Permafrost thaw threatens to release vast quantities of labile carbon (C) into northern aquatic ecosystems, with significant implications for greenhouse gas (GHG) production and evasion. Melting permafrost may create a positive feedback loop wherein higher greenhouse gas emissions lead to increased temperatures and further thaw. A recent study of thaw ponds in subarctic Stordalen Mire found that GHG emissions were lower in ponds with vegetation overall, but more notably in moss-dominated ponds (Kuhn et al. 2018). These data suggest that functional group-specific vegetation cover promotes carbon retention in thaw ponds, and dampens the rising temperature, permafrost thaw, and GHG emission feedback. Here, we examined the influence of two dominant pond primary producers, mosses and sedges, which differ in several functional traits, on carbon dynamics and pond metabolism. Eight thaw ponds representing a gradient of moss, sedge, and open water cover were sampled from Stordalen Mire in July 2019 using a combination of methods: line transects of vegetation cover and pond morphology; biomass harvest; gas sampling from surface and pore waters; floating chambers; and oxygen sensors. Metabolism was modeled using the maximum likelihood estimation LakeMetabolizer model.
Results/Conclusions
Moss and sedge ponds differed in several aspects of carbon cycling in relation to functional group-specific traits. Gas transfer velocity (K600) was faster for sedge dominated ponds and increased with sedge cover, suggesting morphological traits (filamentous/prostate growth form) influence surface-air exchange. Biomass tended to reduce both CO2 and CH4 gas concentrations across pore and surface water samples. Surface water concentrations of CH4 were lower in moss dominated ponds relative to ponds with more open water or sedge cover, likely due to its methanotroph symbiont. However, CO2 concentrations in surface water were higher relative to sedge-dominated ponds. Sedge cover was positively correlated with pore water CO2 and is expected to be the result of root processes, including root respiration and oxygen leakage. Preliminary analyses of ecosystem metabolism (gross primary production, GPP, and net ecosystem production, NEP) were primarily driven by variation in moss biomass, which was positively correlated with GPP. These data suggest the abundance of particular primary producers can influence C cycling processes in subarctic thaw ponds, potentially reducing the feedback to global warming.