Benthic fluxes of methane along the salinity gradient of a tropical monsoonal estuary: Implications for CH4 supersaturation and emission

Observations in the Mandovi estuary, a typical tropical monsoonal estuary located on the west coast of India, revealed that waters were perennially supersaturated with CH4, indicating that the estuary could be a significant emitter of this greenhouse gas. In order to test the hypothesis that the estuarine sediments serve as a major source of CH4 to the overlying water column, a series of intact core incubations were carried out at three sites along the salinity (S) gradient of the estuary during the summer (May-June) of 2014. The three sites - off Betim, Amona and Ganjem - represented different salinity regimes (33, 18 and 1, respectively). The benthic CH4 fluxes exhibited a progressive increasing trend from 4.71 μmol m−2 d−1 at the marine end (S > 20) to 16.01 μmol m−2 d−1 in the brackish water zone (20 ≥ S ≥ 5) and 93.90 μmol m−2 d−1 at the freshwater end (S < 5). The total benthic flux of CH4 from the Mandovi estuary was computed to be 9.79 × 106 g y−1. The observed upstream increasing trend may be controlled by a combination of salinity, sedimentary Corg and sediment cohesiveness. High rates of CH4 oxidation (10-1364 μmol m−2 d−1) were observed at all the sites with an increasing trend upstream indicating salinity control on methanotrophy. Despite high methanotrophic activity at the sediment-water interface, the net benthic CH4 release to the water column still remained significant, accounting for high CH4 supersaturation of the estuarine water and making the estuary a potential CH4 source to the atmosphere. Our results imply that during the monsoon (wet) season, the total area integrated benthic CH4 flux may be even higher as the entire estuary becomes freshwater dominated.