Coupled methane and carbon dioxide fluxes in coastal marshes along a salinity gradient

Background/Question/Methods

Carbon fluxes in tidal marshes vary spatially and temporally due to vegetation cover, subsurface biogeochemical processes, and tidal inundation. Methane (CH₄) emissions, in particular, have been shown to vary widely with changing salinity and vegetative species composition. The objective of this study is to use measured carbon dioxide (CO₂) and CH₄ fluxes at various light levels to model net ecosystem exchange, the balance between gross ecosystem CO₂ fixation and ecosystem respiration, over an annual time scale and across a salinity gradient. Three marsh sites in the Mobile Bay estuary were selected that varied in salinity (5-27 ppt) and vegetative species composition (Cladium and Spartina alterniflora). CO₂ and CH₄ fluxes were measured monthly in triplicate plots in the field at several different light levels to generate light response curves. Sediment nutrient profiles and hydrogen sulfide concentrations were measured using porewater wells at 10 and 25 cm depth.  Porewater CH₄ concentrations were also measured from sediment cores every 8 cm down to 25 cm. Short term CO₂ and CH₄fluxes measured in the field will be used to generate gross ecosystem exchange vs. PAR and ecosystem respiration vs. air temperature relationships at each site. 

Results/Conclusions

Mean ecosystem respiration rates of CO₂ ranged from 1.1 μmol m^-2 sec^-1 at the high salinity site to 11.4 μmol m^-2 sec^-1 at the fresh site. Depth integrated porewater CH₄ inventories did not vary significantly between sites; however, the higher salinity site generally had higher concentrations of porewater CH₄ in the sediments compared to the fresh site. For example, porewater CH₄ inventory for the high salinity site during May was 10.1 ±2.7 mol m^-2 compared to 5.1 ±3.9 mol m^-2 for the fresh site. Diffusive CH₄ fluxes from the fresh site were highly variable by month and by plot, with the largest flux of 58.7 ±57.9 mg CH₄ m^-2 d^-1 measured in March.  Diffusive CH₄ fluxes in the brackish and salty sites were higher in May compared to March, when soil temperatures were higher. Combined flux data coupled with sediment nutrient profiles and environmental data will allow us to specify ecosystem controls on C fluxes in coastal marshes. This on-going study will continue to examine C exchange in Alabama’s coastal marshes for a total of 13 months.