Nitrous oxide and hydroxylamine measurements in the Southwest Indian Ocean

Abstract The southwestern basin of the Indian Ocean (SWIO) remains a rather under-sampled region with regard to nitrogen-cycle processes. Here we present the results of extensive nitrous oxide (N 2 O) measurements as well as the first reported open ocean measurements of hydroxylamine (NH 2 OH). Enhanced N 2 O sea-to-air fluxes were found in the zonal band between 5°S and 10°S as a result of wind-driven upwelling, and N 2 O depth profiles showed supersaturation throughout the water column with a distinct maximum at about 1000 m. Excess N 2 O (ΔN 2 O) was found to be positively correlated with apparent oxygen utilization (AOU) and nitrate. Although the water column distribution of NH 2 OH was highly variable, combined analysis with N 2 O and nutrient data allows us to argue for nitrification as the major formation pathway of N 2 O in the SWIO.