Dynamics and exchange fluxes of methane in the estuarine mangrove environment of the Sundarbans, NE coast of India

The distribution and exchange fluxes of methane (CH4) were measured in a mangrove vegetated island and its bordering estuarine system of the Sundarbans mangrove biosphere from June 2010 to December 2011 on monthly basis. The onset of methane production is evident in the forest sediment at about 25 cm deep sediment layer under strong redox condition having an average Eh value of −175.7 mV and showing a 2.8 folds increase in the pore water dissolved methane concentration at that depth in comparison to the surface layer. The average diffusive flux of methane from this methane producing layer to surface was calculated to be 591 ± 106 nmol m−2 d−1. The depth profiles of NO2−–N, SO4−2–S, acid volatile sulphide, organic carbon and dissolved methane in the sediment cores from inter-tidal zones showed distinct trends representing signatures of denitrification, sulfate reduction and methanogenesis in the sediment layers. The methane emission from the sediment–atmosphere interface was observed to be maximum during monsoon and higher emission rates was recorded from upper littoral zone. The annual average atmospheric methane mixing ratio was 2.038 ± 0.07 ppmv. This mangrove biosphere was found to act as source for methane during monsoon while as sink during pre and postmonsoon seasons. Estuarine surface water showed a very high degree of super saturation about 2748 ± 730% for dissolved methane at an annual basis and act as a significant source of methane having an annual average exchange flux of 408 ± 110 nmol m−2 h−1. A box model approach has been adopted at annual basis to understand the distribution and dynamics of methane in this mangrove environment.