The Impact of Marsh Impoundment on Methane Production in a Phragmites Wetland

Coastal wetlands are a natural carbon sink via storage in peat deposits. However, anthropogenic disturbances, such as impoundment, may alter environmental conditions to favor invasive fresh and brackish salinity plant species and the biogeochemical conditions favorable for methanogenesis. As a result, altered coastal wetlands may become a methane source and potentially warm the climate. However, there is a wide range of reported methane fluxes across low salinity coastal wetlands, thus large uncertainty about which environmental factors control methane production and what the most appropriate methane emission factors are. For this study, we evaluated porewater methane within Phragmites australis patches at Sage Lot Pond, a natural marsh, and Herring River, an impounded marsh, in Cape Cod, Massachusetts. At each site, four porewater profiles from 0 to 90 cm of methane, salinity, dissolved inorganic and organic carbon, and oxidative reduction potential (ORP) were collected in July 2019. In addition, plant biomass was measured at each site and water level was measured 30 days prior to sampling. Across the natural and impounded sites porewater salinity was similar (natural 0.8-17.5, impounded 1.0-9.1), water level was consistently at or above the marsh surface, and biomass was similar (natural 960-3960 g/m², impounded 1140-3390 g/ m²), however the methane concentration at the impounded marsh was 150 times greater than the concentration of the natural marsh (natural 1.8 uM, impounded 260 uM). The impounded site was much more reducing, consistent with methanogenesis being the main decomposition pathway. It is likely that iron or sulfate reduction decomposition was favored at the natural site, while the impounded site potentially lacked these electron acceptors. Understanding the environmental drivers of methanogenesis across natural and altered wetlands is essential to managing anthropogenic methane emissions in altered wetlands.