Greenhouse Gas Fluxes in a Natural and Restored Wetland before and after Hurricane Sandy

Abstract:

Recent concerns about wetland restoration have highlighted the potential conflict of the hydrological and ecological benefits of wetlands and greenhouse gas emissions particularly methane. Therefore it is pivotal to quantify emission rates and effects of meteorological, hydrological and ecological drivers of methane and carbon dioxide fluxes in wetlands. Fast methane (CH₄) gas analyzers are now enabling continuous ecosystem scale measurements and assessment. We have set up two eddy flux stations – one in a natural and one in a restored tidal salt marsh in the Meadowlands of New Jersey (MNJ). Continuous methane fluxes were measured with the LI7700 over two growing seasons at the restored site and one season at the natural wetland site. Additional measurements with chambers were conducted at several locations in each site. In October 2012, hurricane Sandy caused a tidal surge 1.6 m above high tide, increasing salinity and water level for two hours at the sites. Methane and carbon dioxide emissions were highly variable in space and time over the two growing season. Whereby, there was a significant correlation between the CO₂ fluxes in the two sites with each other, the temporal dynamics of methane fluxes in each of the sites was not correlated with the other site, suggesting small-scale site-specific controls on methane emissions, but ubiquitous controls on carbon dioxide uptake and release. At both sites methane production and emission is highest during the growing season. In general, ecosystem scale methane emissions at the natural wetland site were lower than at the restored wetland site in 2013. Before and after hurricane Sandy, carbon dioxide fluxes at the restored site were comparable, but slightly decreased at the natural wetland site, signaling great resilience in the system due to external perturbations. Methane emission as measured with the eddy covariance technique as well as the chamber technique increased at the restored site from 2012 to 2013 consistent with an increase in dissolved organic carbon belowground, potentially signaling an effect of hurricane Sandy.