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Essential to all aquatic life
For decades, the signal remained invisible. Yet beneath the surface of rivers, a silent imbalance is taking hold: dissolved oxygen?essential to all aquatic life?is declining on a global scale. A comprehensive analysis based on forty years of satellite observations now reveals the full extent of the phenomenon.
Using 3.4 million Landsat images, a team led by Kun Shi (Nanjing Institute of Geography and Limnology) has reconstructed changes in dissolved oxygen levels in more than 21,000 river segments between 1985 and 2023. The findings: 78.8% of the rivers studied showed a decline, with an average decrease of 0.045 mg/L per decade. This may seem like a minimal change, but it is sufficient to disrupt the biological balance. For many species (fish, invertebrates, or larvae), a difference of just a few tenths of a milligram can trigger a shift toward hypoxia.
Regions Most Affected
Contrary to expectations, it is the tropical regions that are experiencing the most rapid declines. Between 20° north and 20° south, rivers that are already warm and naturally low in oxygen are reaching critical thresholds more quickly. The Ganges illustrates this trend: its oxygen levels are declining twenty times faster than the global average, followed by the Amazon. Conversely, rivers in high latitudes still retain some resilience thanks to their colder waters.
The main mechanism is well understood: as water warms, its ability to dissolve oxygen decreases, a factor that alone accounts for nearly two-thirds of the phenomenon. Extreme heat events further exacerbate this trend, while the biological respiration of ecosystems also contributes to oxygen consumption.
Water infrastructure plays an ambivalent role. Shallow reservoirs promote stagnation and oxygen depletion, while deep reservoirs can, in some cases, limit oxygen loss thanks to cooler water masses.
Alarming projections
Looking ahead to 2100, the projections remain concerning. Without a reduction in CO? emissions, several major regions (the Amazon, India, the eastern United States, and the Arctic) could lose up to an additional 10% of dissolved oxygen. This trend would increase the frequency of mass fish deaths already observed in certain tropical basins.
Invisible yet essential, this depletion of oxygen is profoundly reshaping the health of rivers. And it serves as a reminder that, far beyond temperature changes, climate change is also altering the most fundamental mechanisms of aquatic life.
