Plants typically absorb carbon dioxide and release oxygen as part of photosynthesis. Most people associate greenery with a lower carbon content, and in the case of the trees in Minnesota, that’s certainly true. But in the thousands of duckweed-covered ponds across the state, the reality is more complicated.
The organic matter carried into ponds by rain and melting snow makes ponds actually emit greenhouse gases. All 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 greater greenhouse gas emissions.
New research published in Frontiers in Environmental Sciencesco-authored 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.
In 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 reduce CO emissions. to measure2 and CH4 over time, a floating chamber, such as an inverted 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 each pond.
The research found:
- The biggest factor leading to increased greenhouse gas emissions between different ponds was the presence of duckweed.
- When ponds are completely covered with duckweed, the duckweed obscures other plants and algae in the water and less oxygen is produced in the water column, while the oxygen produced by duckweed in photosynthesis is mainly released directly into the atmosphere.
- 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 if we can’t see it happening. It’s a bit like opening a can of pop,” Cotner said. “The fizzy pop is oversaturated with CO2, and when you open the can, you can hear the gas escaping into the air. The same is happening in these ponds.”
“We were a bit surprised by the great effect the duckweed had,” Rabaey added. “Ponds that were completely covered in duckweed really pump out a lot of methane.”
Stormwater ponds are numerous in urban settings, and there are tens of thousands of ponds in the Twin Cities alone. 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 prevent mixing water could help reduce methane emissions, while still allowing stormwater ponds to function in their role of runoff control and nutrient capture.”
The next steps in the study are to determine whether ponds emit more gas at certain times of the day and to measure the total emissions over the course of a year.
Reduction of methane emissions from lakes possible with new approach
Joseph Rabaey et al, Greenhouse gas emissions in ponds controlled by duckweed cover, Frontiers in Environmental Sciences (2022). DOI: 10.3389/fenvs.2022.889289
Quote: Floating duckweed on ponds fuels increased greenhouse gas emissions (2022, Oct. 20) retrieved Oct. 20, 2022 from https://phys.org/news/2022-10-duckweed-ponds-greenhouse-gas-emissions.html
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