Nitrous oxide flux from a Tropical estuarine system (Cochin estuary, India)

The Cochin estuarine system (CES) has experienced rapid changes over the last few decades from extensive urbanisation, industrialisation, agricultural development activities and aquaculture practices. Intense pollution and other anthropogenic disturbances have led to an accelerated increase in organic matter (OM) and dissolved inorganic nitrogen (DIN) in the estuarine complex. This study explores nitrous oxide (N₂O) fluxes and its drivers in the CES. Relatively higher N₂O concentrations were observed in the inner estuary when compared to the outer estuary. Higher DIN with varying salinity gradients likely controlled the production and flux of N₂O into the inner portions of the system. Hypoxia was never established in the water column indicating that conditions were favourable for nitrification, which in turn possibly led to increased N₂O concentrations. It was observed particularly during the monsoon season that the inorganic nutrients and dissolved greenhouse gases from the river Periyar at the head of the estuary, contributed to the increased N₂O concentrations. The N₂O water to air flux ranged from 0.49 to 29.73μmol m⁻² day⁻¹, which was higher than what was documented in earlier reports. Although the CES exports a substantial amount of nutrients to the adjacent Arabian Sea, the estuary effectively processes nitrogen inputs as N₂O making CES a net source of N₂O emissions.