In montane streams with enriched nutrient content changes were observed in the structure of algal communities as compared with unpolluted waters, especially during autumn and winter, characterized by intensified tourism and low water level. The numbers of Achnanthes minutissima var. minutissima and A. biasolettiana var. biasolettiana fell while those of Cymbella affinis, C. silesiaca, C. sinuata, Navicula cryptocephala, Nitschia palea, Cladophora glomerata, Ulothrix zonata, and species of the order Chlorococcales rose. The increase mass of algae was expressed by a higher index of diatom biomass and a higher content of chlorophyll ɑ.