Hourly methane and carbon dioxide fluxes from temperate ponds

Ponds are regarded as greenhouse gas (GHG) emission hot spots, but how hot are they? We examined this question by measuring methane (CH₄) and carbon dioxide (CO₂) fluxes in six forest and open land ponds on grasslands in Denmark during summer and winter. We used floating chambers with do-it-yourself sensors and automated headspace venting, allowing for 7404 hourly measurements. We found highly variable gas fluxes within ponds and between seasons and pond types. Ebullitive CH₄ fluxes were more variable than diffusive CH₄ fluxes. Ebullition was absent when total CH₄ fluxes were lowest (15 µmol m⁻² h⁻¹), dominant (> 90%) at the highest fluxes (> 400 µmol m⁻² h⁻¹), and increased with water temperature. In summer, a minor daily increase in diffusive fluxes was found on days with high wind speed, while CH₄ ebullition remained constant. CO₂ fluxes paralleled the day-night balance of photosynthesis and respiration. Mean CH₄ ebullition in open and forest ponds exceeded CH₄ diffusive fluxes 4.1 and 7.1-fold in summer (avg. 22.5 °C) and 2.3 and 2.5-fold in winter (9.6 °C), respectively. CO₂ emissions were higher on a molar basis than CH₄ emissions, both in summer and winter, while their annual global warming potentials were similar. Mean annual gas emissions from open and forest ponds (1092 and 2527 g CO₂e m⁻² y⁻¹) are naturally high due to extensive external input of dissolved CO₂ and organic carbon relative to pond area and volume.