Abstract
Abstract The size structure of zooplankton in Kolsay Lakes was characterized with the use of the values of Clarke W-statistic, ∆-Shannon–Weaver, and the average individual mass of the organism. The species composition remaining relatively stable, the year-to-year dynamics of the size and abundance characteristics of zooplankton suggested more rapid eutrophication processes in three out of four Kolsay Lakes. The year-to-year changes in the structure of zooplankton communities were largely determined by the immigration of an allochthonous fish species, a drop in water level, and an increase in the biogenic load caused by the recreation use of the lakes. The effect of very low concentrations of heavy metals on the zooplankton size characteristics can be due to the oligotrophic status of the Kolsay Lakes.