The paper deals with the results of an LM and TM study of diatoms in the largest high-mountain lakes of the Ukrainian Carpathians: lakes Nesamovyte and Brebenskul of the Chornohora mountain range. Samples of plankton and periphyton were collected according to the conventional technique and cleaned using concentrated H2O2. A total of 130 species (133 infraspecific taxa, ist) of diatoms were identified; they belong to 3 classes, 5 sub-classes, 14 orders, 26 families, and 46 genera. At Nesamovyte Lake, 114 species (116 ist) were recorded; in Brebeneskul Lake 44 species (45 ist) were found. Thirty-two (32) taxa were common for both lakes. Eight taxa from Nesamovyte Lake are first cited for Ukraine and 20 taxa are new records for the Ukrainian Carpathians. They include rare species Cymbella lange-bertalotii Krammer, Encyonema neogracile Krammer, Eunotia tetraodon Ehrenb., Pinnularia macilenta Ehrenb., P. subanglica Krammer, and Skabitschewskia peragalloi (Brun et Hérib.) Kulikovskiy et Lange-Bert. Pinnularia falaiseana Krammer is a noteworthy record: this species is known from a few locations in the world. Dominant complexes of the species and ecological peculiarities of the revealed diatoms are analyzed. Most of the species are benthic and plankton-benthic forms. In relation to salinity, a predominance of indifferent species is noted. In relation to the pH of water, in the diatom flora of Nesamovyte Lake, acidophilic and neutrophilic species prevail. In Brebenskul Lake, dominant complexes are formed by alkaliphiles. In terms of water trophicity, diatoms of the Nesamovyte Lake indicate oligotrophic and oligomesotrophic conditions; most of the species in Brebeneskul Lake are indicators of oligoeutrophic and oligomesotrophic waters. Species composition of diatoms indicate the satisfactory ecological state of the studied lakes.
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