Here we present the results of the study Bacillariophyta in the water bodies of the Nyzhniosulsky National Nature Park (NsNNP). Samples of plankton, benthos, and periphyton were collected according to the conventional technique and cleaned using concentrated H2O2. Diatoms were investigated by means of light and scanning electron microscopy. In the NsNNP, a total of 242 species of diatoms were identified; they belong to 3 classes, 14 orders, 31 families, and 62 genera. The number of diatom species varied widely in the studied water bodies: from 26 (in the Borys River) to 223 (in the Sula River) species depending on the type and ecological state of the water body. In taxonomic structure, class Bacillariophyceae (95.9%) was the most diverse compared to Mediophyceae (2.5%) and Coscinodiscophyceae (1.6%). Leading families include Cymbellaceae, Naviculaceae, Bacillariaceae, Gomphonemataceae, Fragilariaceae, Stauroneidaceae, Pinnulariaceae, and Surirellaceae. Genera Navicula, Gomphonema, Nitzschia, and Cymbella were the richest in species. Our study increased the number of NsNNP diatoms by 57.4%; taking the literature into account, a total of 285 Bacillariophyta species are known for the park water bodies. We also revealed a number of new and interesting taxa confirming the role of this protected area in nature conservation of the region. Fifteen species of Bacillariophyta found in the water bodies of the park are first cited for Ukraine. Sixteen species are new for the Forest-Steppe zone of Ukraine and 10 for the Left-Bank Forest-Steppe. Two new nomenclature-taxonomic combinations are proposed: Surirella hibernica (W.Smith) D.Kapustin & O.Kryvosheia comb. nov. and Iconella amphioxys (W.Smith) D.Kapustin & O.Kryvosheia comb. nov.
Keywords: Bacillariophyta, new and rare taxa, Nyzhniosulsky National Nature Park, Sula River, Ukraine