This paper addresses a longstanding debate regarding the factors that limit nitrogen fixation by diazotrophic plankton—the primary source of an essential nutrient to the ocean. Multiple lines of evidence show that diazotroph growth can be locally limited by the atmospheric iron supply, but large-scale rates of N2 fixation are ultimately controlled by N deficits generated within the ocean. These findings can reconcile the conflicting observations of biologists and geochemists, while implying a new sensitivity of the marine N cycle to anthropogenic climate warming.