Factors controlling diazotroph biogeography

We present a theoretical framework, which considers the competition for iron, phosphorus and nitrogen resources to explain the biogeography of nitrogen fixing autotrophs (diazotrophs). Here we assume that diazotrophs require higher iron quotas, and have slower maximum growth relative to other phytoplankton. The theory then suggests that the rate of supply of iron or phosphorus relative to the supply of fixed nitrogen sets the regions where diazotroph can coexist with other phytoplankton. The framework also indicates that iron, phosphate and fixed nitrogen concentrations can be strongly controlled by the local phytoplankton community. We show that the framework allows us to divide the ocean of an earth system model into five clearly defined biogeographical provinces. We consider how these province boundaries shift in a warming world based on the changes to the relative supplies of fixed nitrogen, iron and phosphate.