M.2 Cell density

The principle of the WQ Module density calculations is that cell density is set to increase during daylight primary production (when carbohydrates are created via photosynthesis) and decrease during darkness respiration (when carbohydrates are consumed). Equations (M.2) and (M.3) are used to describe these processes, respectively.

\[\begin{equation} \rho_{PHY\langle computed \rangle} = \rho_{PHY} + \frac{0.124 \times \left(1.0 - e^{\frac{-PAR}{I_K}}\right) - 0.0230}{60} \tag{M.2} \end{equation}\]

\[\begin{equation} \rho_{PHY\langle computed \rangle} = \rho_{PHY} - \frac{0.0230}{60} \tag{M.3} \end{equation}\]

\(\rho_{PHY\langle computed \rangle}\) and \(\rho_{PHY}\) are the computed and initial (previous timestep) cell densities, respectively and \(PAR\) and \(I_K\) are the ambient cell photosynthetically active radiation and half saturation light intensity, respectively. \(I_K\) is specified via the light limitation function, or if not, then via the specification of the stokes settling model, or as the default. Computed cell densities are checked against specified (or default) global minima and maxima and reset to these limits if exceeded. Cell density is only used within the WQ Module to compute stokes settling, and need not be included as a computed variable if other settling models are deployed.