On the basis of modern approaches of quantitative data analysis, syntaxonomy of the Lemnetea class of the Southern Bug valley was developed. It has 10 associations from two unions and one order. It was established that the type and trophicity of water bodies, as well as their flow regime, are the leading factors of the territorial differentiation of the Lemnetea class plant communities. The highest syntaxonomical diversity of free-floating vegetation is characteristic of the upper and lower reaches of the Southern Bug River. In the middle reaches of the river valley, communities of Lemnetea occur mainly in anthropogenic ecotopes. Coenoflora the Lemnetea has been analyzed which includes 42 species of vascular plants belonging to 19 families and 29 genera. The leading families are Lemnaceae, Potamogetonaceae, Hydrocharitaceae and Nymphaeaceae. In the geographic spectrum of the coenoflora of the Lemnetea, the wide-range chorological elements predominate – plurizonal, circumpolar and indifferent species. Using DCA-ordination and ecological scales of Ya.P. Didukh, it has been established that the distribution of Lemnetea communities in hyperspace of abiotic factors occurs under their complex effect. At the same time, for some syntaxa, light regime of ecotope (Stratiotetum aloidis), variability of damping (Lemnetum trisulcae), organic elements content in water and bottom sediments (Lemno-Hydrocharitetum morsus-ranae and Hydrocharitetum morsus-ranae), carbonate content, soil acidity and total soil salt regimes of the trophotope (Lemno gibbae-Wolffietum arrhizae) are very important. The results of phytoindication analysis demonstrated that coenoses of the Lemnetea class in the valley of the Southern Bug River by the relation to soil acidity are neutrophilic, to the total salt regime are eutrophic, to carbonate content in a soil are carbonatophobic, and to nitrogen content in a soil are nitrophilic.
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