The phosphorus and nitrogen loads on Lake Onego, including individual components of the nutrients from different sources, were estimated from the data on the chemical composition and the water volume of the sources that form the hydrochemical regime of Lake Onego, and from the data on the volumes of commercial trout farming in the lake catchment. Natural and anthropogenic load on the lake was determined, and the latter was compared to the lake’s assimilative capacity. Nutrient load estimates based on observed data are in good agreement with the results of mathematical modeling carried out at the Institute of Limnology RAS by means of the ILLM model.