We have constructed a basic closed stock and flow model of lake phosphorus cycling that includes three stock concentrations: inorganic phosphorus, phosphorus in living organic material, and phosphorus in dead organic material. Stocks are connected by flows that are made stock dependent in a plausible manner. The model exhibits two equilibrium points, one stable and the other unstable, with an interesting crossover between them. Below a certain total phosphorus threshold, stable equilibrium is characterized by all of the phosphorus stock residing in the inorganic form, suggesting the collapse of the biological system. Above the threshold, this is not true. We then used STELLA® II, a computer program for dynamic modeling, to study the dynamics of the basic model and some extensions and modifications to it, including the addition of another stock and external loading of the system. When the system is perturbed from equilibrium, the ·model shows eutrophication blooms and other interesting behavior
