Shallow lakes and ponds emit significant amounts of greenhouse gases into the atmosphere, but emissions from these systems vary considerably and are not well understood.
Now, a new Cornell University-led study measures methane and carbon dioxide emissions from 30 small lakes and ponds (one acre or less) in temperate areas of Europe and North America, revealing that the smallest and shallowest bodies of water exhibit the greatest variability over time.
The paper marks an important step toward calibrating climate models so they better predict emissions from inland waterbodies, and it points to the need to study small waterbodies more closely.
"This study helps understand both the drivers of greenhouse gas concentrations, and importantly, what makes some ponds more variable in their concentrations," said Meredith Holgerson, assistant professor of ecology and evolutionary biology and senior author of the study, published in the journal Limnology and Oceanography.
"The paper points to patterns across a broad geographic range, such that we can actually get in and predict which waterbodies are going to vary and will be most variable, and it confirms that we need to go out and sample frequently," said Nicholas Ray, a postdoctoral researcher in Holgerson's lab and the paper's first author.
Holgerson and colleagues have previously estimated that shallow lakes and ponds may contribute 5% of the global methane emissions to the atmosphere. But without accurate measurements across many water bodies, they said, the true number could be as little as half or as much as twice that percentage.
While some small lakes and ponds emit greenhouse gasses in consistent, predictable amounts, others are highly variable. Understanding these dynamics is important as carbon dioxide and methane act as greenhouse gases in the atmosphere, with methane being 25 times more potent at trapping heat than carbon dioxide.
Each body of water analyzed was sampled over the 2018 and 2019 summers at three times in three locations, including the deepest point and then two locations on opposite ends (but not too close to the shore).
"One key result we found was that the smaller the system is, in regard to surface area, the higher emissions are likely to be," Ray said.
For carbon dioxide, samples were consistent in all parts of the waterbody, which revealed that researchers likely only needed to collect a sample from one location to get an accurate prediction of the whole body of water. Methane, on the other hand, required samples from multiple locations to get an accurate measure. Also, for methane, shallower systems were more variable, suggesting stratification of the water column in deeper water may prevent gases from rising to the surface.
For carbon dioxide, the amount of plant life in the water played a large role in variability over time. For methane, variability was more driven by the water depth and likely associated with stratification in the water column.
Among other uses, the study sets the groundwork for informing a New York state climate mitigation strategy to build more ponds to help farmers better handle droughts.
"We're working to identify how ponds can be built, or if there are simple management strategies people can employ, to minimize emissions," Ray said.
Source: https://www.cornell.edu/