J.1 General

Several functions are available within the WQ Module that allow for modification of computed phytoplankton rates in response to environmental conditions. These functions were mentioned in passing in previous Appendices and are called limitation functions because they describe how temperature, nitrogen, phosphorus, silicate, light and salinity potentially limit phytoplankton rates. Their computed values are always dimensionless and between zero and one, and are applied multiplicatively to modify initially computed rates. Limitation functions of zero completely shut off initially computed rates, and conversely, limitation functions of one do not affect these rates whatsoever. Limitation functions that are valued between zero and one linearly affect computed rates as per Equation (I.2).

The key principle of limitation functions is that they provide a quantitative comparison of phytoplanktonic rate requirements with ambient conditions, and then modify phytoplankton behaviour accordingly. For example, an initially computed \(R_{prod\langle computed\rangle}^{phy}\) may make a productivity demand on ambient nitrogen that exceeds instantaneous nitrogen availability. In this case, primary production will need to be reduced to reflect this nitrogen scarcity. This is affected via the multiplicative application to the initial primary productivity rate of a (computed) nitrogen limitation \(L_{nit}^{phy}\) that is less than one. A similar principle applies to temperature, phosphorus, silicate, light and salinity limitation functions: they modify computed phytoplanktonic rates based on a relative assessment of these rates to instantaneous ambient environmental conditions.

There are multiple limitation functions available within the WQ Module for each of temperature, nitrogen, phosphorus, silicate, light and salinity. These are described below.