Work by a University of Wyoming professor and a recent UW Ph.D. graduate has provided a more complete picture of the role of rivers and streams in the global carbon cycle.

Robert Hall Jr., professor in UW's Department of Zoology and Physiology, and former UW Program in Ecology Ph.D. student Erin Hotchkiss, now at the University of Quebec in Montreal, joined colleagues from Sweden's Umea University, the Swedish University of Agricultural Sciences and the University of Washington to analyze the emission of carbon dioxide from running waters in the United States.

Their findings are published in a scientific paper, "Sources of and processes controlling CO2 emissions change with the size of streams and rivers," in the journal Nature Geoscience.

CO2 from streams and rivers constitutes a small percentage of overall emissions of CO2 into Earth's atmosphere, but it's important to properly quantify that contribution, the researchers say. "Without considering the diverse controls on carbon dynamics in integrated land-water budgets, we cannot anticipate how these dynamics will respond to future environmental change," they wrote.

CO2 emitted by rivers and streams comes from two basic sources: from CO2 in soils adjacent to the waters, and from respiration of CO2 by organisms in the waters themselves. By analyzing existing data from streams and rivers across the U.S., and using mathematical modeling, the scientists found that the latter source of CO2 may be larger than has been thought.

"Running waters are not just passive conduits of water and terrestrial carbon, but also function as reactors that generate and release carbon dioxide during its transport downstream," says Hotchkiss, who led the analysis while working as a postdoctoral researcher at Umea University.

The scientists demonstrated that while most CO2 emitted from small streams is derived from surrounding soils, in-stream respiration becomes a larger proportion of CO2 emissions as rivers become larger.

Understanding the mechanisms behind the production of CO2 in running water is essential because it increases the ability to predict how changes in land use and a warming climate could affect the sources and global concentrations of greenhouse gases in the atmosphere.

"It is very important to know the source of carbon dioxide in streams and rivers, as well as the processes controlling respiration and emissions, if we are to understand what happens when the environment changes," Hotchkiss says.