TEMPORAL AND SPATIAL VARIABILITY, AND THE ROLE OF DISSOLVED ORGANIC-CARBON (DOC) IN METHANE FLUXES FROM THE SABINE RIVER FLOODPLAIN (SOUTHEAST TEXAS, USA)

The Sabine River floodplain, in southeast Texas, U.S.A., is a broad fl at watershed which contains a diverse assemblage of blackwater wetland habitats. Methane fluxes were compared across a gradient of three dif ferent habitats, within the floodplain, that extended from the outer r eaches of the basin to the river proper (i.e., blackgum Nyssa aquatica , cypress Taxodium distichum, and arrowhead Sagittaria spp.), as well as a fourth station (a Spartina marsh) within the Sabine-Neches estuar y. Concentrations of dissolved organic carbon (DOG) in the floodplain were very high (15-50 mg/l). Methane emissions were highly variable an d ranged from 0.0007 to 1640 mg m(-2) d(-1), with the highest rates in July 1993. These highest CH4 emission rates were observed just after the longest period of flooding (ca. 1 month) at each of the stations. Fluxes were significantly correlated (both positively and negatively) with precipitation (p<0.01); natural periods of flooding and drought w ere primarily responsible for the wide range of emissions. Methane con sumption rates had little effect on total emissions because of losses by ebullition and plant-mediated transport mechanisms. Plant-mediated transport accounted for as much as 96% of the total CH4 emissions at c ertain stations. Laboratory experiments indicated that UV-B treated hu mic substances were preferentially utilized by methanogens over untrea ted humic compounds. Thus, variability of light penetrance across a fl oodplain may affect the quality and ultimately the availability of DOC to methanogens. Finally, the similarity in peak CH4 emissions, after a 1 month lag period of flooding, in both agricultural and natural wet lands of southeast Texas, may for the first time demonstrate a similar chronological sequence of microbial reactions for CH4 production in d ivergent systems.