Influence of river discharge on the distribution and flux of methane in the coastal Bay of Bengal

Rivers are known to be strong sources of methane (CH4) to the atmosphere and their discharges may also enhance its flux from the coastal ocean. In order to examine this, CH4 fluxes to the atmosphere were examined along the coastal Bay of Bengal during peak discharge period. The saturation of CH4 in the Ganges river was relatively lower (208 ± 7%) than that of Indian peninsular rivers (907 ± 136%). The Ganges discharges into the northwestern (NW) while the peninsular rivers discharge into the central and southwestern (SW) regions of the coastal Bay of Bengal and influence the concentration and saturations of CH4 in the western coastal Bay of Bengal. Methane was higher (4.4 ± 0.9 nM) and more supersaturated (208 ± 37%) in the SW coastal Bay of Bengal than in the NW region (2.5 ± 0.1 nM and 116 ± 2.6% respectively). The higher concentrations of CH4 in the SW region were also influenced by mild coastal upwelling. The efflux of CH4 is estimated as 57.4 ± 19.3 and 14.5 ± 2.3 μmol m− 2 d− 1 from SW and NW regions, respectively, and an average of 38.9 ± 11.3 μmol m− 2 d− 1. Our study suggests that the western coastal waters of the Bay of Bengal act as strong source of CH4 to atmosphere. The total flux from Indian coastal waters (estuaries, mangroves and coastal waters) is estimated as 21.9 × 109 g CH4 y− 1 out of which ~ 70% is from the coastal Bay of Bengal and 20% from estuaries.