Plants typically absorb carbon dioxide and release oxygen as part of photosynthesis. Most people associate greenery with reduced carbon levels, and in the case of Minnesota's trees, that is certainly true. But in the thousands of duckweed-covered ponds across the state, the reality is more complicated.

Due to organic matter carried into ponds by rain and melting snow, ponds actually emit greenhouse gasses. All of that organic matter is eventually broken down by microbes into methane and carbon dioxide, which can then escape into the atmosphere. Duckweed, a small floating aquatic plant, can completely take over the surface of some ponds and cause even higher greenhouse gas emissions.

New research published in Frontiers in Environmental Science, co-authored by Joseph Rabaey, a fellow in the Graduate School, and James Cotner, a professor in the College of Biological Science, examined the causes of greenhouse gas emissions in ponds.

During the summer of 2021, the research team sampled 26 ponds around the Twin Cities for carbon dioxide (CO₂) and methane (CH₄) emissions and concentrations in the water. To measure emissions of CO₂ and CH₄ over time, a floating chamber, like an upside-down bucket, was placed on the surface of the water and attached to a portable gas analyzer. Using this setup and a kayak or canoe, gas emissions were measured at multiple points on every pond.

The research found:

• The biggest factor leading to increased greenhouse gas emissions among different ponds was the presence of duckweed.
• When ponds become fully covered in duckweed, the duckweed shades out other plants and algae in the water, and less oxygen is being produced in the water column, while the oxygen produced by duckweed in photosynthesis is primarily released into the atmosphere directly.
• Because duckweed ponds had less oxygen and methane forms in the absence of oxygen, these ponds produced more methane than ponds without duckweed.

"It can be hard to imagine gas escaping from a pond or lake when we can't see it happening. It's a bit like opening a can of pop," said Cotner. "The carbonated pop is supersaturated with CO₂, and when you open the can you can hear the gas escaping into the air. The same thing is happening in these ponds."

"We were a bit surprised by the large effect the duckweed was having," added Rabaey. "Ponds fully covered in duckweed were really pumping out a lot of methane."

Stormwater ponds are numerous in urban environments, and in the Twin Cities alone there are tens of thousands of ponds. There is potential to better manage these ponds to reduce greenhouse gas emissions.

According to Rabaey, "we now know that duckweed coverage is a warning sign that management is needed, and it really comes down to oxygen. Management strategies that focus on keeping ponds oxygenated, such as fountains or bubblers that discourage duckweed growth and mix the water, could help reduce methane emissions while allowing stormwater ponds to still function in their roles of runoff control and nutrient capture."

Next steps in the research will involve determining whether ponds emit more gas at certain times of day and measuring total emissions over the course of a year.

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