The copper toxic effect was studied on two species of Black Sea diatoms Cerataulina pelagica and Phaeodactylum tricornutum, and two species of dinoflagellates Levanderina fissa and Prorocentrum nanum. A wide range of variability in the resistance of cultures to copper as a toxicant is demonstrated. The EC50 values for different species varied from 1.8 to 43.7 mg·L-1. An increase in the algae initial biomass within one species caused a corresponding decrease in the toxic effects of Cu2+. The two species of dinoflagellates with similar initial biomass and practically equal values of the specific growth rate, demonstrated significant difference in the EC50 parameter. In the small - celled species P. nanum it was equal to 43.7 mg·L-1, whereas, in the large species L. fissa, it decreased by almost 10 times. In the small-cell strain of the diatom C. pelagica, the semi-inhibition growth parameter was 13.5 mg·L-1, which is 2.4 times higher than in the large-cell strain with all other conditions being equal. The copper toxicological effect at low concentrations is determined, first of all, by a decrease in the electronic transport rate values (rETR) in cultures. The second parameter in terms of sensitivity to copper is Fv/Fm (maximum efficiency of photosystem II). The algae morphometric parameters are the most resistant to its impact: the volume and cell’s surface area, as well as the specific surface area. The significant changes on the third day of experiments were observed most often, at a copper concentration near EC50 or above.