Model ecosystems coefficients

Leidy, G.R. Jenkins, R.M. "The development of fishery compartments and population rate coefficients for use in reservoir ecosystem modeling. Appendix J. Digestive efficiencies and food consumption of fish." Final Report, Agreement No. WES-76-2, USDI Fish and Wildlife Service National Reservoir Research Program, Fayetteville, AR,... 

Fig. 9.4. The new mixed model for metals in lakes. Load (or dose) parameters are related to the input of metals to the lake (direct load and load from the catchment), the metal amount in the lake water is distributed into dissolved and particulate phases by the partition coefficient (Kd). Sedimentation is net sedimentation per unit of time (the calculation unit is set to 1 year for Hg and 1 month for Cs). The sensitivity parameters influence biouptake of metals from water to phytoplankton (but they may also be used in other contexts, e.g., to influence the Kd-values, as illustrated by the dotted line, or the rate of sedimentation). The biological or ecosystem variables include pelagic and benthic uptake, bioaccumulation and retention time in the five compartments (lake water, active sediments, phytoplankton, prey and predator fish). The ejfect parameter is the concentration of the metal in predatory fish (used for human consumption). One panel gives the calculation of concentrations, another the driving parameters (model variables should, preferably, not be altered for different lakes, while environmental variables must be altered for each lake). The arrows between these two panels illustrate the phytoplankton biomass submodel...

There are other ways of defining the partition coefficient, e.g., not as amount in solution divided by total amount in suspension, but as concentration in water relative to concentration on suspended matter. Here, Kd is set to 0.1 as a default value, which means that 10% of X is in solution and 90% is particulate. In practice, it is evident that the Kd-value is not a constant, but a variable, which depends on, e.g., (1) the given substance X, (2) lake water pH (and all cluster parameters linked to pH, such as hardness, conductivity, alkalinity, etc.), which influences the equilibrium between X and the particulate phase and the aggregation processes of the carrier particles, (3) the presence of colored substances (humus), which often have a strong affinity to many types of suspended substances (like metals, organic toxins, radioisotopes, etc.), and (4) the character of the particulate phase (if this is clays, humic matter, Fe-oxides/hydroxides, etc.). So, in more extensive ecosystem models, one would need comprehensive submodels to predict the partition coefficient. 

---