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Resting stages are common for the life cycle of some phytoplankton species, including blooming and potentially toxic species. The “seed bank” accumulated in the sediments can initiate blooms in the water column and could be an early warning signal of harmful algal blooms (HABs). In order to identify the phytoplankton cyst assemblages, thirteen surface sediment samples were collected from different sites in the Black Sea. The diversity of the resting stages was assessed using high-throughput sequencing metabarcoding (V7-9 hypervariable region of the 18S rDNA). One hundred and eighty microalgal species were identified with high level of similarity to the reference sequences. Dinoflagellates were dominated by Biecheleria, Gymnodinium and Karlodinium. Within diatoms, Skeletonema, Chaetoceros and Thalassiosira were the most abundant genera. Sixteen of the detected operational taxonomic units (OTUs) were assigned to harmful microalgae (12 dinoflagellates species, 1 diatom, 1 haptophyte and 2 raphidophytes). No pattern of microalgal sequences depth distribution was discriminated. The results show that DNA metabarcoding has a great potential for assessment of the phytoplankton diversity in environmental sediments.