Methane cycling dynamics in sediments of Alaskan Arctic Foothill lakes

Abstract

We measured aspects of sediment methane (CH4) cycling dynamics in 3 shallow (mean depth, z̄= 2.1 m) and 3 deep (z̄ = 6.5 m) Alaskan Arctic Foothill lakes to establish reference data to evaluate future climate-mediated changes in these systems and to identify lake-size-dependent differences in rates and controls on CH4 production and consumption or sensitivity to changing climate. The mean sedimentation rate and sediment oxygen (O2) penetration depth were significantly higher and lower, respectively, in shallow versus deep lakes. The molar carbon to nitrogen (C:N) ratio of 12 for sedimenting material across lakes indicated a dominance by phytodetritus. Pore water dissolved organic C and CH4 concentrations were higher in shallow than in deep lakes at comparable depths below the sediment surface. The average area-based rate of methanogenesis was significantly higher in shallow lakes, exceeding the mean of deep lakes by a factor of 4; however, the mean potential area-based rate of CH4 oxidation was comparable between lake classes due to the reduced sediment O2 penetration depth in shallow lakes. All lake sediments responded similarly to chemical amendments. Hydrogen addition significantly stimulated rates of methanogenesis relative to unamended controls, while rates were unchanged by alternate electron acceptors (SO42-, Fe3+, NO3-, Mn4+), suggesting that other microbial groups did not compete with methanogens for common substrates or produce toxic intermediates. Across all lakes, 30% of assimilated CH4 was converted to methanotophic biomass, and methanotrophic C production could be as much as 23% of epipelic primary production, pointing to the potential importance of CH4-derived C in Arctic lacustrine food webs.