Bubbles coming from freshwater sources, new research suggests, may be a key and currently unaccounted for source of methane, the second-largest greenhouse gas contributor to human-driven global climate change.
In a May 16 paper published in the journal Global Change Biology, University of Wisconsin-Madison graduate student John Crawford and his colleagues, including his advisor Emily Stanley, a UW-Madison professor in the Department of Zoology and the Center for Limnology, show freshwater may be contributing more methane gas to the environment than has previously been measured.
The work has the potential to change how climate scientists and others determine the greenhouse gas budget. It also has implications for agricultural regions, where nitrogen and sulfur-based runoff may impact local methane production.
“There have been recent suggestions that freshwater streams, rivers and lakes are important sources of methane to the atmosphere,” says Crawford, who also works for the U.S. Geological Survey in Boulder, Colo.
In freshwater environments, methane gas comes from the metabolic byproducts of bacteria living in the organic-compound-rich, oxygen-poor sediments. Where oxygen, nitrogen or sulfur are high, methane is low because of the chemistry involved in its formation.
Wetlands are known sources of methane but the streams and rivers that drain them may also contribute to the overall methane budget. Just how much is little understood.
Unlike carbon dioxide, which is highly soluble in water, methane exists in two forms in these freshwater sources: as a dissolved gas and encapsulated in bubbles that rise from sediments “like bubbles coming up in a can of soda,” says Crawford. Few studies have measured the methane trapped in those bubbles.
Crawford and the research team studied the methane in bubbles emitted from Allequash Creek, a tributary of Trout Lake in Vilas County, Wisconsin, where the creek bed is a mix of mucky, organic wetland components and sandy glacial sediment. They also looked at three other area creeks: Mann Creek, Stevenson Creek and North Creek.
They found there was as much methane in bubbles emitted from Allequash Creek and the surrounding area as has been measured in other wetland and lake environments. The researchers estimate at least 50 percent more methane can be emitted by bubbles in the region as is dissolved in the water.
“We are missing half the story, at least in this area, if we don’t include bubbles,” Crawford said. The team believes the creek is representative of other similar bodies of water in the Northern Highlands Lake District of Wisconsin.