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The statistical background for the relationship between the taxonomic diversity of macrozoobenthos and water mineralization is presented in the example of the benthic communities of plain rivers in the Lower Volga River basin. Significant differences have been found for a complex of 35 indices of species and phylogenetic α- and β-diversity in the tributaries of the Saratov and Volgograd reservoirs compared to the saline rivers of the Elton Lake basin. The hypothesis of phylogenetic niche conservatism [5] is confirmed by a significant effect of phylogenetic overdispersion at low and medium salinity of watercourses and phylogenetic clustering at high water salinity. Mean salinity tolerance has been assessed for each of the 356 species considered; this index was interpreted as the statistical probability of inhabiting the waters of certain mineralization by particular species. A statistically significant phylogenetic signal corresponding to the species traits was revealed by Pagel's lambda and Bloomberg's K tests. Mean salinity tolerance for taxa of various systematic levels has been presented as a cladistic tree. [ABSTRACT FROM AUTHOR]