In this article the mathematical model of transformation of biogenous substances forms, such as phosphorus, nitrogen and silicon, in water ecosystems by the example of the Azov Sea is constructed. The model considers absorption and excretion of nutrients by phytoplankton, and also transition of substances from one form to another. The equations system that describing model was studied for what linearization of system was made, the square functional was constructed. As a result of research sufficient conditions of uniqueness of the problem solution were received, the theorem was formulated.
Keywords: phytoplankton, phosphorus, nitrogen, silicon, biogene, chemical and biological source, equation of convection-diffusion-reaction, linearization, uniqueness of the decision of equations system