Values of rarefied species richness with standard deviation (sample rarefaction with Mao Tau estimation) for groups of sites differed in terms of biocoenotic type (IV − a, b; V − c, d; VI − e, f), method of quality assessment (BMWPpl and MMI) and values of environmental quality (white circles − low, triangles − moderate, black circles − high). Values of fit accuracy of logarithmic regression curves are presented as R². Differences in richness between sites with moderate and extreme values presented on plots (a)–(d) are statistically significant (Kolmogorov–Smirnov's test).