Description of the Phosphorus Model.
This model is very simple. It is stripped down to the simplest form that can still reproduce the essential aspects of how atoms of phosphorus enter the ocean, are transported about within it, and then finally leave the ocean. It also represents how atoms of phosphorus enter and leave different substances (for instance, bodies of living organisms or molecules of phosphate) during their time in the oceans. The main aim of the model is to examine how the amount of phosphate in the oceans is controlled over long (geological) timescales.
The diagram to the right shows how the oceanic cycle of phosphorus is represented in the model. The model is a two-box model of the global ocean. Or, in other words, the model consists of two boxes, one on top of the other. The top box represents the surface ocean down to the limit of the deepest wind-induced mixing during the year and the bottom box represents the deep ocean below the influence of wind and waves.
The largest fluxes (transports) of phosphorus are included in the model. Phosphorus is almost totally absent from the atmosphere. The only significant input of phosphorus to the oceans comes as part of the flow of water down rivers and into the ocean. Various chemicals are dissolved in river water (it isn't completely pure) and one of those chemicals is phosphorus. The most significant loss of phosphorus from the oceans is in organic debris (remains of living organisms) sinking to the ocean floor and becoming incorporated into sedimentary rocks. As well as these inputs to and outputs from the ocean as a whole, the model also includes the main processes that shift phosphorus between surface and deep waters within the oceans. These are:
(1) uptake of dissolved phosphate to form new phytoplankton biomass;
(2) subsequent release (after cells get attacked by bacteria or predators, or after cells break apart for other reasons) of most of this organic phosphorus back into dissolved nutrients in the surface ocean;
(3) release of a further portion of the organic phosphorus back into deeper waters after small particles of organic material sink out of the surface oceans and are then are decomposed by deeper-dwelling bacteria; and
(4) slow mixing between the surface and deep oceans, partly due to the physical movement of large masses of water between the surface and deep layers (upwelling and downwelling), and partly due to diffusion.
Click here for a more detailed description of the phosphorus model.