Tiny ponds play a
disproportionately large role in global greenhouse gas emissions from
inland waters,
according to a new study by Yale’s School of Forestry & Environmental
Studies.
(February 1, 2016) Although
ponds less than a quarter of an acre in size make up only 8.6% of the surface
area of the world’s lakes and ponds, they account for 15.1% of carbon dioxide
(CO2) emissions and 40.6% of diffusive methane (CH4) emissions. The findings
appear in the Feb. 1 online edition of the journal Nature Geoscience.
“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 Yale
doctoral student Meredith Holgerson, the study’s lead author.
Holgerson and co-author Peter Raymond, professor of
ecosystem ecology at Yale, conducted their analysis by combining 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 has to do with the physical makeup of very small
ponds and the way they cycle carbon. Small ponds have a high
perimeter-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. Lastly, 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.
“That makes small ponds an important player in the carbon
cycle,” Holgerson said.
The findings warrant additional research to more accurately
estimate the number of tiny ponds around the world, she added. Such spots don’t
typically show up on satellite images, but they can be mapped using aerial
images and LiDAR, a remote sensing technology that uses reflected laser light.
The researchers also said their 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.