Small ponds emit huge share of greenhouse gases: study

Washington: Tiny ponds less than a quarter of an acre in size play a disproportionately large role in global greenhouse gas emissions from inland waters, a new study has found.

Although such small ponds make up only 8.6 per cent of the surface area of the world's lakes and ponds, they account for 15.1 per cent of carbon dioxide (CO2) emissions and 40.6 per cent of diffusive methane (CH4) emissions.

"Our study is the first to include these small ponds in global estimates of CO2 and CH4 emissions, largely because they are difficult to map and were thought to play a small role in carbon cycling," said Meredith Holgerson from Yale University in US.

For the study, researchers combined recent estimates on the global number of lakes and ponds with a compilation of direct measurements of CO2 and CH4 concentrations from 427 lakes and ponds.

They found that concentrations were greatest in smaller ponds and decreased as the ponds and lakes grew larger.

The reason for this has to do with the physical makeup of very small ponds and the way they cycle carbon.

Small ponds have a high perimetre-to-surface-area ratio, for example, and accumulate a higher load of terrestrial carbon - so-called 'leaf litter,' sediment particles and other material.

Small ponds also tend to be shallow, which means their terrestrial carbon loads are highly concentrated compared to larger lakes, researchers said.

Gases produced at the bottom of these ponds are able to reach the top more often than what occurs in larger lakes, due to greater water mixing and shallower waters. Because of this, CO2 and CH4 generated in sediments affects the entire pond, they said.

"That makes small ponds an important player in the carbon cycle," said Holgerson.

The findings suggest that small ponds are likely breaking down terrestrial carbon that is not factored into assessments of the world's carbon stocks and fluxes.

"The carbon cycling that happens in freshwater systems needs to be accounted for in estimates of terrestrial production," said Holgerson.

"These numbers are important to quantifying the global carbon cycle and making predictions about future stocks and flows of carbon," she said.

The findings were published in the journal Nature Geoscience.