Deoxygenation: Climate warming driving sustained oxygen loss in global rivers

A study published in Science Advances reveals that global rivers are experiencing widespread and sustained deoxygenation

Led by Professor Shi Kun of the Nanjing Institute of Geography and Limnology at the Chinese Academy of Sciences, the research warns that tropical rivers are the most vulnerable ecosystems and face an urgent need for mitigation strategies.

Tracking four decades of oxygen decline

Dissolved oxygen is vital for river ecosystems, supporting aquatic life, maintaining ecological health, and regulating biogeochemical cycles. To track long-term oxygen trends, the research team used a machine-learning stacking algorithm to analyze data from 21,439 river reaches worldwide across a nearly 40-year period from 1985 to 2023.

The findings showed that global river ecosystems are losing oxygen at a rate of -0.045 mg L⁻¹ per decade. Overall, 78.8% of the studied rivers experienced deoxygenation during the four-decade timeline, indicating a widespread decline in water quality that threatens river biodiversity.

The vulnerability of tropical rivers

The study revealed that the most severe deoxygenation is occurring in tropical rivers located between 20°S and 20°N, such as those in India. This finding contradicts previous assumptions that high-latitude rivers would be the primary hotspots due to amplified climate warming in those regions.

Instead, the combination of naturally lower baseline oxygen levels and faster rates of oxygen loss makes tropical rivers highly susceptible to hypoxia events. These low-oxygen crises can devastate local aquatic populations and alter entire river habitats.

Primary drivers of deoxygenation

The researchers broke down the primary causes of the global decline, identifying climate-driven factors as the dominant forces:

• Reduced oxygen solubility (62.7%):
  • As river water warms due to climate change, it naturally holds less dissolved gas, which accounts for the majority of the oxygen loss.
• Heatwave events (22.7%):
  • Extreme heatwaves accelerated the deoxygenation rate by an additional 0.01 mg L⁻¹ per decade compared to average temperature conditions.
• Ecosystem metabolism (12%):
  • Biological processes driven by changes in temperature, light, and water flow accounted for the remaining decline.

Effects of water flow and dams

The study also quantified how physical changes to river systems alter oxygen levels. Both low-flow and high-flow conditions were found to partially mitigate deoxygenation, lowering the rate of oxygen loss by 18.6% and 7.0%, respectively, compared to normal-flow conditions.

Furthermore, dam impoundment had contrasting effects depending on the structure of the reservoir. Dams built with shallow reservoirs accelerated deoxygenation in the impoundment area, whereas deep reservoirs actually helped to mitigate oxygen loss.

The researchers conclude that these findings provide a systematic baseline for global policymakers. The data emphasizes the negative consequences of climate warming on flowing water systems and highlights the critical need to protect tropical river networks from ongoing oxygen depletion.