Tropical coastal mangroves being one of the most productive ecosystems of the world1 higher leaf litter deposition together with faster tidal sedimentation gives litter little chance for aerobic decomposition, resulting organic carbon rich anoxic condition in the subsurface sediment layer. Mangroves sediments rich in clay content further reduces the porosity of the sediment and induce anoxic condition.

Such anoxic condition encourages microbial processes in the subsurface mangrove sediments like denitrification, sulphate reduction, methanogenesis, and other redox reactions, which may contribute significantly to nutrient turnover2 and production of gases like methane, nitrous oxide, and hydrogen sulphide.

Research findings on Methane emission in Sundarban mangroves:

  • Sundarbans is a sink for methane during pre and post monsoon months; but during the monsoon it becomes a source. The annual contribution of this subtropical mangrove as a source of methane was calculated to be10.9 Gg.3
  • The onset of methanogenesis in this mangrove environment primarily occurs at a shallow depth (20–25 cm) of the sediment. 3
  • The produced methane undergoes vertical diffusive transport resulting methane emission (7.056 mg m−2d−1) from sediment surface to the atmosphere. 3
  • Horizontal transport as pore water dissolved methane enriching estuarine water column dissolved methane, which partially escapes to the atmosphere through air–water exchange (0.157 mg m−2d−1) and the remaining conveyed to the coastal water. 3

References:

  1. E. Odum & Heald, E.J. The detritus-based food web of an estuarine mangrove community. L.E. Cronin (Ed.), Estuarine Research, Academic Press, New York, pp. 265–286 (1975).
  2. M. Alongi & Christoffersen, P. Benthic infauna and organism–sediment relations in a shallow, tropical coastal area: influence of outwelled mangrove detritus and physical disturbance. Marine Ecol. Prog. Ser., 81, pp. 229–245 (1992).
  3. Dutta, M. K., Chowdhury, C., Jana, T. K. & Mukhopadhyay, S. K. Dynamics and exchange fluxes of methane in the estuarine mangrove environment of the Sundarbans, NE coast of India.  Environ. 77, 631–639 (2013).
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