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).