Nitrogen fixation rates in the eastern Arabian Sea

The Arabian Sea experiences bloom of the diazotroph Trichodesmium during certain times of the year when optimal sea surface temperature and oligotrophic condition favour their growth. We measured nitrogen fixation rates in the euphotic zone during one such event in the Eastern Arabian Sea using 15N2 tracer gas dissolution method. The measured rates varied between 0.8 and 225 μmol N m−3 d−1 and were higher than those reported from most other oceanic regions. The highest rates (1739 μmol N m−2 d−1; 0-10 m) coincided with the growth phase of Trichodesmium and led to low δ15N (<2‰) of particulate organic matter. At stations not experiencing Trichodesmium bloom nitrogen fixation rates were low (0.9-1.5 μmol N m−3 d−1). Due to episodic events of diazotroph bloom, contribution of N2 fixation to the total nitrogen pool may vary in space and time.