-Between 1991 and 1995 aquatic macrophyte composition was observed in the lower part of the reservoir of the hydroelectric power plant of Balbina (Amazonas, Brazil). After closure of the dam in 1987, vegetation cover -mostly Eichhornia crassipes -was high, but was not associated with drift wood, produced by the decomposing, emergent forest. Comparison of the (1989 -1995) suggests that the succession was the result of a relatively mild and short period of eutrophication, followed by declining nutrient levels. Annual variation of water levels, followed by aquatic and terrestrial decomposition of the marginal vegetation, may allow for the maintenance of relatively productive vegetation belts along the shore lines of islands and inundated stream valleys. Key-words:Hydroelectric powerplants, Amazonas, Balbina, water plants. Observações Sobre a Dinâmica de Macrófitas Aquáticas no Lago da Usina Hidrelétrica deBalbina, Amazonas, Brasil.RESUMO -Entre 1991 e 1995 a composição das macrófitas aquáticas da porção inferior do reservatório da hidrelétrica de Balbina foi observada. Após o fechamento do lago pela barragem em 1987 a cobertura do lago por macrófitas aquáticas, principalmente por Eichhornia crassipes, foi alta, mas não existem dados quantitativos. A partir de 1990 a redução da cobertura foi rápida, e notou-se uma sucessão nítida e consistente em diversas áreas do lago: Eichhornia ® Utricularia e Cyperaceae ® Salvinia. As ciperáceas e outras plantas menos dominantes foram associadas com madeiras flutuantes provenientes da floresta emergente em decomposição. O monitoramento contínuo das condições químicas da água (1983 -presente) sugere que a sucessão é o resultado de um período relativamente curto de eutrofização, seguido por um declínio rápido do teor de nutrientes. Devido as variações anuais do nível d'água, a faixa marginal das ilhas e dos igarapés inundados é caraterizada por solos relativamente ricos, graças à decomposição periódica de plantas terrestres inundadas na cheia, e de macrófitas aquáticas na seca. Portanto, espera-se que o lago, com suas milhares de ilhas e igarapés inundados, sustente uma certa produtividade pesqueira, apesar do baixo nível de nutrientes do lago. Palavras-chaves:

The genus Euploca (Heliotropiaceae) was proposed by Nuttall (1836) and in its current circumscription, following Diane et al. (2003), also encompasses all the species in Heliotropium section Orthostachys, as well as the species belonging to the genera Hilgeria Förther and Schleidenia Endl. On the basis of Diane et al. (2016), Euploca includes about 100 cosmopolitan species. They grow especially in dry zones with centers of taxonomic diversification in Mexico and South America, the later in which it is represented by 33 species. The majority of these (28 spp.) are endemics, corresponding to approximately 85% of all the species, various of them occurring in the Southern Cone (Southern Brazil, Uruguay, Argentina, Chile, and Bolivia). Only one species occurs in Chile (E. procumbens (Mill.) Diane & Hilger), and two species are reported from Ecuador, one of them endemic to the Galapagos Archipelago. Euploca species, vegetatively, are herbs, subshrubs, and rarely shrubs. The leaves are alternate to pseudo-opposite, rarely pseudoterminate, linear to broadly ovate or obovate. Inflorescences are 1-pluribranched, rarely ebracteose (encompassing a small group of species predominantly found in South America, e.g., E. barbata (DC.) J. I. M. Melo & Semir, a species restricted to the Caatinga vegetation, in Northeastern Brazil), many-flowered or presenting single flowers as in E. lagoensis (Warm.) Diane & Hilger, a species largely distributed in the Neotropics, and E. parciflora (Mart.) J. I. M. Melo & Semir, an endemic species from Brazil (Caatinga and Cerrado). Fruit are dry, separating into four 1-seeded mericarpids (Diane et al., 2016) adapted to hydrochory (water-dispersed as probably in E. paradoxa