Using our own data and data from the literature, we assessed the total biomass of the biotic component of the ecosystem of the Ivankovo Reservoir (Upper Volga, Russia), a eutrophic reservoir which is becoming overgrown with macrophytes. The biotic component of freshwater ecosystems is formed by communities of multicellular and unicellular organisms and viruses in the water layer (plankton) and bottom sediments (benthos) and also macrophytes and autotrophic and heterotrophic organisms growing on their surface (epiphyton). The biomass of the biotic component of the Ivankovo Reservoir equaled 39,853 tons С. Plankton, benthos and macrophytes with epiphyton equaled 3.6%, 41.6% and 54.8% of the total biomass respectively. We determined the contribution of higher aquatic plants, algae, cyanobacteria, heterotrophic bacteria, viruses, protozoans, multicellular invertebrates and fish to the formation of total biomass. The largest share was taken up by higher aquatic plants (54.5%). The second largest share was taken by heterotrophic bacteria (37.4%), most of which live in the bottom sediments. The high concentration of bacteria and invertebrates in the bottom sediments indicate significant provision of the organic substrates from the water column. The biomass of fish, the highest trophic link in the reservoir, equaled 15.0% of the biomass of their potential food substrates, invertebrate animals, and 0.7% of the total biomass of the biotic component. The greater part of the autochthonous organic compound in the reservoir is formed as a result of activity of phytoplankton, which provides 69.4% of total primary production of macrophytes, phytoepiphyton, phytoplankton and phytobenthos. The total primary production during the vegetation period was approximately forty times higher than the annual production of the fish. Currently, the share in the phytoplankton of large colonial cyanobacteria not consumed by zooplankton, the share of non-heterocystic species of cyanobacteria capable of heterotrophic feeding and the share of mixotrophic flagellates is increasing. Eutrophication of the reservoir is significantly stimulated by the development of macrophytes, and, presumably, the contribution of macrophytes to the total primary production of the reservoir will continue to increase.