Scientists warn that the world’s rivers are running out of oxygen

The study was led by Prof. Kun Shi of the Nanjing Institute of Geography and Limnology (NIGLAS) at the Chinese Academy of Sciences. Dr. Qi Guan served as the first author, and the project also involved a researcher from Tongji University.

Rivers Losing a Vital Ingredient for Life

Dissolved oxygen plays a critical role in maintaining healthy river ecosystems. It supports aquatic organisms, helps sustain biodiversity, and influences important biogeochemical processes. When oxygen levels fall, river health can deteriorate, putting fish and other freshwater species at risk.

To examine how river oxygen levels have changed over time, the researchers used a machine-learning stacking algorithm to analyze observations from 21,439 river reaches worldwide collected over nearly four decades (1985-2023).

Their analysis revealed a clear global trend. River oxygen levels declined at an average rate of -0.045 mg L-1 decade-1, and 78.8% of the rivers included in the study showed signs of deoxygenation.

Tropical Rivers Hit Hardest

The strongest oxygen losses were found in tropical rivers located between 20°S and 20°N, including rivers in India. This result surprised researchers because scientists had previously expected rivers at higher latitudes, where warming is often more intense, to face the greatest deoxygenation risks.

Instead, the study showed that tropical rivers already tend to have lower oxygen concentrations, making them especially vulnerable when oxygen levels continue to drop. Combined with faster deoxygenation rates, these conditions increase the likelihood of hypoxia events, when oxygen becomes too scarce to support many forms of aquatic life.

River Flow and Dams Influence Oxygen Loss

The researchers also examined how river flow patterns and dam impoundment affect oxygen decline.

Both low-flow and high-flow conditions appeared to partially reduce deoxygenation compared with normal-flow conditions. Rivers experiencing low-flow conditions had an 18.6% lower deoxygenation rate, while high-flow conditions were associated with a 7.0% lower rate.

Dam impoundment produced different effects depending on reservoir depth. In shallow reservoirs, impoundment accelerated oxygen loss. In deeper reservoirs, however, it helped reduce deoxygenation in the impounded area.

Heatwaves Accelerate River Deoxygenation

Further analysis showed that declining oxygen solubility caused by climate warming was the primary driver behind the global oxygen decline, accounting for 62.7% of the observed changes.

Ecosystem metabolism, influenced by factors such as temperature, light, and water flow, contributed 12% of the deoxygenation.

The team also investigated the role of heatwaves. Their results showed that heatwave events accounted for 22.7% of global river deoxygenation. Heatwaves increased the deoxygenation rate by 0.01 mg L-1 decade-1 compared with conditions under average climatological temperatures.

Overall, the findings highlight the growing impact of climate warming on flowing freshwater ecosystems, also known as lotic ecosystems. The researchers say tropical rivers should be considered a top priority for mitigation efforts aimed at preventing worsening oxygen depletion. The study also provides a scientific foundation that policymakers can use when developing strategies to address river deoxygenation worldwide.