A simple model has the potential to resolve the long-running debate amongst
oceanographers over whether nitrogen or phosphorus exerts overall control
on oceanic primary production. A representation of the competition between
nitrogen-fixing and other phytoplankton is inserted into a two-box global m
odel of the oceanic nitrogen and phosphorus cycles. Homeostatic regulation
of both nitrate and phosphate concentrations results, with surface waters m
ore deficient in nitrate than phosphate in the steady state, but with exter
nal phosphate inputs controlling longer-term primary production in the glob
al ocean.