The results of a comparative study of the phytoplankton assemblage structure in two parts of the Kanev Reservoir, characterized by different levels of urban pollution, are presented. Analysis of pattern variability for several types of size spectra, namely the biomass size spectrum (BSS), the normalized biomass size spectrum (NBS) and the traditional taxonomic size spectrum (TTSS), was performed on the basis of a dataset produced by routine monitoring during a 24–month period at two stations in the Kanev Reservoir (Ukraine). Station 1 was situated in a relatively pure riverine part of the reservoir, whereas station 2 was located further down, at the mouth of the Syrets River, which carries the outflow of Kiev sewage. Both stations were characterized by markedly different phytoplankton composition and dynamics. While the BSS curves were very different, the annual NBS and TTSS curves exhibited a similar general pattern, with some differences evidenced in the fine structure of each spectrum. The obtained indices of the traditional taxonomic size spectrum (TTSS) that characterize the state of phytoplankton in a relatively pure area of the Kanev Reservoir (st. 1) and in st. 2, which was contaminated by urban drains of Kiev sewage at the mouth of the Syrets River, are compared with the same indices of a clean fresh-water reservoir, in particular Lake Kinneret (Israel). The impact of this urban pollution, as evidenced through the deformations of the size spectrum patterns, is discussed in detail.

Keywords: phytoplankton, anthropogenic pollution, size-frequency distribution, size spectra

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