Drivers of carbon dioxide and methane supersaturation in small, temporary ponds

Inland waters are an important component of the global carbon cycle, but there is a poor understanding of carbon dynamics in very small ponds. In this study, I evaluated the concentrations and drivers of carbon dioxide (CO2) and methane (CH4) in six small (< 1000 m(2)), temporary, forested ponds in Connecticut, USA. The six ponds were on average 19-fold supersaturated in CO2 and 504-fold supersaturated in CH4 relative to the atmosphere. For both gases, this level of supersaturation is among the highest reported for lentic freshwaters. The physical, chemical, and biological parameters in the ponds differ from larger lakes, and may explain the supersaturation. Specifically, the ponds have high terrestrial carbon loadings, are shallow, and polymictic, meaning much of the water is in contact with the carbon-rich sediments. Pond CO2 concentrations were best predicted by a negative relationship with dissolved oxygen (DO), indicating that substantial respiration, likely from the sediments, drew down oxygen (O-2) and produced CO2. The stoichiometric relationship between CO2 supersaturation and O-2 undersaturation also implicates anaerobic respiration as a significant source of CO2 production in the ponds. The high carbon, low oxygen, and shallow nature of small ponds were also prime for CH4 supersaturation. Methane concentrations were best predicted by a negative relationship with precipitation, likely because precipitation increased pond depth, diluted dissolved organic carbon and gas concentrations, and increased DO. If the respiration of terrestrial carbon in small ponds is not accounted for in carbon budgets, current estimates of terrestrial net ecosystem productivity may be overestimated.