Toward resolving disparate accounts of the extent and magnitude of nitrogen fixation in the Eastern Tropical South Pacific oxygen deficient zone

Examination of dinitrogen (N 2 ) fixation in the Eastern Tropical South Pacific oxygen deficient zone has raised questions about the range of diazotrophs in the deep sea and their quantitative importance as a source of new nitrogen globally. However, technical considerations in the deployment of stable isotopes in quantifying N 2 fixation rates have complicated interpretation of this research. Here, we report the findings of a comprehensive survey of N 2 fixation within, above and below the Eastern Tropical South Pacific oxygen deficient zone. N 2 fixation rates were measured using a robust 15 N tracer method (bubble removal) that accounts for the slow dissolution of N 2 gas and calculated using a conservative approach. N 2 fixation was only detected in a subset of samples (8 of 125 replicated measurements) collected within suboxic waters (< 20  μ mol O 2 kg −1 ) or at the oxycline. Most of these detectable rates were measured at nearshore stations, or where surface productivity was high. These findings support the hypothesis that low oxygen/high organic carbon conditions favor non‐cyanobacterial diazotrophs. Nevertheless, this study indicates that N 2 fixation is neither widespread nor quantitatively important throughout this region.