Biogeochemical dynamics of exogenous methane in an estuary associated to a mangrove biosphere; The Sundarbans, NE coast of India.

Manab Kumar Dutta, Rishmita Mukherjee, Tapan Kumar Jana, Sandip Kumar Mukhopadhyay

 Marine Chemistry 170, 1-10 (2015)

The distribution, sources and fate of dissolved CH4 were monitored in a mangrove associated estuary of the Sundarbans from January 2011 to December 2012. This tropical estuary was polyhaline (salinity range between 19.06 ± 4.33 and 27.09 ± 0.59), well mixed and net heterotrophic. Average dissolved CH4 concentrations in es- tuarine surface and bottom waters were 69.90 and 56.17 nM, respectively, showing a very high degree of super- saturation ( ≈ 2869 ± 571%). Dissolved CH4 was significantly correlated with salinity but not with other physico- chemical and biological properties of the estuarine water, indicating its exogenous origin. Average dissolved CH4 concentration in sediment pore water was 53.4 times higherthanestuarine water, suggesting significant CH4in- flux from the sediment to the estuary via advective and diffusive transports.Average advectiveand diffusive CH4 fluxes from intertidal and sub-tidal sediments were 159.52 and 8.45μmol m−2d −1, respectively. The average dissolved CH4 oxidation rate in the estuarine water column was 20.59nmolL −1d−1Being highlysupersaturated with respect to atmospheric equilibriumthe estuary acts as a source of 0.093 GgCH4 annually to the atmosphere with an average flux of 8.88 μmol m−2d−1.In the estuarine CH4 cyclethe advective flux and CH4 oxidation were recorded as the predominant input and output processes,respectively.Finally,a box model representing the CH4 biogeochemical cycle in this estuarine mangrove habitat derived a CH4 export flux from the estuary to the adjacent continental shelf of 0.485 Gg yr− 1.