Two studies by Cornell University researchers have revealed that ponds release more greenhouse gas than they store, shedding new light on the carbon sequestration capabilities of these water bodies.
According to the researchers, while human-made ponds sequester and release greenhouse gases, when added up, they may be net emitters. Ponds are water bodies measuring 12 acres or less and there could be a billion of them on Earth.
“Global climate models and predictions rely on accurate accounting 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 studies. Nicholas Ray, a postdoctoral researcher in Holgerson’s lab, is a co-author of both papers.
Ponds may contribute 5% of the global methane emissions to the atmosphere; however, in the absence of accurate measurements across many water bodies, the true number could be as little as half or as much as twice that percentage.
The paper by Holgerson and colleagues, titled High Rates of Carbon Burial Linked to Autochthonous Production in Artificial Ponds, published in the journal Limnology and Oceanography Letters, examines how much carbon is sequestered in 22 Cornell Experimental Ponds. Constructed in 1964, the 50 identical ponds provide highly controlled environments, with detailed records from previous studies, which allowed Holgerson and Ray to evaluate how management activities contributed to carbon storage.
Taking sediment cores and measurements of sediment thickness for each of the 22 study ponds, the researchers calculated the amount of carbon sequestered annually per square metre. They also found that carbon burial rates were influenced by aquatic plants, fish and additions of high nitrogen levels relative to phosphorus, nutrients that may not get renewed in a static pond and become limited.
The study concluded that natural and artificial ponds sequester 65% to 87% of the total amount estimated to be stored by all lakes.
The second study, by Ray and Holgerson, titled High Intra-Seasonal Variability in Greenhouse Gas Emissions From Temperate Constructed Ponds, published in the journal Geophysical Research Letters, examined seasonal emissions of greenhouse gases from four of the Cornell Experimental Ponds.
In the study, the researchers measured gas emissions from the ponds approximately every two weeks over the course of an ice-free period in 2021.
“Global estimates of greenhouse gas budgets from ponds are highly uncertain, in part due to lack of temporal measurements,” said Ray, who is lead author of the study. The researchers found that methane – a greenhouse gas 25 times more potent than carbon dioxide – accounted for most of the gas emitted annually, and carbon dioxide and methane emissions varied greatly by season.
Carbon dioxide was absorbed during early summer months when plants were growing, and emitted later in the year, when plants decomposed.
Methane was emitted throughout the warm months. When water was stratified (a layer of warm water sitting on top of cold waters), methane built up and led to overall higher emissions than when water was mixed by wind or sudden cooling.
Put together, the two studies establish that ponds are net emitters of greenhouse gases due to methane release overwhelming the amount of carbon stored in the sediments. But the findings also offer the possibility of reducing methane emissions with bubblers or underwater circulators.
Source: Cornell University
Image: Two of 50 Cornell experimental ponds, built in 1964, and used for research