Studying stream bubbles isn't exactly a walk in the park — what with the mud and ticks, the long days hiking and swimming through mucky streams, the sun exposure and scratching brush.
But in the end, it may prove to be worthwhile. The bubbles coming from freshwater sources may be an important and currently unaccounted for source of methane, the second-largest greenhouse gas contributor to human-driven global climate change.
In a paper published May 16 in the journal Global Change Biology, University of Wisconsin-Madison graduate student John Crawford, his colleagues, and advisor Emily Stanley, a 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.
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, whose bed is a mix of mucky, organic wetland components and sandy glacial sediment. The creek is a tributary of Trout Lake in Vilas County, Wisconsin. They also looked at three other area creeks.
A bubble trap collecting freshwater bubbles from Allequash Creek. (Photo courtesy John Crawford, Center for Limnology)
To measure the methane in bubbles as they rose through the water, the team created traps to capture them and the air they contained. They ran the air through a sensor called a gas chromatograph to determine how much methane was contained within it. The team also looked at the rate of bubble release across sample sites.
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 says. The team believes the creek is representative of other similar bodies of water in the Northern Highlands Lake District of Wisconsin based on years of observation.
The study ran from May through November of 2013. The team trudged out to the sampling sites every other day through that period, measuring the composition of bubbles every other week.
The team was surprised to find methane-containing bubbles coming from sandy sediments, though the concentration of methane and release rate of bubbles was highest in the mucky, organic-rich sediments, which the team refers to as "hot spots" of methane emissions. Crawford says it's surprising to find methane at all in these highly oxygenated regions.
Methane concentrations are kept low in areas with high oxygen, nitrogen and sulfur content because of the chemistry involved in its formation. The methane from the creeks they sampled is coming from sediments mostly devoid of these elements.
While this study cannot adequately assess contribution of bubble-contained methane from freshwater sources to the environment, it shows it's an area worth further investigation. Methane can lead to ozone production and levels of the gas in the atmosphere are 150 percent higher than they were before the Industrial Revolution.
For Crawford, his interest lies in helping explain things that people can't see, though that bubbles can be seen and felt makes the work more fun, he says. He is driven by the idea that really small ecosystems — like the freshwater lakes, streams and rivers Wisconsin is known for — can potentially play a major role in the overall greenhouse gas pool.
"You are never going to get the budget right if you don't get all the pieces," he says.
The study was funded by the National Science Foundation under Cooperative Agreement and by the U.S. Geological Survey Water, Energy and Biogeochemical Budgets Program.