Although ponds that are 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.
This study (by Yale’s School of Forestry & Environmental Studies) is the first to include these small ponds in global estimates of CO2 and CH4 emissions, basically because they are difficult to map (they don’t typically show up on satellite images) and were thought to play a small role in carbon cycling. The analysis was conducted 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. It was 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 affect the entire pond.