Methane and carbon dioxide dynamics in peatland lake sediments from arctic Sweden

Northern peatland lakes are sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere. Accelerated warming in the sub-arctic is projected to increase emissions of CO2 and CH4 through diffusive loss and ebullition (bubbling). In order to quantify carbon gas emissions through ebullition, and link them to underlying processes in the sediment, we have examined two sub-arctic lakes in Stordalen Mire. This long-term biogeochemical research site is located in the discontinuous permafrost zone in northern Sweden. Ebullition events were quantified during four consecutive summers and were compared to lake sediment cores taken from four lake zones. Coring sites differed in both water depth and ebullition rates. A range of geochemical and microbial measurements have been made using sediment material: (a) incubations of lake sediment at three depths and two temperatures for CO2 and CH4 production rates; (b) sediment and porewater CH4 concentrations and δ13C-CH4; (c) porewater dissolved inorganic carbon (DIC); (d) microbial community profiling via 16S rRNA gene amplicon sequencing; (e) total organic carbon, total organic nitrogen, and calcium carbonate; and (f) the isotopic signature (δ13C and δ15N) of the organic matter. Results indicate that spatial variability of CO2 and CH4 concentrations in the sediment are closely related to ebullition rates measured at the lake surfaces. Based on sampling at different water depths and locations, CO2 potential production rates follow DIC patterns in the cores. These results are consistent with the dissolved CH4 concentrations and potential methane production rates also measured during incubations. This highlights the need for more studies that link sediment characteristics to CO2 and CH4 release from high latitude aquatic ecosystems.