The researchers found that ponds, both natural and artificial, can be net emitters of greenhouse gases, particularly methane. By studying carbon burial rates and greenhouse gas emissions, the study found that ponds may account for a large portion of global methane emissions and capture large amounts of carbon. However, the release of methane, a gas more powerful than carbon dioxide, exceeds the amount of carbon sequestered by these ponds.
Research from Cornell University shows that ponds may be net emitters of greenhouse gases, especially methane. Addressing methane releases has the potential to transform ponds from greenhouse gas sources to greenhouse gas sinks.
While man-made ponds can both absorb and release greenhouse gases, combined, they can be net emitters of greenhouse gases, according to two related studies by Cornell University researchers.
These two studies begin to quantify the significant impact of man-made and natural ponds on the global greenhouse gas budget, the measurement of which is not well understood.
"Global climate models and projections rely on accurate calculations of greenhouse gas emissions and carbon storage," said Meredith Holgerson, assistant professor of ecology and evolutionary biology in the College of Agriculture and Life Sciences and senior author of the study. Nicholas Ray, a postdoctoral researcher in Holgerson's lab, is a co-author of both papers.
Pond impact and carbon burial rates
Holgerson and colleagues have estimated that ponds (defined as 5 hectares or less, and there may be 1 billion ponds on Earth) may account for 5% of the global total methane emissions into the atmosphere. However, because many bodies of water are not accurately measured, the true number may be only half or twice this percentage. At the same time, there are few estimates of carbon burial rates in ponds.
On August 18, the journal "Limnology and Oceanography Letters" published a paper titled "High Rates of Carbon Burial Linked to Autochthonous Production in Artificial Ponds", which studied the carbon burial rates of 22 Cornell experimental ponds. These same ponds (50 in total) were built in 1964 and provide a highly controlled environment with detailed records from previous studies. The data allowed Holgerson and Ray to assess the contribution of management activities to carbon storage.
For the study, the researchers looked at past management activities while also conducting sediment coring and sediment thickness measurements in each of the 22 study ponds. They measured the amount of carbon in the sediment, extrapolated those measurements to the entire pond, and then divided that number by the age of the pond to arrive at the amount of carbon sequestered per square meter per year, a number that was the same order of magnitude as wetlands and mangroves and higher than lakes.
They also found that carbon burial rates were affected by aquatic plants (those large enough to be seen), fish, and high nitrogen additions relative to phosphorus. Adding the right types and proportions of nutrients promotes plant growth, which uses carbon to make cells, and when plants die, these nutrients are deposited at the bottom of the pond.
While data on organic carbon sequestration in natural ponds is lacking, the scientists extrapolated their findings to estimate total carbon burial rates in natural and artificial ponds worldwide. They concluded that natural and artificial ponds account for 65% to 87% of the total estimated storage in all lakes, suggesting that scientists are underestimating the amount of carbon sequestered by ponds and lakes globally.
Seasonal changes in gas emissions
The second study, titled "High Intra-Seasonal Variability in Greenhouse Gas Emissions From Temperate Constructed Ponds," published on September 19 in the journal Geophysical Research Letters, examined seasonal emissions of greenhouse gases (mainly carbon dioxide and methane) from four experimental ponds at Cornell University.
In the study, researchers measured gas emissions from these ponds approximately every two weeks during the ice-free period of 2021.
"Global estimates of pond greenhouse gas budgets are highly uncertain, in part because of a lack of temporal measurements," said Lei, the study's lead author. "The researchers found that methane - a greenhouse gas 25 times more powerful than carbon dioxide - accounts for the majority of annual gas emissions, with emissions of carbon dioxide and methane varying significantly by season."
In early summer when the plants are growing, the pond absorbs carbon dioxide, and later in the year when the plants decay, the pond releases the carbon dioxide. Methane is emitted throughout the warm months, but weekly emissions vary widely, illustrating the need for frequent sampling for accurate accounting.
The researchers found that methane can accumulate when water is stratified (a layer of warm water sits on top of a colder bottom layer), resulting in higher overall methane emissions than when water is mixed by wind or sudden cooling. This is because methane-producing pond bed microbes require low-oxygen conditions, and mixing disrupts these conditions.
Potential solutions and funding
When the results from both papers are combined, ponds are net emitters of greenhouse gases because the amount of methane released exceeds the amount of carbon stored in the sediments. But the findings also raise the possibility of using bubblers or underwater circulators to reduce methane emissions.
"If we can reduce the amount of methane, we could potentially turn these ponds from net emitters to net absorbers, but we have to control the amount of methane emissions," Holgerson said.