N.3 Breakdown

Breakdown is the pelagic biological conversion of refractory particulate organic matter (which is a single computed variable) to three labile particulate organic matter computed variables. It is therefore a source of labile particulate organic carbon, nitrogen and phosphorus, and a sink of refractory particulate organic matter. Breakdown neither consumes or produces dissolved oxygen or bioavailable inorganic nutrients. Breakdown is implemented within the refractory organics constituent model of the WQ Module only, so the following does not apply if the labile organics constituent model is used.

The pelagic breakdown rate of refractory particulate organic matter is computed via Equation (N.5).

\[\begin{equation} R_{bdn\langle computed\rangle}^{RPOM} = R_{bdn}^{RPOM} \times \underbrace{\frac{\left[DO\right]}{K_{hyd-O_2}^{POM} + \left[DO\right]}}_{\text{Influence of oxygen}} \times \underbrace{\vphantom{\frac{\left[DO\right]}{K_{sed-O_2}^{NO_3} + \left[DO\right]}} \hphantom{\text{ab}} \left[\theta_{hyd}^{POM}\right]^{\left(T-20\right)}}_{\text{Influence of temperature}} \tag{N.5} \end{equation}\] \(R_{bdn}^{RPOM}\)is the user specified (or default) refractory particulate organic matter breakdown rate at 20\(^o\)C without the influence of dissolved oxygen, \(\left[DO\right]\) is the ambient dissolved oxygen concentration, \(K_{hyd-O_2}^{POM}\) is the user specified (or default) half saturation concentration of dissolved oxygen for labile particulate organic matter hydrolysis, \(\theta_{hyd}^{POM}\) is the corresponding temperature coefficient, and \(T\) is ambient water temperature. The parameters \(K_{hyd-O_2}^{POM}\) and \(\theta_{hyd}^{POM}\) used in Equation (N.5) are the same as those used for the hydrolysis calculations described in Section N.2 and specified via the associated labile organic matter hydrolysis command.

N.3.1 Consumption

The breakdown rate from Equation (N.5) is multiplied by the ambient refractory particulate organic matter concentration to compute its consumptive flux (loss) at each model timestep in each model cell via Equation (N.6).

\[\begin{equation} \href{AppDiags.html#WQDiagBdn}{F_{bdn\langle computed\rangle}^{RPOM}} = R_{bdn\langle computed\rangle}^{RPOM} \times \left[ RPOM \right] \tag{N.6} \end{equation}\]

N.3.2 Production

The consumptive flux from Equation (N.6) results in the labile particulate organic carbon, nitrogen and phosphorus productive fluxes in Equation (N.7).

\[\begin{equation} \left.\begin{aligned} F_{bdn\langle computed\rangle}^{POC} =& F_{bdn\langle computed\rangle}^{RPOM} \\ \\ F_{bdn\langle computed\rangle}^{PON} =& F_{bdn\langle computed\rangle}^{RPOM} \times X_N^{RPOM} \\ \\ F_{bdn\langle computed\rangle}^{POP} =& F_{bdn\langle computed\rangle}^{RPOM} \times X_P^{RPOM} \end{aligned}\right\} \tag{N.7} \end{equation}\] \(X_N^{RPOM}\) and \(X_P^{RPOM}\) are the user specified (or default) molar ratios of nitrogen and phosphorus to carbon within refractory particulate organic matter. These should not differ significantly from the Redfield ratios of 16/106 and 1/106, respectively.