Minnesota Ponds Covered in Duckweed Causes Less Oxygen Production, More Greenhouse Gas Emissions

Duckweed covering Minnesota ponds is causing increased greenhouse gas emissions and reduced oxygen production, as research reveals.

As part of photosynthesis, plants typically take in carbon dioxide and exhale oxygen.

Greenery typically has a negative connotation with carbon emissions, and Minnesota's trees certainly fit that description.

The situation is more complicated in the thousands of ponds throughout the state that is covered in duckweed.

Greenhouse Gas from Ponds

Ponds release greenhouse gases because of the organic matter that rain and snowmelt carry into them.

All of the organic material eventually decomposes into carbon dioxide and methane by microbes, which are then released into the atmosphere.

Small floating aquatic plants like duckweed can completely cover some ponds, increasing greenhouse gas emissions.

The contributing factors of greenhouse gas emissions in ponds were investigated in new research co-authored by James Cotner, a professor from the College of Biological Science, and Joseph Rabaey, a fellow from the Graduate School.

According to the US Environmental Protection Agency (EPA), greenhouse gases are those that sustain heat in the atmosphere.

Greenhouse gases include carbon dioxide, methane, nitrous oxide, and fluorinated gases.

26 Ponds

The research team took water samples from 26 ponds near the Twin Cities in the summer of 2021 to measure methane (CH4) and carbon dioxide (CO2) emissions and concentrations.

A floating chamber, resembling an upside-down pail, was positioned on the water's surface and connected to a portable gas analyzer to track CO2 and CH4 emissions over time.

Multiple locations on each pond were used to measure gas emissions using this set-up and a canoe or kayak.

The Findings:

• The presence of duckweed in various ponds was the main cause of increased greenhouse gas emissions.
• Since most of the oxygen made by duckweed during photosynthesis is directly released into the atmosphere when ponds are completely covered in duckweed, less oxygen is produced in the water column and other plants and algae are shaded out.
• Ponds with duckweed produced much more methane than ponds without because these ponds had less oxygen. Consequently, methane forms in the lack of oxygen.

Can of Pop and Ponds

Cotner said that because gas cannot be seen escaping out of a pond or lake, it can be difficult to imagine it happening.

It's similar to opening a can of carbonated soda, where the gas is heard escaping into the air as the can is opened because the soda is supersaturated with CO2.

This is what occurs in these ponds.

Rabaey added that they were a little taken aback by the significant impact the duckweed was producing.

Ponds that were completely covered in lush duckweed were producing a lot of methane.

Heed the Duckweed

There could be tens of thousands of stormwater ponds in the Twin Cities alone.

Stormwater ponds are common in urban settings. These ponds could potentially be managed more effectively to cut greenhouse gas emissions.

According to Rabaey, their research, published in Frontiers in Environmental Science, showed that coverage of duckweed is a clear signal that management is required, and oxygen is the key factor.

Management practices that emphasize maintaining oxygen levels in ponds, such as installing fountains or bubblers to mix the water and prevent the growth of duckweed, may help lower methane emissions while maintaining the ability of stormwater ponds to control runoff and capture nutrients.

The research's next steps will involve measuring annual total emissions and figuring out whether ponds emit higher amounts of gas at particular times of the day, Phys Org reported.