Based on the results of the calculation of 55‑year-long changes in mineral phosphorus in the Mozhaisk storage reservoir, the structure of its balance over the annual cycle and during the spring refill are considered. The flowchart of the processes modeled in the simulation model and the general equation for calculating mineral phosphorus concentrations are presented. The components of the phosphate balance studied here are phosphorus influx from bottom sediments during anoxic conditions near the bottom, phosphorus influx resulting from the metabolism of phytoplankton, zooplankton and fish, phosphorus influx as a result of dissolved organic matter mineralization, phosphorus discharge to the reservoir’s tailwaters, phosphorus intake in bottom sediments as a result of sorption and co-deposition, phosphorus consumption during phytoplankton photosynthesis. The ratio of external and internal mineral phosphorus flows was determined, and the dominant influence of phosphate inflow with river runoff on the amount of its accumulation in the reservoir was revealed. Variations of the above components of the phosphate balance across phases of the reservoir’s water regime were analyzed. The patterns in the fluctuations of the phosphate stock and the main components of phosphorus balance during the refill period are highlighted. The maximum phosphate storage ranged from 8.3 to 54.9 tons, with an average at 22.9 tons. The trend line for the maximum content of phosphates in spring since the initial filling of the reservoir until now has the form of a parabola with the maximum in the late 1990s. It is shown that the change of the mineral phosphorus stock in the reservoir is controlled by serial changes in inflow, sedimentation and consumption by phytoplankton.