A schematic of the modelled nitrogen and phosphorus cycles. The unlabelled arrows leading into the phytoplankton (O and NF) indicate the biological uptake of nitrate and phosphate.
The nitrogen-phosphorus model is a first-order model of the major source and sink terms in the ocean's nitrogen and phosphorus cycles, and feedback loops that connect them. The model builds upon the simpler phosphorus model.
This model is very simple. It is stripped down to the simplest form that can still reproduce the nitrogen (N) and phosphorus (P) cycles. That is to say, the simplest form that is still able to capture the main aspects of how atoms of nitrogen and phosphorus enter the ocean, are transported about within it, and then finally leave the ocean. The aim of this model is to understand how the nitrate:phosphate ratio of the ocean is regulated, and also how the amount of phytoplankton in the ocean, and therefore the fertility of the ocean as a whole, are controlled.
The diagram to the right shows how the oceanic cycles of nitrogen and phosphorus are represented in the model. As well as including the same cycling of phosphorus as in the phosphorus-only model, the model also represents how atoms of nitrogen enter and leave different substances (for instance, bodies of living organisms or molecules of dinitrogen and nitrate) during their time in the oceans.
The model is a two-box model of the global ocean, where the upper box represents the surface ocean down to the seasonal limit of the deepest wind-induced mixing and the lower box represents the deep ocean below the influence of wind and waves.
Click here for a more detailed description of the nitrogen-phosphorus model.
Tyrrell, T. (1999). The relative influences of nitrogen and phosphorus on oceanic primary production. Nature 400, 525–531.