Toward resolving disparate accounts of the extent and magnitude of nitrogen fixation in the Eastern Tropical South Pacific oxygen deficient zone
Corday R. Selden, Margaret R. Mulholland, Brittany Widner, Peter Bernhardt, Amal Jayakumar
Abstract
Abstract 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.