Rivers are rapidly warming, losing oxygen

Essential indicators of water quality and river ecosystem health are dissolved oxygen levels and water temperature. However, they could be better known since they are difficult to quantify due to the lack of consistent data across rivers and the numerous factors that might alter the oxygen levels in each watershed.

A new study by Penn State scientists reconstructed daily water temperature and dissolved oxygen in 580 rivers across the United States and 216 rivers in Central Europe by training a deep learning model using temporal weather and water quality data and static watershed attributes.

They found that rivers are warming and losing oxygen faster than oceans. The study shows that of nearly 800 rivers, warming occurred in 87%, and oxygen loss occurred in 70%.

Additionally, the study predicts that, during the next 70 years, river systems, particularly those in the American South, are likely to see times when oxygen levels are so low that they could “induce acute death” in some fish species and endanger aquatic diversity.

Li Li, Penn State’s Isett Professor of Civil and Environmental Engineering and corresponding author on the paper, said, “This is a wake-up call. We know a warming climate has led to warming and ocean oxygen loss, but we did not expect this to happen in flowing shallow rivers. This is the first study to take a comprehensive look at temperature change and deoxygenation rates in rivers — and what we found has significant implications for water quality and the health of aquatic ecosystems worldwide.”

For this study, scientists used AI and deep learning techniques to reconstruct historically sparce water quality data from nearly 800 rivers across the U.S. and central Europe. They discovered that rivers are warming and losing oxygen more quickly than seas, which may have significant ramifications for aquatic life and human survival. According to the National Oceanic and Atmospheric Administration, most Americans live less than a mile from a river or stream.

Li said, “If you think about it, life in the water relies on temperature and dissolved oxygen, the lifeline for all aquatic organisms. We know that coastal areas, like the Gulf of Mexico, often have dead zones in the summer. This study shows us this could also happen in rivers because some rivers will no longer sustain life like before.”

“Declining oxygen in rivers, or deoxygenation, also drives the emission of greenhouse gases and releases toxic metals.”

The study found that urban rivers warmed up the fastest, while agricultural rivers warmed up the slowest but deoxygenated the fastest. Additionally, they forecasted future rates using the model and discovered that across all the rivers they examined, future deoxygenation rates will be between 1.6 and 2.5 times higher than current rates.

Because it is typically believed that rivers do not lose oxygen at a rate comparable to that of large bodies of water like lakes and oceans, the oxygen loss in rivers is unexpected. However, this study shows that rivers are fast losing oxygen. That was highly concerning because if the oxygen levels fall low enough, aquatic life becomes deadly.

Li said, “The model predicted that, within the next 70 years, certain species of fish could die out completely due to longer periods of low oxygen levels, which would threaten aquatic diversity broadly.”

Journal Reference:

Zhi, W., Klingler, C., Liu, J. et al. Widespread deoxygenation in warming rivers. Nat. Clim. Chang. (2023). DOI: 10.1038/s41558-023-01793-3

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