Abstract <p>Hydrochemical studies carried out in Lake Kroshnozero in 1953–1954, 1986–1987, and 2021–2023 under different intensities of human economic activity in the catchment area and directly on the reservoir revealed changes in the chemical composition of the water and identified the main limnological processes by which one can judge the restructuring of the lake ecosystem over the past 70 years. In the early 1950s, Lake Kroshnozero was already eutrophic. The average annual concentration of P_tot in the lake water was 32 μg/L. River runoff plays the main role in the transport of biogenic elements into the lake (primarily phosphorus). Compared to the 1950s, the export of P_tot with river water over a 70-year period changed from 5.17 to 8.45 t/year. The external phosphorus load on the reservoir currently amounts to 0.92 g/m² year, which is 2.2 times higher than in the 1950s. Currently, during the period of summer temperature stratification in deep-water areas of the lake with an acute O₂ deficiency, environmental conditions (Eh, <i>t</i>, pH) are created in the bottom layers of water, under which P-PO₄ begins to mobilize from bottom sediments. Additional (internal) phosphorus load promotes an increase in production processes in the reservoir. Kinetic patterns of transformation and turnover of substances in the lake water are considered. It was found that the transformation of organic matter, N_org, P_tot and Si proceeds more slowly than the turnover of mineral forms of nitrogen compounds and phosphorus.</p>