The methane (CH4) flux was measured during a field campaign in August 2021 on the Kenai Peninsula, Alaska. We chose locations in coastal wetlands along transects to cover a gradient from freshwater flooded into tidal, saltwater flooded wetlands. In total, we measured at 27 different locations including saltwater tidal regularly flooded bare grounds, temporarily irregularly flooded, as well as seasonally flooded, vegetated coastal wetlands.
Flux measurements were made with a micro-portable LosGatos greenhouse gas analyzer (LosGatos Research) and a light-weight custom-made bucket chamber consisting of non-transparent PVC (diameter = 26 cm, volume ~ 19,000 cm3). We chose this small, lightweight equipment in order to be highly mobile in muddy, shrubby terrain and measure at field sites which were not easy to access. At each site we measured three replicates for 7 to 10 minutes, described the vegetation cover (if present), and measured the chamber and air temperature. The bucket chamber was equipped with a venting tube and a small fan to have well-mixed conditions in the chamber.
The CH4 flux (in mg CH4 m-2 h-1) for each replicate measurement was calculated based on the volume and temperature of the bucket chamber, and the ideal gas law. We manually removed the first 30 sec of each measurement, because of potential disturbance while placing the bucket chamber on the site (we did not use pre-installed collars). Fluxes were calculated by applying a linear regression to the CH4 concentration and were given in mg CH4 m-2 h-1. The r squared was used to determine the quality of the linear regression. Fluxes that had a linear regression with a r squared below 0.9 were discarded. However, we did not want to exclude all the near-zero measurements as these indicate important data as well (no CH4 fluxes). Therefore, all measurements, which were below the precision of the greenhouse gas analyzer of 0.5 ppb (~0.44 mg CH4 m-2 h-1, depending on chamber volume) were included in the analysis as zero fluxes.
In a final step, we averaged all the replicates from a measurement site in order to have one CH4 flux value per measurement site and calculated the CH4 flux for the growing season of 153 days in g CH4 m-2 yr-1, which served as input data for the bootstrapping of the Beringia coastal wetland CH4 estimation.
