NANJING, CHINA / MENA Newswire / — A global study has found that climate warming is driving sustained oxygen loss in rivers, adding pressure to freshwater ecosystems that support aquatic life, water quality and biogeochemical cycles. The peer-reviewed research, published on May 15 in Science Advances, examined long-term dissolved oxygen trends in river systems worldwide and identified a broad decline across most of the reaches analyzed.
Researchers led by Prof. Shi Kun at the Nanjing Institute of Geography and Limnology of the Chinese Academy of Sciences analyzed 21,439 river reaches over the period from 1985 to 2023. The team used satellite observations, climate data and a machine-learning stacking algorithm to reconstruct dissolved oxygen patterns across nearly four decades, providing a global assessment of changes in flowing waters.
The study found that river ecosystems are losing oxygen at an average rate of 0.045 milligrams per liter per decade, with 78.8 percent of the studied river reaches experiencing deoxygenation. Dissolved oxygen is essential for fish, invertebrates and other aquatic organisms, while also influencing nutrient cycling and the chemical processes that shape river health.
Tropical rivers show largest vulnerability
The most severe oxygen loss was identified in tropical rivers between 20 degrees south and 20 degrees north, including river systems in India. The study found that these rivers face heightened vulnerability because many already have lower oxygen levels while also recording faster deoxygenation trends. That combination increases exposure to hypoxia, a condition in which oxygen concentrations fall below levels needed by many aquatic organisms.
The findings differed from expectations that high-latitude rivers would be the main deoxygenation hotspots because those regions are warming rapidly. Instead, the study identified tropical rivers as the areas with the strongest combined risk from existing low oxygen and continuing oxygen decline. The authors also assessed the role of flow conditions, finding that both low-flow and high-flow periods were linked with lower deoxygenation rates compared with normal flow conditions.
Warming identified as main driver
The study attributed 62.7 percent of observed oxygen loss to climate-driven declines in oxygen solubility, reflecting the physical limit that warmer water holds less oxygen than cooler water. Ecosystem metabolism, measured through factors including temperature, light and water flow, accounted for 12 percent of the decline. Heatwave events were responsible for 22.7 percent of global river deoxygenation and increased the deoxygenation rate by 0.01 milligrams per liter per decade compared with average temperature conditions.
The research also found that dam impoundment altered oxygen trends in reservoir areas, with different effects depending on reservoir depth. Shallow reservoirs were linked with accelerated deoxygenation, while deep reservoirs were associated with mitigation of oxygen loss. The study provides a broad baseline for measuring climate-related changes in global rivers and highlights the importance of tracking dissolved oxygen as a core indicator of freshwater ecosystem health.