Mountain streams emit surprisingly large amounts of CO2

Mountain streams play a surprisingly significant role in global carbon fluxes, according to a new study. Pound for pound, mountain streams emit more CO2 than the wider waterways below.

In studying the relationship between flowing freshwater and carbon cycles, scientists have mostly focused on streams and rivers in low-altitude regions. But mountains account for a quarter of Earth's surface, and the streams that drain the planet's peaks collect and organize a third of global runoff.

To butter understand how this high-elevation water cycle influences global carbon fluxes, researchers compiled a variety of environmental measurements at dozens of mountain stream sites. Importantly, scientists used a new method to precisely calculate the gas exchange velocities across the air-water interface at each site.

The research team organized the data by comparing each site's hydrologic and geological properties, including the site's soil organic carbon content, to measure gas exchange velocities. Using the data, scientists extrapolated patterns to build a global model to estimate carbon emissions for 1.8 million mountain streams worldwide.

The simulation results -- published this week in the journal Nature Communications -- showed that mountain streams, though they cover for just 5 percent of the surface area of Earth's fluvial networks, account for between 10 percent to 30 percent of the CO2 emissions released by rivers and streams.

"We have known for a number of years that freshwater ecosystems emit roughly the same amount of CO2 that oceans absorb, but we had never before done rigorous studies on the role of the countless mountain streams for the global CO2 fluxes. Until now they were aqua incognita," study co-author Tom Battin, head of EPFL's Stream Biofilm and Ecosystem Research Laboratory in France, said in a news release. "But our latest findings now open up exciting new research avenues, such as to better understand where all that CO2 comes from and how we can more accurately account for the world's alpine regions in our assessments of the global carbon cycle."

River and streams work to release carbon dioxide via erosion and chemical reactions. Many of the world's fluvial networks carve into deposits of carbonate rock, formed millions of years ago by the remains of marine microorganisms.

Scientists hope to improve the accuracy of their new model by acquiring more data from mountain streams around the world.

"We are just starting to discover the role of mountain streams for the global carbon cycle," said Battin. "These are exciting times for environmental sciences."

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