There are (currently) three models that represent different parts of the environment; atmosphere, river and soil. These models only model the processes occurring in the media and not biota or fauna living therein.
The concentrations calculated for media can be used by other models (eg. the food models). In the case where the user do not wish to dynamically model these concentrations, but use measurements instead, there are "measurement" equivalents of each model into which the user can enter concentrations that can be used elsewhere.