Biogeochemistry of methane in a tropical monsoonal estuarine system along the west coast of India

Estuaries are known to be important sources of methane (CH4) to the atmosphere. However, a lack of adequate field studies in understanding the sources/sinks of CH4 in estuaries hampers the global atmospheric budgeting. Therefore, more studies are needed to bridge the gap and improve our understanding of fluxes of CH4 to the atmosphere. We report here results of a year-long study of the Mandovi-Zuari estuarine system of Goa, on the west coast of India. Methane was measured along with the other ancillary parameters such as temperature, salinity and oxygen in the water column of both the estuaries along the salinity gradient. The estuarine waters were supersaturated with CH4, which exhibited significant spatial (along the salinity gradient) and temporal variability. The concentrations varied from 6 to 901 nM (saturation: 300-46,000%) in the Mandovi estuary and from 8 to 1022 nM (saturation: 400-41,000%) in the Zuari estuary from the estuary mouth to the fresh water end. The water-air CH4 fluxes increased towards the freshwater end in both the estuaries. The annual freshwater CH4 input was higher in the Zuari (209 ± 182 × 106 μmol d−1) than in the Mandovi (153 ± 144 × 106 μmol d−1) estuary. The high build-up of CH4 could be due to a combination of supply from mangrove swamps, sedimentary inputs and river runoff.