Low rates of dinitrogen fixation in the eastern tropical South Pacific Limnology and Oceanography

Abstract Recent work has suggested that the oxygen deficient zone (ODZ) and overlying surface waters of the eastern tropical South Pacific (ETSP) is a potential niche for dinitrogen (N2) fixation. Rates of dinitrogen fixation were measured in the ETSP above and within the ODZ in July 2013 using a modified 15N2 bubble addition method, wherein a bubble was added, mixed, and then removed, and the isotopic enrichment of the dissolved N2 was measured directly for each incubation. N2 fixation rates in the euphotic zone ranged from below detection to 3.9 nmol L−1 d−1 and were below detection at all depths surveyed within the ODZ. Depth-integrated rates ranged from below detection to 289.7 μmol m−2 d−1. DNA and RNA of diverse nifH genes were detected at both surface waters and in the ODZ. However, the results of this study suggest that N2 fixation rates were low and contribute little to N cycling in the ETSP.