Large Methane Emissions during Ice-Melt Periods in a Temperate Pond

Large methane (CH4) emissions during ice-melt periods have been frequently reported in lakes, but whether similar processes occur in ponds remains largely unknown. Here, we conducted observations in a temperate pond across two consecutive ice seasons (2022-2024), with daily sampling during ice melt, to characterize ice-melt CH4 dynamics, reveal potential CH4 production pathways, and estimate the contribution of ice-melt emissions to the annual CH4 budget. CH4 concentrations increased sharply prior to ice melt, resulting in a diffusive pulse during the ice-melt periods (20.1 ± 14.1 mg m-2 h-1), approximately 11 times higher than that during observed ice-free periods (1.8 ± 2.5 mg m-2 h-1). In contrast, CH4 ebullition during ice melt remained low (0.21 ± 0.15 mg m-2 h-1). Despite the brief duration (∼7 days), ice-melt CH4 emissions accounted for 8.7% of annual total CH4 emissions (diffusion + ebullition) and 22.3% of annual diffusive CH4 emissions from June 2022 to May 2023. Variations in water chemistry and sediment microbial communities suggest that both anaerobic and aerobic CH4 production may have contributed to these emission pulses. Particularly, CH4 concentration increased concurrently with chlorophyll a and dissolved oxygen before ice melt, suggesting possible phytoplankton-derived CH4 production under oxic conditions. Given the ubiquity of seasonal freeze-melt cycles in temperate ponds, incorporating ice-melt CH4 fluxes is necessary to refine global CH4 budget estimates.