Dynamics of dissolved organic and mineral fractions of nutrients(C, SI, N and P) is simulated for ten regions of the Caspian Sea with the help of an ecological model. The modeling results are analyzed for the most productive northern part of the sea. Assessments of biogenic substance inputs to the sea with the river run off (Volga, Ural, Terek) as well as the internal fluxes of the compound biotransformations in the marine ecosystem are presented and discussed. The values of primary production, calculated from the modeling results, are in good agreement with experimental data available from the literature.