They may be small, but new research from the University of Minnesota finds that ponds — especially those covered by floating plants — can be a significant contributor of greenhouse gases into the atmosphere.
In general, plants absorb carbon dioxide and release oxygen into the atmosphere during photosynthesis.
But when it comes to ponds and small lakes, Jim Cotner, a professor of ecology, evolution and behavior at the U of M, found it's more complicated.
Cotner and graduate student Joseph Rabaey sampled 26 ponds around the Twin Cities, including both urban stormwater ponds and more natural forested ponds. They measured the amount of carbon dioxide and methane in the ponds’ water and emissions.
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“In almost all cases, they are sources of greenhouse gases into the atmosphere, rather than (carbon) sinks,” Cotner said.
Small ponds tend to contain a lot of nutrients and organic matter from the surrounding land, Cotner said. Microbes break down that organic matter into methane and carbon dioxide, which can escape into the atmosphere.
The team found that ponds covered with a floating plant called duckweed produced more methane, one of the most potent greenhouse gases that contribute to climate change.
Duckweed stays on the water’s surface, shading algae and other plants so they can't produce oxygen, Cotner said. Meanwhile, most of the oxygen produced by duckweed escapes directly into the atmosphere, he said.
"The fact that you don't have oxygen in the water makes it pretty much ideal for the production of methane,” he said.
There are more than 30,000 stormwater ponds in Minnesota, which play an important role in managing runoff, reducing flooding and filtering out nutrients before stormwater reaches rivers and lakes.
Cotner said increasing turbulence in those ponds, either by installing an aerator or fountain or removing surrounding trees to increase wind speed, could help circulate oxygen and reduce duckweed growth.
Much research has focused on the role oceans and other large water bodies play in greenhouse gas emissions, but Cotner said it’s clear that small ponds also are significant.
“It's interesting that the smallest systems seem to be the ones that have the hugest impact,” he said.
The study was published in the journal Frontiers in Environmental Science.