When Meredith Holgerson arrived at Cornell in 2020, she began searching for the perfect ponds. Poring over maps, taking tips, she'd traipse into the woods of central New York with an inflatable kayak strapped to her back.
By 2021, she had found Mud Pond and Texas Hollow Pond, two bodies of water similar in size but different in key ways that would help her team understand why and how ponds - often understudied and overlooked in carbon budgeting - emit such a high volume of greenhouse gases.
Now a study of the two ponds, published Nov. 26 in Limnology & Oceanography, reveals surprising and complex mechanisms for how carbon dioxide and methane in ponds are built up, stored and released. The researchers found that Texas Hollow, while only a little over 1 meter deeper than Mud Pond, emitted more than twice the amount of carbon dioxide. They also examined how the ponds' stratification - the extent to which the water forms layers of different temperatures - impacts methane emissions, finding that the ponds' slight differences in depth and light led to unexpected results, with nearly twice as much methane bubbling up from the sediments of Mud Pond than Texas Hollow.
The research will contribute to a more accurate calculation of ponds' carbon emissions and sequestration - now poorly understood - and to local management.
"It's important to figure out these mechanisms," said Holgerson, associate professor of ecology and evolutionary biology in the College of Agriculture and Life Sciences. "Identifying mechanisms will allow us to model carbon fluxes more accurately, and then we can improve predictions about how different types of lakes and ponds influence regional or global emissions."
Relative to their size, ponds are "biogeochemical hotspots," Holgerson said, storing and emitting high levels of greenhouse gases. That tracked for Texas Hollow Pond and Mud Pond, which the researchers found stored large amounts of carbon in their sediments and had high emissions of both carbon dioxide and methane. In fact, the "bubbling" of methane out of Mud Pond is among the highest readings ever reported for ponds or lakes.
The team initially hypothesized that the stronger the stratification, or the more distinct the layers and the longer they persist, the more methane gets produced and emitted.
But the emissions from Mud Pond and Texas Hollow Pond surprised them. Carbon dioxide emissions, which they expected to be higher in Mud Pond due to higher oxygen levels throughout the pond, were more than double in Texas Hollow. And despite Texas Hollow having much stronger stratification than Mud Pond, they found that twice as much methane bubbled up from the bottom of Mud Pond. Furthermore, the diffusive methane emissions, from methane dissolved in the water, were nearly the same in the two ponds.
"In our previous work, we found that if a pond was stratified, more gasses built up in the bottom waters, and those gases would transport up to the surface waters and get emitted," Holgerson said. "So we were surprised that the diffusive methane emissions between the two ponds were similar and that Mud Pond had basically double the bubbles; this did not match our previous results in other ponds."
The researchers hypothesize that other factors are at play - Mud Pond, which is more weakly stratified than Texas Hollow, has light penetration throughout the pond that allows plants to grow and thrive. The plants absorb carbon dioxide, possibly contributing to the lower emissions; but when the plants die, they drop into the sediment, decompose, and the resulting methane bubbles up. Meanwhile, the lack of oxygen at the bottom of Texas Hollow lends itself to methane production, but the stratified layers trap some of that methane and prevent it from being emitted.
"We think that the relationship between stratification and gas emissions may be a Goldilocks situation. If you have really strong stratification, the gasses are produced but some become trapped, and under no stratification, the pond mixes often and has a lot of oxygen, which may reduce methane production," Holgerson said. "But in between, if you have this weaker stratification and light throughout the water column and plants, like Mud Pond has, you might get more methane. There are a lot of follow up questions that we have now."
Holgerson's team is continuing to analyze data from the ponds. In 2024, they used a generator and circulator to "turn over" Texas Hollow Pond, simulating a summer storm to measure the impact on emissions. They're also including the ponds in a broader survey of 16 ponds around New York state to assess if the patterns in Mud Pond and Texas Hollow hold across a broader sample size.
"We're especially interested in how stratification influences methane emissions in the context of climate change - as warming temperatures can strengthen stratification," Holgerson said.
Co-authors of the study include former postdoctoral researcher Nicholas Ray, former lab technician Kathryn Gannon and Adam Heathcote from the St. Croix Watershed Research Station.
The study was funded by the New York State Water Resources Institute and the National Science Foundation.
