Phytoplankton productivity is regulated by the ocean nitrogen inventory. Over long timescales fixed nitrogen is removed by denitrification, but this loss is balanced to some extent by biological fixation of dissolved nitrogen gas. Although the global distribution and productivity of nitrogen fixing organisms has been shown to be influenced by the availability of nitrogen, phosphorus and iron, the complex behaviour of the system as a whole is not yet fully understood. Here we present a simplified ecological theory that predicts the global distribution of nitrogen fixers, and explains observed global variability in surface nitrate, phosphate and iron. The model unites prior observations and previously conflicting theories. Using the theory to divide the ocean into four observable provinces, we make testable predictions regarding how ocean ecology and biogeochemistry are controlled by atmospheric iron deposition and oceanic nutrient supply. Our model provides a consistent quantitative framework through which we may better understand the ecological niche of nitrogen fixers, and their potential role in regulating Earth's climate.