Study tracks methane emission from streams

Snake River, Summit County Colorado
Freshwater streams disturbed by human activity are significant sources of heat-trapping methane pollution. @bberwyn photo.

Protecting water quality has climate benefits

Staff Report

Climate models may be significantly underestimating the amount of greenhouse gases produced by fresh water streams, researchers said in a new study released this week.

Published in the journal Ecological Monographs, the findings suggest that human disturbances on watersheds are a key factor in raising concentrations of methane, a particularly potent heat-trapping pollutant. Based on the research, the world’s rivers and streams pump about 10 times more methane into our atmosphere than previously estimated.

“Scientists know that inland waters, like lakes and reservoirs, are big sources of methane,” said Emily Stanley, a professor at the UW-Madison Center for Limnology and lead author of the paper.

Yet accurately measuring emissions of methane from these sources has remained a challenge. Rivers and streams haven’t received much attention because they don’t take up much surface area on a global scale and, with respect to methane, didn’t seem to be all that gassy. But over the years, measurements taken by Stanley and her lab members seemed to indicate these sources may produce more methane than scientists had previously known.

Data from the U.S. Geological Survey’s LandCarbon Project helped create some baseline figures for the exchange of methane between water and the atmosphere, and the research team also used data on methane concentrations  from 111 publications and three unpublished datasets.

The result was “very surprising,” Stanley said. “I thought maybe we’d be off by a factor of two, so I was taken aback by how much higher the estimate was.”

The analysis showed noticeably higher methane emissions from streams and rivers in watersheds marked with heavy agriculture, urban development or the presence of dams. This suggests efforts to improve stream health may have the added benefit of reducing greenhouse gases.

“The fact that human activity in a watershed leads to high methane concentrations in those rivers and streams underscores yet another reason to pay attention to water quality,” Stanley said. “On top of everything else, it adds to this climate problem, too.”

Methane from freshwater is often a byproduct of bacterial metabolism, as they break down organic matter under low-oxygen conditions, like in the sediment at the bottom of a lake. As the climate warms, the contribution of greenhouse gases from natural sources likes rivers, streams and wetlands is expected to increase because higher temperatures accelerate this bacterial breakdown, releasing more carbon dioxide and methane.

The next step, says Stanley, is figuring out where all that methane comes from. Running rivers and streams are usually better aerated and full of oxygen, making all that methane a bit of a mystery.

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