N.5 Activation

Activation is the pelagic biological conversion of refractory dissolved organic carbon, nitrogen and phosphorus to their respective labile dissolved organic equivalents. It is therefore a source of dissolved labile organic carbon, nitrogen and phosphorus, and a sink of the corresponding refractory dissolved organics. Activation does not consume or produce any other computed variables, although its computed rate does depend on dissolved oxygen concentration.

The pelagic activation rate is computed within the WQ Module for refractory dissolved organic carbon, nitrogen and phosphorus via Equation (N.17).

\[\begin{equation} R_{act\langle computed\rangle}^{RDOM} = R_{act}^{RDOM} \times \underbrace{\left[\underbrace{\frac{\left[DO\right]}{K_{miner-O_2}^{DOM} + \left[DO\right]}}_{\text{aerobic}} + \underbrace{f_{an} \times \frac{K_{miner-O_2}^{DOM}}{K_{miner-O_2}^{DOM} + \left[DO\right]}}_{\text{anaerobic}}\right]}_{\text{Influence of oxygen}} \times \underbrace{\vphantom{\left[\underbrace{\frac{\left[DO\right]}{K_{miner-O_2}^{DOM} + \left[DO\right]}}_{\text{aerobic}} + \underbrace{f_{an} \times \frac{K_{miner-O_2}^{DOM}}{K_{miner-O_2}^{DOM} + \left[DO\right]}}_{\text{anaerobic}}\right]} \hphantom{\text{ab}} \left[\theta_{miner}^{DOM}\right]^{\left(T-20\right)}}_{\text{Influence of temperature}} \tag{N.17} \end{equation}\] \(R_{act}^{RDOM}\) is the user specified refractory dissolved organic matter (i.e. carbon, nitrogen and phosphorus) activation rate at 20\(^o\)C without the influence of dissolved oxygen, \(\left[DO\right]\) is the ambient dissolved oxygen concentration, \(K_{miner-O_2}^{DOM}\) is the user specified half saturation concentration of dissolved oxygen for dissolved organic matter mineralisation, \(\theta_{miner}^{DOM}\) is the corresponding temperature coefficient, \(f_{an}\) is a fractional multiplier that weights the Michaelis-Menten contribution of non-O\(_2\) processes within the calculation of total mineralisation and \(T\) is ambient water temperature. The parameters \(K_{miner-O_2}^{DOM}\), \(f_{an}\) and \(\theta_{miner}^{DOM}\) used in Equation (N.17) are the same as those used for the mineralisation calculations described in Section N.4 and specified via the associated labile organic matter mineralisation command. If this is the case and labile organic matter mineralisation is not required as a simulated process, then the command can still be issued but with the first argument, \(R_{miner}^{DOM}\), equal to zero.

N.5.1 Consumption

The activation rate from Equation (N.17) is multiplied by the ambient refractory dissolved carbon, nitrogen and phosphorus concentrations to compute their respective consumptive fluxes (losses) at each model timestep in each model cell via Equation (N.18).

\[\begin{equation} \left.\begin{aligned} \href{AppDiags.html#WQDiagActC}{F_{act\langle computed\rangle}^{RDOC}} =& R_{act\langle computed\rangle}^{RDOM} \times \left[ RDOC \right] \\ \\ \href{AppDiags.html#WQDiagActN}{F_{act\langle computed\rangle}^{RDON}} =& R_{act\langle computed\rangle}^{RDOM} \times \left[ RDON \right] \\ \\ \href{AppDiags.html#WQDiagActP}{F_{act\langle computed\rangle}^{RDOP}} =& R_{act\langle computed\rangle}^{RDOM} \times \left[ RDOP \right] \end{aligned}\right\} \tag{N.18} \end{equation}\]

N.5.2 Production

The consumptive flux from Equation (N.18) results in the labile dissolved organic carbon, nitrogen and phosphorus productive fluxes in Equation (N.19).

\[\begin{equation} \left.\begin{aligned} F_{act\langle computed\rangle}^{DOC} =& F_{act\langle computed\rangle}^{RDOC} \\ \\ F_{act\langle computed\rangle}^{DON} =& F_{act\langle computed\rangle}^{RDON} \\ \\ F_{act\langle computed\rangle}^{DOP} =& F_{act\langle computed\rangle}^{RDOP} \end{aligned}\right\} \tag{N.19} \end{equation}\]