The construction of reservoirs alters the life cycle of many organisms, causes changes in biodiversity patterns and worst scenarios can lead to species reduction or extinction. We sought to investigate the changes in the phytoplankton community caused by the alteration of the environmental filters imposed by the implementation of the Hydroelectric Plant of Estreito, the middle Tocantins river. The assembly was investigated using Taxonomic and Functional Groups (GF) analyzes. Five sampling points were involved in this study, located along the Tocantins river, later converted into the reservoir of Estreito HPP. The monitoring period was 4 years (December 09 to May 13) and the collections performed quarterly in the Tocantins River, during the pre-filling, pre-filling, and post-filling phases. Samples for the study were performed in the subsurface of the water (30 cm), and processed in the laboratory for identification and counting of organisms. The species were first identified taxonomically and later classified into functional groups. The water samples for physical and chemical analyzes were collected concomitantly with the biological samples and processed according to the recommended methodology. As a result, we noticed that the implantation of the hydroelectric plant caused alterations in the alteration of the assembly structure responding to the new environmental filters, with a marked reduction of biovolume and taxonomic richness and GF. Temporally there was a significant difference between the phases, obtaining the Pre phase as the most different of the others. Longitudinally there were no significant differences. In the taxonomic composition the Pre phase was dominated by the ambiguous Aulacoseira species, the Enc stage by Cylindrospermopsis raciborskii and the Post phase by Aulacoseira sp. In the study of GF 27 GF were recorded. The Pre phase obtained the highest values of biovolume and GF C (diatoms) as dominant. In Enc the GF Sn (cyanobacteria) was dominant. In the Post GF P (diatoms); N (Desmidiaceae); W2 (Euglenaceae); Sn and Lm (cyanobacteria) co-dominated with values of similar biovolumes. This study revealed the important roles of environmental filters in the selection of species and functional groups of phytoplankton assemblages, in face of a new regime of fluvial functioning.
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