METHANE FLUX FROM MANGROVE SEDIMENTS ALONG THE SOUTHWESTERN COAST OF PUERTO-RICO

Although the sediments of coastal marine mangrove forests have been co nsidered a minor source of atmospheric methane, these estimates have b een based on sparse data from similar areas. We have gathered evidence that shows that external nutrient and freshwater loading in mangrove sediments may have a significant effect on methane flux. Experiments w ere performed to examine methane fluxes from anaerobic sediments in a mangrove forest subjected to secondary sewage effluents on the southwe stern coast of Puerto Rico. Emission rates were measured in situ using a static chamber technique, and subsequent laboratory analysis of sam ples was by gas chromatography using a flame ionization detector. Resu lts indicate that methane flux rates were lowest at the landward fring e nearest to the effluent discharge, higher in the seaward fringe occu pied by red mangroves, and highest in the transition zone between blac k and red mangrove communities, with average values of 4 mg CH4 m-2 d- 1, 42 mg CH4 m-2 d-1, and 82 mg CH4 m-2 d-1, respectively. Overall mea n values show these sediments may emit as much as 40 times more methan e than unimpacted pristine areas. Pneumatophores of Aviciennia germina ns have been found to serve as conduits to the atmosphere for this gas . Fluctuating water level overlying the mangrove sediment is an import ant environmental factor controlling seasonal and interannual CH4 flux variations. Environmental controls such as freshwater inputs and incr eased nutrient loading influence in situ methane emissions from these environments.