Influence of Groundwater Connectivity and Sulfate on Boreal Lake Methane Dynamics in the Sporadic Permafrost Zone

Rapid thawing of perennially frozen ground (i.e., permafrost thaw) in northern regions alters hydrologic flow paths, changing lake biogeochemistry and carbon turnover, potentially causing elevated emission of methane (CH4) to the atmosphere. Shifts in hydrologic flow can lead to excess input of nutrients and organic material to lakes, thus increasing dissolved CH4 and resulting lake CH4 emission. However, shifting flow paths could also lead to a higher input of terminal electron acceptors for anaerobic microbial respiration (TEAs; including sulfate), potentially inhibiting CH4 production. The Taiga Plains in western Canada contains vast stores of organic soil carbon, is experiencing rapid permafrost thaw, and has heterogenous surficial geology. Thus this region has complex and variable groundwater-surface water interactions that influence hydrology and lake biogeochemistry. To assess the impact of permafrost thaw and altered hydrology on lake biogeochemistry, we measured CH4 fluxes, sediment redox sensitive solutes, CH4 concentrations, and water quality parameters monthly from three lakes over a three ice-free seasons and used ex-situ sediment incubations to calculate potential rates of CH4 production. The study lakes are small (surface area = 0.4-2.1 ha), shallow (0.5-2.5 m max. depths), and have varying levels of groundwater input, as indicated by specific conductivity in the water columns (150-1440 μS cm-1). Potential CH4 production rates in the sediments were similar across the lakes. However, relatively high in-situ concentrations of sulfate in surface waters (~350 mg L-1) and hydrogen sulfide in the sediments (~1000 μg L-1) negatively correlated with sediment CH4 concentrations and net CH4 emission, suggesting the presence of sulfate in the surface waters mitigates CH4 emission. As permafrost thaw accelerates, further altering hydrologic flow paths, excess delivery of TEAs may moderate net emission of CH4 from lakes. Future research should explore regional differences in landscape characteristics, groundwater connectivity and quality in connection to lake CH4 dynamics.