Abstract: The paper presents the results of experiments with mesomodeling of water pollution in a lowland freshwater body by a mixture of salts of heavy metals (Cd, Zn, Cu). The response of phytoplankton (in terms of primary production characteristics) to the presence of heavy metals in water and to the introduction of a sorbent based on mechanochemically oxidized humic acids has been studied. The removal of heavy metals by phytoplankton on the 16th day of the experiment has reduced the concentration of Cd(II) to 62, Zn(II) to 58- and Cu(II) to 46% of their initial levels. The application of a sorbent under the same conditions has led to a decrease in the concentration of Cd(II) to 21, Zn(II) to 27, and Cu(II) to 10% of the initial level. The examined sorbent does not cause blooming in the water body, thus it differs from the sorbents that contain humic acids with a native structure of molecules.