Methane Diffusion and Ebullition From Ecohydrological Patches in a Freshwater Estuarine Marsh: Underscoring the Spatial Heterogeneity of Methane Fluxes Within Wetland Ecosystems

Abstract:

Better measurements of the patch-level methane flux heterogeneity in wetlands are needed not only to help parameterize bottom-up biogeochemical models and constrain global estimates. They are also becoming critical to understand methane emissions at the ecosystem-level for the design of carbon credit projects aiming to restore or preserve wetlands. Here, we designed an innovative sampling approach, combining a spatially balanced survey of different ecohydrological patches and rapid chamber deployments for high-frequency measurements of methane from the water column in a temperate freshwater, estuarine marsh. We used the data from chamber deployments to further partitioning diffusion and ebullitive fluxes. Three ecohydrological patches were distinguishable in the wetlands defined by the type and presence/absence of vegetation, namely: emergent, floating-leaved, and open water. Net methane fluxes from the water column (diffusion plus ebullition) in emergent vegetation patches were larger than in the floating-attached vegetation and open water patches (p<0.001). The median [and interquartile] fluxes in emergent patches (1.65 [0.72 – 2.67] µmol m^-2 s^-1) was more than 5 times higher than in floating-attached (0.29 [0.012 – 1.31] µmol m^-2 s^-1) and close to 18 times than in the open water patches (0.092 [0.005 – 1.11] µmol m^-2 s^-1). Diffusive fluxes were not significantly different between patches, while ebullitive fluxes were different (p<0.001), mirroring differences at the patch level. Our results indicate that differences at the patch-level are likely due to differences in ebullitive fluxes. Our findings also underscore the challenges that the modeling community and scientists and managers conducting measurements in the field, face when dealing with methane emissions from similar freshwater wetlands and possibly other types of wetlands.