Marine nitrogen: Phosphorus stoichiometry and the global N:P cycle

Nitrogen supply is often assumed to limit marine primary production. A global analysis of total nitrogen (N) to phosphorus (P) molar ratios shows that total N:P is low (<16:1) in some estuarine and coastal ecosystems, but up to 100:1 in open oceans. This implies that elements other than N may limit marine production, except in human impacted, estuarine or coastal ecosystems. This pattern may reconcile conflicting enrichment studies, because N addition frequently increases phytoplankton growth where total N:P is expected to be low, but P, Fe, or Si augment phytoplankton growth in waters where total N:P is high. Comparison of total N:P stoichiometry between marine and freshwaters yields a model of the form of the aquatic N:P cycle.