The results of simulation of changes in the phytoplankton biomass in Mozhaiskoye reservoir during the 55-year period of its operation as a source of water supply forMoscowwere analyzed. The principal design equations for the phytoplankton unit of the reservoir’s hydrological model and their updated parameters are presented. The results of model calculations of multiannual fluctuations of the phytoplankton growth rate during the growing season were used for a multidimensional correlation analysis of the factors for the reservoir’s productivity. We consider a set of abiotic factors grouped according to the type of impact and manifestations in the ecosystem of the reservoir into hydrological, meteorological, hydrochemical and factors related to flow regulation by hydropower engineering. A steady long-term upward trend was revealed in the phytoplankton growing-season biomass in Mozhaiskoye reservoir. Statistical calculations of linear relations detected the most important factors for among-year variations in production processes and phytoplankton growth in the reservoir. It was shown that the level of phytoplankton growth in the reservoir in summer is determined by peak flow, which generates the external nutrient loading on the reservoir, while meteorological factors show little correlation with phytoplankton biomass in the growing season. Diatoms are the least predictable group of phytoplankton.