Surplus supply of bioavailable nitrogen through N2 fixation to primary producers in the eastern Arabian Sea during autumn

Abstract

Diazotrophs have received recent appreciation as a major source of bioavailable nitrogen in the global oceans. They mostly flourish in warm, stratified, calm and nutrient depleted conditions in the ocean. Such conditions prevail during spring and autumn seasons in the Arabian Sea. Some previous experimental studies conducted during spring have suggested the highest rates of N2 fixation among other oceans in the eastern Arabian Sea, but there are no such records during autumn. In addition, modelling studies have suggested high rates of annual N2 fixation in the Arabian Sea. In this study, we conducted isotope labeling incubation experiments in the eastern Arabian Sea in autumn 2010 to estimate N2 fixation rates and primary production. Unlike previous studies conducted in this region, we did not witness any diazotrophic bloom, but our N2 fixation rates (1300–2500 μmol N m−2 d−1) were still comparable to the rates reported in previous studies conducted in spring, and among the highest rates observed in the global oceans. Our data suggest an important role of excess phosphate to sustain N2 fixation during autumn. Most intriguingly our study shows that N2 fixation supplies a surplus amount of bioavailable nitrogen required for primary producers during autumn in this region.