Abstract. Ponds smaller than 10 000 m2 likely account for about one-third of the global lake perimeter. The release of methane (CH4) and carbon dioxide (CO2) from these ponds is often high and significant on the landscape scale. We measured CO2 and CH4 fluxes in a temperate peatland in southern Ontario, Canada, in summer 2014 along a transect from the open water of a small pond (847 m2) towards the surrounding floating mat (5993 m2) and in a peatland reference area. We used a high-frequency closed chamber technique and distinguished between diffusive and ebullitive CH4 fluxes. CH4 fluxes and CH4 bubble frequency increased from a median of 0.14 (0.00 to 0.43) mmol m−2 h−1 and 4 events m−2 h−1 on the open water to a median of 0.80 (0.20 to 14.97) mmol m−2 h−1 and 168 events m−2 h−1 on the floating mat. The mat was a summer hot spot of CH4 emissions. Fluxes were 1 order of magnitude higher than at an adjacent peatland site. During daytime the pond was a net source of CO2 equivalents to the atmosphere amounting to 0.13 (−0.02 to 1.06) g CO2 equivalents m−2 h−1, whereas the adjacent peatland site acted as a sink of −0.78 (−1.54 to 0.29) g CO2 equivalents m−2 h−1. The photosynthetic CO2 uptake on the floating mat did not counterbalance the high CH4 emissions, which turned the floating mat into a strong net source of 0.21 (−0.11 to 2.12) g CO2 equivalents m−2 h−1. This study highlights the large small-scale variability of CH4 fluxes and CH4 bubble frequency at the peatland–pond interface and the importance of the often large ecotone areas surrounding small ponds as a source of greenhouse gases to the atmosphere.

<p><strong>Abstract.</strong> Ponds smaller than 10000 m<sup>2</sup> likely account for about one third of the global lake perimeter. The release of methane (CH<sub>4</sub>) and carbon dioxide (CO<sub>2</sub>) from these ponds is often high and significant on the landscape scale. We measured CO<sub>2</sub> and CH<sub>4</sub> fluxes in a temperate peatland in southern Ontario, Canada, in summer 2014 along a transect from the open water of a small pond (847 m<sup>2</sup>) towards the surrounding floating mat (5993 m<sup>2</sup>) and in a peatland reference area. We used a high-frequency closed chamber technique and distinguished between diffusive and ebullitive CH<sub>4</sub> fluxes. CH<sub>4</sub> fluxes and CH<sub>4</sub> bubble frequency increased from a median of 0.14 (0.00 to 0.43) mmol m<sup>−2</sup> h<sup>−1</sup> and 4 events m<sup>−2</sup> h<sup>−1</sup> on the open water to a median of 0.80 (0.20 to 14.97) mmol m<sup>−2</sup> h<sup>−1</sup> and 168 events m<sup>−2</sup> h<sup>−1</sup> on the floating mat. The mat was a summer hot spot of CH<sub>4</sub> emissions. Fluxes were one order of magnitude higher than at an adjacent peatland site. During daytime the pond was a net source of CO<sub>2</sub> equivalents to the atmosphere amounting to 0.13 (−0.02 to 1.06) g CO<sub>2</sub> equivalents m<sup>−2</sup> h<sup>−1</sup>, whereas the adjacent peatland site acted as a sink of −0.78 (−1.54 to 0.29) g CO<sub>2</sub> equivalents m<sup>−2</sup> h<sup>−1</sup>. The photosynthetic CO<sub>2</sub> uptake on the floating mat did not counterbalance the high CH<sub>4</sub> emissions, which turned the floating mat into a strong net source of 0.21 (−0.11 to 2.12) g CO<sub>2</sub> equivalents m<sup>−2</sup>h<sup>−1</sup>. This study highlights the large small-scale variability of CH<sub>4</sub> fluxes and CH<sub>4</sub> bubble frequency at the peatland-pond interface and the importance of the often large ecotone areas surrounding small ponds as a source of greenhouse gases to the atmosphere.</p>

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