKU hydrologist.

The results of the study do not even imply a complete glacier retreat - which would mean a further worsening in some waters, at least in terms of temperature increase.

These circumstances would be detrimental to aquatic fauna, among others, as their oxygen requirements usually also increase with rising water temperatures.

"One consequence of this is, for example, the shift of the fish regions towards the source, which will reduce the size of the trout region in particular," Klingler concludes.