Methane Flux from an Active Delta within the Mississippi River Coastal Deltaic Plain.

Methane (CH₄) has significant global warming potential, which is ~ 28 times greater than carbon dioxide (CO₂) over a century.  Coastal systems are generally considered to be low contributors to the global atmospheric pool of CH₄ due to the activity of sulfate-reducing bacteria, however measurements in river deltas are underrepresented in the literature.  Methane flux was investigated seasonally at three different geomorphic settings (newly forming island, river bottom and established freshwater marsh) within the Wax Lake Delta located in the Mississippi River deltaic.  The CH₄ flux rates were 385±327 mg C m^-2 d^-1 in March and 2858 ±1286 mg C m^-2 d^-1 in June at the freshwater marsh site. At the island site, CH₄ flux was substantially smaller at 7.94±3.57 mg C m^-2 d^-1 in March and 215±153 mg C m^-2 d^-1 in June while at adjacent river bottom site, CH₄ flux was lowest at 1.54±0.78 mg C m^-2 d^-1 in March and 15.2±8.31 mg C m^-2 d^-1 in June at sediment-water interface, and 3.73±3.67 mg C m^-2 d^-1 in March and 18.2±1.04 mg C m^-2 d^-1 in June at the air-water interface .  Overall, CH₄ emission rates were significantly higher, by up to two orders of magnitude, for the established marsh compared to both the newly forming island and adjacent submerged channel bottom sites. Predictably, C content was significantly higher at the marsh site compared to the other two sites and was a major factor stimulating CH₄ production. As the delta ages and marsh vegetation take hold, the CH₄ emissions can be expected to increase. The CH₄ emission rates at the marsh site, located more distally from the active delta, are in close agreement with other rates measured throughout the Mississippi Deltaic plain with a mean value of 111.2±84.9 g CH₄-C m^-2 yr^-1. These data suggest that areas located within the sphere of influence of an active delta do not provide a significant source of methane, despite the fresh water conditions, due to a lack of soil C. However, as the delta matures and marsh establishment leads to increased primary productivity, these areas can become hotspots of CH₄ emissions within the coastal zone.  The current status of rising sea level may lead to conversion, over time, of the fresh marsh to brackish and salt marsh, which would significantly reduce CH₄ emissions over the longer term.