The frequency of the individual phytoplanktontaxa was estimated, and species were classified ac-cording to the Tichler scale (Trojan 1975): incidental(0-25%), accessory (26-50%), permanent (51-75%),and absolutely permanent (76-100%). The Shan-non-Weaver species diversity index was calculatedbased on the number (H
n
’) and biomass (H
b
’) of thetaxa. The Pearson correlation coefficient was used todetermine the dependence between the values of thediversity index. Significant changes in the value of the biomass of total phytoplankton in the waters of