The composition of aquatic plant communities in shallow freshwater bodies, including floating plants, submerged plants and phytoplankton, can have important effects on greenhouse gas production, transport and emissions, according to a new study.
The findings could lead to aquatic plant management strategies that help mitigate the release of gases such as methane, carbon dioxide and nitrous oxide.
And that's important as about half of all the methane emissions on the planet originate from aquatic sources, with wetlands, ponds and shallow lakes accounting for most of it. Methane is a powerful greenhouse gas that is roughly 28 times more potent over 100 years than carbon dioxide.
"A lot of public and privately owned water bodies can be heavily managed, based on what people want," said Meredith Theus, the lead author of the study, published Aug. 5 in the journal Aquatic Botany. For example, people often remove submerged plants (or water weeds) and algae from ponds when they simply affect the ability to swim comfortably, she said.
"Our study suggests that we can potentially manage these systems to mitigate climate change or reduce overall emissions," said Theus, a doctoral student in the lab of co-author Meredith Holgerson, associate professor in the Department of Ecology and Evolutionary Biology in the College of Agriculture and Life Sciences.
In the study, Theus set up a summer field experiment from late spring to early fall at the Cornell Experimental Ponds Facility. Within each of three ponds, she set up three corrals (mesocosms) to establish the following three treatments: submerged plants (whose roots are in sediment); submerged and floating plants (such as duckweed that float freely on the surface); and phytoplankton (tiny plants like algae that float in the water column).
Once the communities were established, she collected water column chemistry measurements, including dissolved greenhouse gas concentrations, every two weeks. She sampled greenhouse gas fluxes (gases emitting from the water into the atmosphere) for methane, carbon dioxide and nitrous oxide using a portable greenhouse gas analyzer.
The experiments revealed that the treatment with submerged plants and floating plants had the highest water column concentrations of carbon dioxide and methane, and the lowest nitrous oxide concentrations, but those results weren't reflected in the fluxes, which showed no differences between the three treatments.
"You'd think surface water concentrations would be similar to fluxes because if you had more of something in the water, you'd have more of that thing coming out, but we didn't see that," Holgerson said.
One reason for this result, she said, could be that floating plants like duckweed, while individually small, can collectively blanket the water and block gases from escaping into the atmosphere.
"Some of our previous research has found the highest concentrations of methane in ponds that are completely covered with duckweed, because the duckweed acts like a lid," Holgerson said.
One caveat: Data wasn't collected every day, so flux measurements may not have captured gases escaping when a big wind pushed duckweed to one side of the pond.
Also, the system is complicated, as tiny duckweed roots have been shown to house bacteria called methanotrophs, which consume methane and break it down. The study takes an important step toward informing future research that might explain the discrepancies between greenhouse gas concentrations and fluxes.
"Plant types and community assemblages might have really big effects on the processes that we don't really see inside the 'black box' of a lake or a pond, which could ultimately have a large effect on overall emissions," Theus said.
The study was funded by the Cornell Atkinson Center for Sustainability; the President's Council of Cornell Women's Affinito-Stewart Grant; and the National Science Foundation.
