“DO dynamics in climate transition zones are more sensitive to temperature fluctuations. For instance, Indian rivers experienced a more substantial oxygen depletion, exceeding 0.2 mg per litre per decade, whereas rivers in the United States and Central Europe showed a decrease of only 0.038 mg per litre per decade,” the researchers observed.
Pointing out that approximately 8.6 per cent of global rivers experience declines in DO concentrations in response to rising air temperatures, with half of these rivers found in tropical regions, the study said that this temperature-driven DO decline is pronounced in India along with other places like the Parana Plateau, southeast United States, Brazilian highlands and Central Europe.
Despite sensitivity and complexity, tropical freshwater systems remain underrepresented in investigations of low-oxygen events when compared to temperate, lake or marine systems, the study said.
Tropical rivers are important freshwater ecosystems in the world, as they encompass nearly 19 per cent of the global land area and transport 66 per cent of global freshwater and 59 per cent of organic carbon into the oceans.
These tropical freshwater ecosystems are heavily affected by climate change scenarios. In particular, regions such as tropical South America and South Asia show increasing tendencies towards drying and droughts.
In addition, deterioration of surface water quality by organic matter input is another important factor. Organic matter contents can be elevated in tropical rivers during monsoon and additional storm events.
“For instance, tropical rivers such as the Ganges and Mekong exhibit four to six fold increases in particulate organic matter during the high-water period,” the researchers said.
In many tropical rivers, organic matter fluxes are further controlled by land-use. Agricultural practices further enhance organic matter, which is considered a stressor of low DO concentrations in aquatic environments. Lack of efficient sewage treatment and construction of dams are other influencing factors.
Undertaken by researchers based in Germany and Sri Lanka, the study, which presented new evidence by stable isotopes, was published by Nature’s Scientific Reports, a pre reviewed journal, on August 10.
Dissolved oxygen, according to the study, is a key indicator of ecological quality and arguably remains the most sensitive tool to quantify and monitor ecosystem water quality. Water temperature is an important parameter that impacts DO saturations and temperature-related influences span physical, chemical and biological functions that drive metabolic rates, nutrient cycles and primary productivity.
The increases in water temperature due to climate change, the researchers said, correlate with a decline in DO concentrations in many surface waters worldwide and analyses of Landsat observations and climate data over the past four decades indicate that global surface waters have experienced a strong decline in DO of 0.045 mg per litre per decade, the study observed.
