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 N-15 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 mu 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.