Phosphorus model cons

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Weaknesses of the Phosphorus Model

In computer modeling there is a continuum between (1) conceptual-type models that aim to simplify a system down to include only its most important elements, in particular those that govern its behaviour, and (2) simulation-type models which are highly detailed and try as much as possible to avoid making assumptions about what is important; instead they try to include as much detail as possible in the expectation that nothing important will be omitted. The phosphorus model is very much a conceptual-type model. As such, it is important to realise that it will not be the most appropriate model to use for many purposes.

Potential weaknesses of a conceptual approach include the likelihood of omitting something important. Every assumption runs the risk of being incorrect in an important way and thereby making model results divorced from reality. It only takes one unreasonable assumption or omission, if it relates to a key part of the system, to invalidate all of the model's results.

Some of the major simplifications made in the phosphorus model are as follows:

(a) horizontal variations are completely ignored. The model is unable to simulate differences between different ocean basins, between high and low latitudes, or between deep and shallow water environments.

(b) vertical variations can only be captured in an approximate way due to only two boxes in the vertical.

(c) only the major fluxes are included. Fluxes deemed to be of lesser importance, such as the delivery of phosphorus to the surface ocean within dust, are not included.

(d) the representation of biology is extremely simplified. Limitation of growth rate due to light, temperature and other nutrients (for instance nitrogen, silicate and iron) are not included at all in the model, despite plentiful evidence that they are important at various times and places in the real ocean.

(e) different types of phytoplankton are not distinguished in the model.

(f) there is no day-night cycle and no seasonal cycle in the model.

(g) Only inorganic phosphate is modelled. Dissolved organic phosphate (DOP) is not.

The motivation for all of these omissions is the expectation that their addition would not fundamentally change the behaviour of the model. It is predicted that adding these features would not greatly alter the model’s general long-term response to, for instance, changes in the amount of phosphorus in the ocean, or changes in the input rate of phosphate down rivers.