Methane emissions along a salinity gradient of a restored salt marsh in Casco Bay, Maine

This study functions as a pilot project to understand the relationship between salinity and methane emissions on a recently restored salt marsh in Casco Bay, Maine. Long Marsh is a 1.5 mile-long, narrow tidal salt marsh located in Harpswell, Maine, that has been tidally restricted by an undersized culvert for over 100 years. Recent restoration efforts on Long Marsh replaced this culvert with a larger one in February, 2014. The salinity gradient has since been restored along much of the marsh, and freshwater vegetation that encroached on the marsh platform has died back. Vegetation and salinity are key indicators and drivers of CH4 emissions on salt marshes. Using static gas chambers, we quantified CH4 fluxes along a salinity gradient at three sites ranging from healthy marsh (salinity of 27 to 31 psu) with Spartina vegetation, to regions invaded by freshwater and Typha vegetation (salinity of 0 to 4 psu). Two transitional sites affected by vegetation changes were also measured. Sampling was executed in the months of July, August and October. CH4 concentrations were determined using a gas chromatograph with a flame-ionization detector. CH4 flux data were determined, and complimented by δ13C and % organic carbon values in 50 cm sediment cores at each site. Lowest CH4 fluxes with least variability were observed at the most saline sites with Spartina vegetation (range of -3.3. to 12.8 μmol CH4/m2 hr). The highest and widest range of CH4 emissions ranged from -0.72 μmol to CH4/m2 hr to 256.3 μmol CH4/m2 hr at the freshest, Typha dominated sites from July to October. The transitional sites exhibited significant variability ranging from -4.6 μmol CH4/m2 hr to 16.9 μmol CH4/m2 hr . For all sites, lowest fluxes were observed in October and highest fluxes in July, suggesting seasonal influence on CH4 emissions. CH4 flux data suggest the reintroduction of healthy tides inhibits methane production and emission. Sediments from transitional sites showed a δ13C shift from C4 signal (Spartina; -15 ‰) to C3 signal (Typha; -25 ‰) in the sediment record, which identifies the encroachment of Typha onto the marsh platform due to tidal restriction. Based on average fluxes at freshwater sites for all sampling periods (61.8 μmol CH4/m2 hr) and a calculated 3.11 ha decrease (92%) in Typha area in the first 15 months of restoration, we project a decrease from 75000gCH4 to 5700 gCH4 ± 3000 gCH4 emitted (per 3 months) since restoration