Determining the meteorological forcing that affect seasonal and diurnal dynamics of methane emissions at a constructed urban wetland in Ohio

Wetlands are considered sinks of CO2, but also produce Methane, a very potent greenhouse gas. As recent concerns over GHG emissions are starting to conflict with the need for wetland restoration, it has become pivotal to assess the GHG budgets from temperate urban wetlands.

We set up a meteorological station for continuous micro-meteorological and eddy-covariance flux measurements of CO2 and methane using an open-path gas analyzer at the Olentangy River Wetland Research Park (ORWRP) in Ohio State University. The sustainability of these wetlands is dependent on their potential benefits provided through their ecosystem services and their GHG budget. Results from the eddy-flux measurements of methane match well with those from chamber-based measurements that are conducted monthly at the ORWRP. A combination of continuous eddy-flux measurements, with chamber-based point measurements, which are sparse in time, is needed due to high spatial and temporal variation in carbon sequestration and methane emission rates across the different sub-habitats of the wetland ecosystem. We used a multi-patch footprint model to determine the emissions and sequestration rates in different components of the wetland, such as the river, the billabong overflow pond, dry upland forest and two wetlands – containing mostly indigenous vs. mostly invasive cattail species.

We find a clear seasonal pattern of increased methane emission during the summer and near zero during the winter. A clear daily pattern exists for carbon flux rates that peak in the early afternoon. However, ecosystem methane emissions do not show strong and predictable daily patterns and they are very poorly correlated with environmental drivers such as soil/water temperature and incoming radiation. Within these weak daily fluctuations, we hypothesize that a daily peak of emissions in the morning hours is associated with venting of high near-ground methane concentrations that accumulate during the night when the wind and turbulent mixing are low.