Inferred N2 fixation rates (nmoles N L−1 d−1) resulting from 15N-labeled contaminants.

N2 fixation rates that would be inferred from 24-h field N2 fixation assays conducted with 15N2 stocks containing the respective concentrations 15N-ammonium contaminants detected in Sigma-Aldrich and Cambridge Isotopes 15N2 gas. In the simulations, microbial plankton assimilate 15N-ammonium rather than fix 15N2. Incubations are simulated in volumes of 0.25 L or 4.5 L equilibrated with 0.1 mL of 15N2 gas, with 2.0×10−7 µmol L−1 of plankton nitrogen (with a δ15N = 0‰) assimilating at a range of specific growth rates, µ (d−1), countered by equivalent recycling rates, at incremental concentrations of ambient ammonium (δ15NNH4 = −2‰). Inferred rates of −1 d−1 are considered undetectable (n.d.).Inferred N2 fixation rates (nmoles N L−1 d−1) resulting from 15N-labeled contaminants.