The role of methane transport from the active layer in sustaining methane emissions and food chains in subarctic ponds

Shallow groundwater flow from the seasonally thawed active layer is increasingly recognized as an important pathway for delivering methane (CH₄) into Arctic lakes and streams, but its contribution to CH₄ emissions from thaw ponds has not been evaluated. Furthermore, the potential influence of the shallow groundwater-derived CH₄ on the trophic support and nutritional quality of thaw pond food chains remains unexplored. In this study, we used a radon-mass balance approach to quantify the CH₄ transport from the active layer into thaw ponds in a sub-Arctic catchment. We analysed stable isotopes and fatty acids of pond macroinvertebrates to evaluate the potential effects of CH₄ inputs through active layer groundwater flows on the aquatic food chains. Our results indicate that CH₄ fluxes from the active layer can sustain CH₄ emissions from the ponds. Consumers in ponds receiving greater CH₄ inputs from the active layer had lower stable carbon isotope signatures that indicates a greater trophic reliance on methane oxidizing bacteria (MOB), and they had lower nutritional quality as indicated by their lower tissue concentrations of polyunsaturated fatty acids. Accurate predictions of CH₄ release from small thaw ponds will thus require improved knowledge of the contributions from various processes including internal production, flow paths of active layer groundwater, and MOB-consumer interactions.