Iron, nitrogen and phosphorus in the ocean

Abstract

It has been proposed that widespread deficits of nitrate in the ocean, like those observed today, are caused by iron limitation of marine nitrogen fixation1. That is, only when iron is sufficiently abundant to satiate nitrogen fixers will the ratio of nitrate to phosphate in the ocean increase to 16, the average for phytoplankton. Tyrrell2 developed a simple two-box model of oceanic nitrogen and phosphorus cycles to describe the regulation of both nitrate and phosphate concentrations in the global ocean. His criterion for nitrate deficit in the ocean, a molar ratio of N:P in surface waters (Rs) of less than 16, is satisfied without recourse to iron limitation, calling into question Falkowski's proposal1 about the biogeochemical significance of iron limitation as it relates to nitrogen fixation and oceanic levels of nitrogen and phosphorus. Here I show that small changes in the assumptions of Tyrrell's model, well within acknowledged uncertainty, can lead to values of Rs greater than 16. Consequently, the consistency of the model with the observed distributions of nutrients in the ocean is uncertain, and the influence of iron may still be considered important.