Nitrous oxide in the western Bay of Bengal

Abstract

Extensive observations on nitrous oxide (N₂O) in the atmosphere and waters of the western Bay of Bengal during March-April 1991 yield surface saturations and atmospheric fluxes ranging from 89.3 to 213.9% (mean 125.2%), and from -0.10 to 10.67 μmol (N₂O)/m²/d (mean 0.65 μmol/m²/d), respectively. The overall N₂O flux from the Bay of Bengal is estimated to be 0.027-0.077 Tg N/yr which is substantially lower than that computed for the Arabian Sea (0.19-0.28 Tg N/yr). Lower surface saturations and consequently smaller air-sea fluxes in the Bay of Bengal may result from strong stratification caused by the immense river runoff. The computed vertical exchange coefficient (0.16 cm²/s) at the top of the thermocline is about 1/3 of the corresponding value (0.55 cm²/s) in the Arabian Sea. The associated smaller nutrient fluxes to the euphotic zone may be significantly responsible for the lower productivity of the Bay of Bengal. As expected, marked accumulation of N₂O occurs in subsurface layers of the Bay of Bengal with the total inventory of excess nitrous oxide (ΔN₂O) estimated as ~ 5.4 Tg N. Since the Bay of Bengal is not an active water-column denitrification site, there appears to be no loss of N₂O through bacterial reduction. Vertical profiles of N₂O are characterized by a pronounced maximum at ~ 200-300 m which intensifies northward. The N₂O distribution seems to be influenced by the subsurface circulation. The relationship between ΔN₂O and the apparent oxygen utilization (AOU) is linear in three ranges: at AOU < 150 μM, from AOU ≈ 150 μM to the N₂O maximum (depth ≈ 300 m), and at depths > ~ 1000 m. Within the depth range ~300-1000 m, however, ΔN₂O and AOU are not significantly correlated due to large changes in N₂O concentrations associated with small variations in the ambient oxygen levels.