Variability in Concentrations and Fluxes of Methane in the Indian Estuaries

Abstract

In order to examine the fluxes of methane (CH4) from the Indian estuaries, measurements were carried out by collecting samples from 26 estuaries along the Indian coast during high discharge (wet) and low water discharge (dry) periods. The CH4 concentrations in the estuaries located along the west coast of India were significantly higher (113 ± 40 nM) compared to the east coast of India (27 ± 6 nM) during wet and dry periods (88 ± 15 and 63 ± 12 nM, respectively). Supersaturation of CH4 was observed in the Indian estuaries during both periods ((0.18 to 22.3 × 103 %). The concentrations of CH4 showed inverse relation with salinity indicating that freshwater is a significant source. Spatial variations in CH4 saturation were associated with the organic matter load suggesting that its decomposition may be another source in the Indian estuaries. Fluxes of CH4 ranged from 0.01 to 298 μmol m−2 day−1 (mean 13.4 ± 5 μmol m−2 day−1) which is ~30 times lower compared to European estuaries (414 μmol m−2 day−1). The annual emission from Indian estuaries, including Pulicat and Adyar, amounted to 0.39 × 1010 g CH4 year−1 with the surface area of 0.027 × 106 km2 which is significantly lower than that in European estuaries (2.7 ± 6.8 × 1010 g CH4 year−1 with the surface area of 0.03 × 106 km2). This study suggests that Indian estuaries are a weak source for atmospheric CH4 than European estuaries and such low fluxes were attributed to low residence time of water and low decomposition of organic matter within the estuary. The CH4 fluxes from the Indian estuaries are higher than those from Indian mangroves (0.01 × 1010 g CH4 year−1) but lower than those from Indian inland waters (210 × 1010 g CH4 year−1).