Table 2. Inferred N₂ fixation rates (nmoles N L⁻¹ d⁻¹) resulting from ¹⁵N-labeled contaminants.

Ambient [NH4+]	Cambridge Isotopes			Sigma-Aldrich			Sigma-Aldrich
(µmol L−1)	Cambridge A			Sigma A1			Sigma B
	Lot # I1–11785A			Lot # SZ1670V			Lot # MBBB0968V
	0.52 µmol 15NH4 +/mol 15N2			25 µmol 15NH4 +/mol 15N2			1900 µmol 15NH4 +/mol 15N2
	µ = 0.1	µ = 0.2	µ = 0.3	µ = 0.1	µ = 0.2	µ = 0.3	µ = 0.1	µ = 0.2	µ = 0.3	Incubation Volume (L)
0.001	0.019	0.019	0.019	9.0	9.0	9.0	310	460	530	0.25
0.01	0.014	0.016	0.016	7.5	8.5	8.6	250	400	470
0.1	n.d.	n.d.	n.d	1.6	2.7	3.6	90	170	220
1	n.d.	n.d.	n.d.	0.17	0.32	0.45	13	24	34
0.001	n.d.	n.d.	n.d.	0.50	0.50	0.50	38	38	38	4.5
0.01	n.d.	n.d.	n.d.	0.42	0.47	0.48	30	35	36
0.1	n.d.	n.d.	n.d.	0.08	0.15	0.19	6.5	11	15
1	n.d.	n.d.	n.d.	n.d.	0.012	0.017	0.7	1.4	1.9

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 (δ¹⁵N_NH4 = −2‰). Inferred rates of <0.01 nmoles N L⁻¹ d⁻¹ are considered undetectable (n.d.).