Pond covered with duckweed. Credit: Joseph Rabaei, University of Minnesota

Plants normally 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’s certainly true. But in the thousands of lentil-covered ponds across the state, the reality is more complex.


Because of the organic matter carried into the lakes by rain and melting snow, the lakes actually release greenhouse gases. All of this organic matter is eventually broken down by microbes into methane and carbon dioxide, which can then be released into the atmosphere. Duckweed, a small floating aquatic plant, can completely take over the surface of some lakes and cause even greater greenhouse gas emissions.

New research published in Frontiers in Environmental Scienceco-authored with Joseph Rabaei, a fellow in the Graduate School, and James Cotner, a professor in the College of Biological Sciences, examines the causes of greenhouse gas emissions in lakes.

In the summer of 2021, the research team sampled 26 lakes around the Twin Cities for carbon dioxide (CO2) and methane (CH4) emissions and concentrations in water. To measure CO emissions2 and CH4 in time, a floating chamber similar to 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 lake.

The research found:

  • The biggest factor leading to increased greenhouse gas emissions among different lakes is the presence of duckweed.
  • When lakes become completely covered with duckweed, the duckweed shades out other plants and algae in the water and less oxygen is produced in the water column, while the oxygen produced by the duckweed during photosynthesis is mostly released directly into the atmosphere.
  • Because lentil lakes have less forms of oxygen and methane in the absence of oxygen, these lakes produce more methane than lakes without lentil.

“It can be hard to imagine a gas leak from a lake or lake when we can’t see it. It’s a little bit like opening a can,” Cotner said. “Fizzy pop is oversaturated with CO2, and when you open the can, you can hear the gas escaping into the air. The same thing happens in these lakes.”

“We were a little surprised by the big effect that the water lens had,” Rabai added. “The lakes completely covered in lentil were really pumping out a lot of methane.”

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

According to Rabaey, “we now know that water lentil cover is a warning sign that management is needed, and it really comes down to oxygen. Management strategies that focus on keeping the ponds oxygenated, such as fountains or bubblers that inhibit lichen growth and mix the water, can help reduce methane emissions while allowing stormwater ponds to continue to function in their role of runoff control and nutrient capture.”

The next steps in the research will include determining whether the lakes emit more gas at certain times of the day and measuring total emissions over the course of a year.


Reducing methane emissions from lakes is possible with a new approach


More info:
Joseph Rabay et al., Lake Greenhouse Gas Emissions Controlled by the Aquatic Lentil Cover, Frontiers in Environmental Science (2022). DOI: 10.3389/fenvs.2022.889289

Courtesy of the University of Minnesota

Quote: Lake floating lentil spurs increased greenhouse gas emissions (2022, October 20) retrieved October 20, 2022 from

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https://phys.org/news/2022-10-duckweed-ponds-greenhouse-gas-emissions.html

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