Seasonal variations of methane fluxes from an unvegetated tidal freshwater mudflat (Hammersmith Creek, GA)

Wetlands are estimated to contribute nearly 40 % of global annual methane (CH4) emissions to the atmosphere. However, because CH4 fluxes from these systems vary spatially, seasonally, and by wetland type, there is a large uncertainty associated with scaling up the CH4 flux from these environments. We monitored seasonal patterns of CH4 cycling from tidal mudflat wetland sediments adjacent to a vegetated freshwater wetland in coastal Georgia between 2008 and 2009. CH4 emissions were significantly correlated with CH4 production and sediment saturation state with respect to CH4 but not with temperature. CH4 cycling displayed distinct seasonal patterns. Winter months were characterized by low CH4 production and emissions. During the spring, summer and fall, CH4 fluxes exceeded CH4 production in the top 40 cm. Comparison of CH4 sources and sinks in conjunction with the interpretation of CH4 concentration profiles using a 1D reactive transport model indicated that CH4 delivered via lateral tidal pumping likely provided additional CH4 to the upper sediment column. Seasonally high CH4 ebullition rates reflected increased CH4 production and decreased CH4 solubility. The annual CH4 flux was estimated to be on the order of 10 mol CH4 m(-2) y(-1) which is 2-4 times the global average for wetland CH4 emissions. Thus, even though tidal freshwater mudflats are of limited spatial extent, these environments may serve as globally significant sources of CH4 to the atmosphere. This study highlights the importance of these dynamic environments to the global CH4 cycle and their relevance to climate change.