“This can be affected by a few environmental factors, such as the velocity of the water flow, wind speed of the region, and the aquatic dissolved oxygen level and dissolved organic carbon, which can be influenced by the internal metabolism of the water bodies,” said Prof. Peng.
To explore the relationship between the magnitude and spatial variations of riverine GHG emissions on a global scale, Profs. He and Peng developed a database of GHG measurements covering 595 rivers across the planet.
The researchers found that global riverine CO2 concentration drops as the stream order goes up. “The availability of carbon and nitrogen as well as dissolved oxygen level could be predominant factors that result in high GHG emissions in headwater streams,”said Prof HE. He explained that most headwater rivers originate from glaciers, permafrost peatlands, forest catchments, or regions with a high density of agricultural cultivation, and thus have a higher concentration of labile carbon and nitrogen.
At headwater streams (i.e., stream orders 1 to 3), high concentration of labile carbon and nitrogen input, low dissolved oxygen concentration and large gas exchange velocity collectively contribute to high GHG emissions.
As the stream order increases from 1 to 7, the concentration of terrestrial carbon and nitrogen loadings declines as does gas exchange velocity. At the same time, dissolved oxygen accumulates in the water bodies, thus resulting in decreasing GHGs emissions.
The situation changes at stream orders 7 to 9. Water discharges at this order are mostly located downstream and closer to organic carbon-rich places such as urban cities and croplands, which consequently raises the CO2 concentration of the water.
Studies like this warn us that more efforts are needed to understand the full scale of human influence on riverine GHG emissions and climate change in general.
“We hope this study will bring us one step closer to the development of a process-based model to simulate the riverine carbon and nitrogen dynamics responding to climate change and anthropogenic disturbance,” said Prof. He.
Editor's note:
the study was supported by STEP, a TPE related science project.
