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

•Distribution of methane in mangrove dominated estuarine water column.•Pore water dissolved methane concentration and its transport.•Dissolved methane consumption and turnover rate•Air–water methane exchange flux from Sundarbans estuaries•Role of Sundarbans estuaries on regional methane budget

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 estuarine 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 physicochemical and biological properties of the estuarine water, indicating its exogenous origin. Average dissolved CH4 concentration in sediment pore water was 53.4 times higher than estuarine water, suggesting significant CH4 influx from the sediment to the estuary via advective and diffusive transports. Average advective and diffusive CH4 fluxes from intertidal and sub-tidal sediments were 159.52 and 8.45 μmol m− 2 d− 1, respectively. The average dissolved CH4 oxidation rate in the estuarine water column was 20.59 nmol L− 1 d− 1. Being highly supersaturated with respect to atmospheric equilibrium the estuary acts as a source of 0.093 Gg CH4 annually to the atmosphere with an average flux of 8.88 μmol m− 2 d− 1. In the estuarine CH4 cycle the 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 Ggyr− 1.