Fig. 4.

Local and basin-scale limitation of N₂ fixation rates under modern-ocean conditions (Q_F/Q_O = 20). (A) Comparison of nutrient limitation factors for model diazotrophs: γ_Fe = Fe/(Fe + K_Fe), ${\mathrm{\gamma }}_P\mathrm{=}\mathrm{P}{\mathrm{O}}_4^{3-}/\left(\mathrm{P}{\mathrm{O}}_4^{3-}\mathrm{+}{K}_P\right)$, where K_Fe and K_P are half-saturation constants for diazotroph growth on Fe and $\mathrm{P}{\mathrm{O}}_4^{3-}$, respectively. Values of γ_Fe/γ_P separate regions where diazotrophs are Fe-limited (γ_Fe/γ_P < 1) and $\mathrm{P}{\mathrm{O}}_4^{3-}$-limited (γ_Fe/γ_P > 1). (B) Change in column-integrated N₂ fixation 30 y after the atmospheric Fe supply is raised by 1 g m⁻² y⁻¹ across the Pacific to eliminate Fe limitation. Regions in which N₂ fixation is raised (positive) are almost exactly offset by those where it is lowered (negative), yielding a net increase less than 10 TgN/y (Fig. S9).