Seasonal methane emissions by diffusion and ebullition from oligohaline marsh environments in coastal Louisiana

Methane is an important atmospheric greenhouse gas that is emitted from many natural and anthropogenic sources. In order to evaluate the global methane budget, precise data are needed from the diverse sources including coastal wetlands. Over 100 time-series determinations of methane emissions from an oligohaline wetland (brackish marsh) in coastal Louisiana show large variability during five seasonal sampling periods. Emission by both diffusion and ebullition (bubbles) was measured, however, neither of these emission modes were strongly dependent on either water depth or temperature (except in winter). Methane emission to static collectors placed over plants (Scirpus olneyi and Spartina patens) was not significantly different from shallow open water or mud. However, considerable heterogeneity in methane emissions and processes occurs even at a single site. Thus, establishing a reasonable estimate of the overall methane emission for a particular marsh environment and season requires multiple measurements at several sites. The average emissions for April, May, July, and September ranged from 31 to 54 mg/m2/h (744-1296 mg/m²/day). This can be separated into emissions from diffusion ranging from 8.3 to 20 mg/m²/h (18-50% of total) and emissions due to ebullition of 20-44 mg/m /h (50-82%). January emissions were much lower, amounting to 0.2 mg/m²/h (6 mg/m²/day), mainly by diffusion with only one episode of ebullition. Extrapolating these data to annual emissions gives total annual methane emissions of 203 g/m²/yr (61 g/m²/yr by diffusion and 142 g/m²/yr by ebullition).