Eunotiaceae groups 12 genera, among them are Eunotia Ehrenberg, Actinella Lewis and Eunotioforma Kociolek & A.L.Burliga. Existing studies point to the predominance of the genus Eunotia in Amazonian waters, being well represented mainly in acidic, oligotrophic waters with low amounts of minerals. Samples from Brazilian aquatic environments, including the region of the state of Pará, point out that genera of the Eunotiaceae family, especially Eunotia, are the most common in the Brazilian flora. However, there are still few studies carried out in the Amazon region and in reservoirs. Therefore, this study aims to record, identify and describe the taxa of the genera Eunotia, Actinella and Eunotioforma in the Reservoir of the Curuá-Una Hydroelectric Power Plant (Santarém, Pará) and thus contribute to the increase in knowledge of the phycological biodiversity of the region amazon. The periphytic samples were obtained through artificial samplers (artificial plastic substrate) immersed in three points of the Reservoir. The substrate was immersed 30 days before each collection, which took place in September (dry season) and December (flood) of 2016. In the field, on the day of collection, the slides of the samplers were scraped with a toothbrush and the material was diluted in 150 ml of water for every three plastic slides. After collection and scraping, the samples were fixed with formalin (4%) and subsequently, the material was oxidized with nitric acid. The counting of diatom valves was performed on permanent slides (looking for an efficiency of 80%). Data analysis showed that Eunotiaceae represented 33.36% of the valves counted, with Eunotia being the most representative genus (16 taxa), followed by Actinella (eight taxa) and Eunotioforma (one taxon). The species with the highest number of registered valves were: Eunotia bilunaris, E. mucophila, Actinella sp. 3 and Eunotioforma curvula. With regard to the records for Pará, 14 taxa are not mentioned in the State List of Flora Species in Brazil, reinforcing the importance of studies like this one for the knowledge of Amazonian biodiversity. Significant variations related to the richness and abundance of taxa were not observed between the analyzed hydrological periods. However, some taxa marked these periods, namely: Actinella gracilis, A. siolii, Eunotia enigmatica, E. minor, E. trigibba and Eunotia sp. 1 (dry season) and Actinella brasiliensis and E. actinelloides (wet season).
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