Nitrogen-phosphorus model pros
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.
As with any model, simplification has both advantages and disadvantages. This article describes some of the nitrogen-phosphorus model pros.
Types of Models
In computer modelling there is a continuum between two caricature classes of models:
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
2. simulation-type models that are highly detailed and try whenever 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
By simplifying to only the most important fluxes and state variables, this model is very much a conceptual-type model.
One of the key advantages of such an approach is fast run times. The most detailed (fine spatial resolution) simulation models can, at the extreme, take many months of real time to simulate only a few decades. In contrast, this conceptual model can simulate hundreds of thousands of years in only a few seconds.
Leading on from this, a second advantage is the ease of experimentation and sensitivity with the quicker models such as this one. If each model run takes many months then the ability to carry out many different runs in series is limited. This is particularly important where uncertainty in the model formulation favours large numbers of sensitivity runs.
Another advantage of simpler models is the ability to understand why the model produces a particular behaviour. This is much harder, and sometimes impossible, with very complex models. It is not always possible to disentangle the large network of interacting effects in a complex model in order to work out which one is key, whereas it is easier in a simple model.
This model of the nitrogen and phosphorus cycles includes the quantitatively largest fluxes in the phosphorus cycle in the same way as the P-only model:
1. Uptake of P by phytoplankton
2. Release of P from phytoplankton organic matter upon decay
3. Downwards transport of P within sinking organic particles
4. Net upwards transport of P due to physical exchange of (nutrient-scarce) surface and (nutrient-rich) deep waters
5. Input of P to the ocean as a whole via river input
6. Loss of P from the ocean as a whole when organic particles sink to the seafloor, eventually get covered by further layers of sinking particles, and finally get compressed into marine sedimentary rocks
It also includes all of the largest fluxes in the nitrogen cycle:
1. Uptake of fixed nitrogen (nitrate, nitrite, ammonium) by phytoplankton
2. Uptake of dinitrogen (N2) by nitrogen-fixing phytoplankton
3. Release of N (initially in the form of ammonium) from phytoplankton organic matter upon decay
4. Bacterial conversion of ammonium to nitrite and then nitrate (nitrification)
5. Downwards transport of N as part of sinking organic particles
6. Net upwards transport of fixed nitrogen due to physical exchange of (nutrient-scarce) surface and (nutrient-rich) deep waters
7. Input of fixed nitrogen to the ocean as a whole via river input
8. Input of fixed nitrogen to the ocean as a whole via atmospheric deposition
9. Loss of fixed nitrogen from the ocean as a whole when organic particles sink to the seafloor, eventually get covered by further layers of sinking particles, and finally get compressed into marine sedimentary rocks
10. Loss of fixed nitrogen from the ocean as a whole through the processes of denitrification and anammox.
The model tracks the population dynamics of two separate groups of phytoplankton: the N2-fixers and the non-fixers (those which are obliged to use fixed nitrogen as their nitrogen source). The model includes the growth rate dependencies on phosphate and fixed nitrogen availability (the N2-fixers are not affected by availability of fixed nitrogen). By including the competition between these two groups, and its relation to nutrient concentrations, the model is able to include a feedback that links the nitrogen and phosphorus cycles together and provides a potentially stabilising negative feedback loop.
Some models are "closed-system" in the sense of forcing all substances to remain within the model ocean. Sinking material that reaches the seafloor, for instance, will be remineralised at the bottom (or top) of the ocean, and rivers will not be modelled. The boundaries of these models are absolute. This model is more realistic in the sense of being open-boundary (open-system). That is to say, it doesn't just simulate internal fluxes (moving N and P around the ocean) but also the fluxes into and out of the ocean as a whole.
Tyrrell, T. (1999). The relative influences of nitrogen and phosphorus on oceanic primary production. Nature 400, 525–531.