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Ponds release more greenhouse gases than they store

Two recent studies led by Cornell University has found that, although human-made ponds both sequester and release greenhouse gases, when added up, they may unfortunately be net emitters. 

These studies aim to elucidate the consequential impacts of both anthropogenic and natural ponds on the global greenhouse gas budget – a field still shrouded in uncertainty.

“Global climate models and predictions rely on accurate accounting of greenhouse gas emissions and carbon storage,” said Meredith Holgerson, an assistant professor of Ecology and Evolutionary Biology at Cornell and the senior author of both studies.

Focus of the research 

Prior estimates by Holgerson and her team suggest that ponds, categorized as bodies of water five hectares (12 acres) or smaller – with an estimated total of one billion such ponds globally – could be responsible for approximately five percent of global methane emissions. 

However, the real figure might range from half to double of this estimate, highlighting the necessity for accurate measurements across diverse water bodies.

The first paper, “High Rates of Carbon Burial Linked to Autochthonous Production in Artificial Ponds,” published on August 18 in the journal Limnology and Oceanography Letters, examined the amount of carbon sequestered in 22 of the Cornell Experimental Ponds. Established in 1964, these 50 identical ponds offer meticulously controlled environments and a wealth of data from preceding research, enabling scientists to assess the impact of management activities on carbon storage.

Carbon sinking rates

Through a comprehensive examination of past management practices and in-depth measurements of sediment cores and thickness, the researchers determined the carbon sinking rates in these ponds. The findings revealed that these rates were comparable to those of wetlands and mangroves and surpassed those of lakes.

The experts also discovered the influential role of aquatic flora, fish, and certain nutrient ratios on carbon sinking rates. Proper nutrient balance enhances plant growth, leading to increased carbon deposition at the pond’s bottom after plant decay.

Despite the paucity of data on organic carbon sequestration in natural ponds, the extrapolation of their findings allowed the researchers to gauge the total carbon sinking rate in both natural and artificial ponds globally. 

The investigation revealed that these ponds might account for 65 percent to 87 percent of the total carbon sequestered by all lakes, underscoring a global underestimation of carbon sequestration in ponds and lakes.

Greenhouse gas emissions 

The second study, “High Intra-Seasonal Variability in Greenhouse Gas Emissions from Temperate Constructed Ponds,” published on September 19 in the journal Geophysical Research Letters, delved into the seasonal variance in greenhouse gas emissions, predominantly carbon dioxide and methane, from four Cornell Experimental Ponds.

“Global estimates of greenhouse gas budgets from ponds are highly uncertain, in part due to lack of temporal measurements,” said study lead author Nicholas Ray, a postdoctoral fellow in Holgerson’s lab. 

The researchers found that methane, a gas 25 times more impactful on global warming than carbon dioxide, constituted the majority of the annual emissions, with significant seasonal fluctuations.

Regular monitoring is needed

The research outlined how ponds assimilate carbon dioxide during the plant growth season in early summer and release it later, with methane emissions being consistent but varying greatly on a week-to-week basis, indicating the necessity of regular monitoring.

Moreover, the study highlighted the correlation between water stratification, methane production, and increased emissions, emphasizing the significance of frequent sampling in mitigating methane levels. 

The microbial production of methane on pond beds was found to be contingent on low-oxygen conditions, which is disrupted when water is mixed by winds or sudden cooling.

Methane mitigation strategies 

Both studies reveal that, due to the substantial methane emissions, ponds are net contributors to greenhouse gases, overshadowing the carbon sequestered in the sediments. 

However, the studies also highlight the potential for transitioning ponds from net emitters to net absorbers of greenhouse gases through the application of methane mitigation strategies, such as the use of bubblers or underwater circulators.

“If we could reduce that methane number, we could potentially flip these ponds from being net emitters to net sinks, but we have to get a handle on that methane,” Holgerson concluded.

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