Determining total emissions and environmental drivers of methane flux in a Lake Erie estuarine marsh

Estuarine freshwater marshes can act as an important ecosystem for carbon storage and flux because of its strategic position in a watershed. We monitored CH4 and CO2 fluxes in Old Woman Creek, an estuarine wetland of Lake Erie, Ohio. The eddy covariance (EC) technique was used to measure fluxes of CH4 and CO2 continuously during the growing seasons of 2015 and 2016. Simultaneously, monthly sampling of gas exchange was conducted using non-steady state chambers in four distinct land-cover types in the wetland: open water, emergent vegetation (Typha spp.), floating vegetation (Nelumbo spp.) and mud flats. Chambers and EC measurements were combined to provide estimates of the continuous contributions of each land cover to the total methane emissions of the wetland. In addition, water and meteorological measurements were used to determine the most important environmental drivers of methane flux in the wetland. We found an average rate of emission from the Typha patch, the most abundant vegetated land cover, of 219.4 g CH4C m−2 y−1, which was much higher than rates reported in similar emergent vegetation types in other wetlands. Mud flats had the highest rates of CH4 emission, followed by Nelumbo and Typha patches, and open water. Mud flats contributed 6.8% of the total CH4 emissions of the wetland despite occupying only 1.5% of the wetland area, whereas open water contributed 16.1% despite occupying 47% of the wetland area. Water temperature and wind speed were the strongest environmental drivers of CH4 flux to the atmosphere. Carbon fluxes were strongly correlated to methane fluxes. Fluctuating water levels above the wetland's surface had a weak effect on overall CH4 emissions in the wetland, with stronger effects during the night than during the day. Providing an empirical model that predicts the influence of different environmental drivers CH4 emissions in the wetland can aid in the design of estuarine wetlands that retain nutrients and reduce coastal eutrophication while minimizing greenhouse gas emissions.