Large contribution to inland water CO2 and CH4 emissions from very small ponds

Inlandwaters are an important component of the global carbon cycle. Although they contribute togreenhousegas emissions(1-5), estimates of carbon processing in these waters are uncertain. The global extent of very small ponds, with surface areas of less than 0.001 km(2), is particularly difficult to map, resulting in their exclusion from greenhouse gas budget estimates. Here we combine estimates of the lake and pond global size distribution, gas exchange rates, and measurements of carbon dioxide and methane concentrations from 427 lakes and ponds ranging in surface area from 2.5 m(2) to 674 km(2). We estimate that non-running inland waters release 0.583 Pg C yr(-1). Very small ponds comprise 8.6% of lakes and ponds by area globally, but account for 15.1% of CO2 emissions and 40.6% of diffusive CH4 emissions. In terms of CO2 equivalence, the ratio of CO2 to CH4 flux increases with surface area, from about 1.5 in very small ponds to about 19 in large lakes. The high fluxes from very small ponds probably result from shallow waters, high sediment and edge to water volume ratios, and frequent mixing. These attributes increase CO2 andCH(4) supersaturation in the water and limit efficient methane oxidation. We conclude that very small ponds represent an important inland water carbon flux.