The sound of running water has long been associated with positive health benefits, and the appealing sound of a babbling brook can be found on many recordings intended to aid relaxation and induce sleep. No countryside picnic is complete without the sweet singing of a shallow stream somewhere nearby, and water features that emulate the delicate rippling of water rivulets over rocks are popular additions to gardens all over the world.
Unfortunately new research suggests that the bubbles coming from freshwater sources 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 recent (May 16, 2014) 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 that 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, however, is not yet fully 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.
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 said. The team believes the creek is representative of other similar bodies of water in the Northern Highlands Lake District of Wisconsin.
The study, which was funded by the National Science Foundation under Cooperative Agreement and by the U.S. Geological Survey Water, Energy and Biogeochemical Budgets Program, ran from May through November of 2013, and the team were had to trudge out to the sampling sites every other day through that period, measuring bubble composition every other week.
They were surprised to find methane-containing bubbles coming from sandy sediments, which are generally less organic-rich than mucky beds. However, the concentration of the gas was lower from sandy sediments than from the mucky methane “hot spots.”
Crawford says it’s surprising to find methane at all in these highly-oxygenated creeks, though the methane is coming from mostly oxygen-poor sediments that form their beds.
While the new study cannot adequately assess the global 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.
If the bubbles from babbling brooks are found to be a significant contributor to climate change, it is difficult to see how this effect can be mediated. Climate change may challenge the existence of mankind, but water, by its very liquid nature, likes to run and will continue to do so until the end of time, a fact appreciated and delightfully described by Alfred Lord Tennyson in his poem, The Brook:
"And out again I curve and flow
To join the brimming river,
For men may come and men may go,
But I go on forever. "