A new analysis of more than 21,000 river reaches has found that dissolved oxygen levels have been declining across most of the world's flowing waters for nearly four decades, with tropical rivers showing the steepest losses. The study, published in Science Advances, points to climate warming as the leading driver of the trend.
The research was led by Prof. Kun Shi of the Nanjing Institute of Geography and Limnology (NIGLAS) at the Chinese Academy of Sciences, with Dr Qi Guan as first author and contributions from Tongji University. Using a machine-learning stacking algorithm, the team analysed observations from 21,439 river reaches worldwide collected between 1985 and 2023. Across the dataset, river oxygen levels fell at an average rate of -0.045 mg L⁻¹ per decade, and 78.8% of the rivers examined showed signs of deoxygenation. Dissolved oxygen is a foundational variable in freshwater ecology: it sustains fish and other aquatic species, supports biodiversity, and shapes key biogeochemical processes. When oxygen drops far enough, hypoxic conditions can set in, and aquatic life can no longer be supported.
Tropical rivers, which already tend to carry lower baseline oxygen concentrations, are losing oxygen faster, raising the likelihood of hypoxia events in regions that can least absorb them
One of the study's more unexpected findings concerns geography. The strongest oxygen losses were observed in tropical rivers between 20°S and 20°N, including river systems in India. Researchers had previously expected higher-latitude rivers, where warming tends to be more pronounced, to face the greatest deoxygenation risk. Instead, tropical rivers, which already tend to carry lower baseline oxygen concentrations, are losing oxygen faster, raising the likelihood of hypoxia events in regions that can least absorb them.
The authors also examined how river flow regimes and dam impoundment influence the trend. Compared with normal-flow conditions, low-flow conditions were associated with an 18.6% lower deoxygenation rate, and high-flow conditions with a 7.0% lower rate. Dam impoundment had divergent effects depending on reservoir depth: in shallow reservoirs, it accelerated oxygen loss, while in deeper reservoirs, it helped slow deoxygenation within the impounded area.
The team attributed the bulk of the decline to climate warming itself. Declining oxygen solubility tied to higher water temperatures accounted for 62.7% of the observed change, ecosystem metabolism, influenced by temperature, light, and flow, for 12%, and heatwave events for 22.7%. Heatwaves alone raised the deoxygenation rate by 0.01 mg L⁻¹ per decade compared with conditions under average climatological temperatures.
The authors describe the findings as a sign of the growing impact of climate warming on lotic, or flowing, freshwater ecosystems, and argue that tropical rivers should be treated as a priority for mitigation efforts. They also frame the work as a scientific foundation for policymakers developing strategies to address river deoxygenation worldwide.
