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Aim Fish diversity in two lentic environments: a lake connected (Barbosa lake) to a river and an artificial reservoir (Ninfeias reservoir) were determined in the dry and rainy periods in 2015. We expected to find more diversity in the connected lake and greater abundance in the reservoir. Methods Specimens were caught using gill nets. Moreover, environmental parameters such as temperature, electrical conductivity, pH, dissolved oxygen and water transparency were measured during the two periods. Results 546 specimens were collected from 9 families, 12 genera and 15 species of the orders Characiformes, Siluriformes, Cichliformes and Gymnotiformes. Pimelodus maculatus was the most abundant species in the connected lake. Two species were caught from the reservoir: Geophagus brasiliensis and Hoplias malabaricus, the former being predominant. The dominant species in the studied environments corresponded to 48.71% of the total number of specimens. The ichthyofaunal composition from the lake showed a high richness of the order Characiformes (66.7%). Siluriformes and Gymnotiformes represented 13.3% each and the order Cichliformes only 6.7% of the total number of species. In the artificial reservoir, there was an equivalent contribution of Cichliformes and Characiformes. The diversity was higher in the lake (H' = 2.05) and lower in the reservoir (H' = 0.34). Significant differences (p

Cap.1: This study tested experimentally the hypothesis that traíra Hoplias aff. malabaricus is a keystone predator in isolated lagoons of the upper Paraná River floodplain. The experiment was conducted from April to October 2002, through an initial manipulation of traíra density (levels of treatment: addition, removal and reference). Treatment effects were evaluated on descriptors of fish assemblage structure (species richness, abundance, equitability and biomass), through repeated measures analysis of variance, in two habitat categories (open and macrophyte-dominated areas). In spite of being recorded in all assemblages and depended on habitat category, reductions in species richness were significantly more pronounced in lagoons where H. aff. malabaricus was removed. In these lagoons, the number of individuals was also significantly lower than in those where the piscivore was added or maintained in natural densities. Equitability showed opposite tendencies. Analysis of relative abundance by size-classes revealed a sharp reduction in the number of small-sized individuals (< 3cm), contributing to the absence of pronounced alterations in biomass. These evidences supported our hypothesis and demonstrated that the understanding of mechanisms by which predators affect the persistence of prey species constitutes a challenge for future researches, especially in tropical ecosystems. ap.1: O presente estudo testou experimentalmente se a traíra Hoplias aff. malabaricus pode ser considerada uma espécie-chave nas lagoas isoladas na planície de inundação do alto rio Paraná, no período de abril a outubro de 2002, através da manipulação inicial em sua densidade (níveis do tratamento: adição, remoção e referência). Os efeitos desse tratamento foram avaliados nos descritores da estrutura das assembléias (riqueza de espécies, abundância, eqüidade e biomassa), através de análises de variância com medidas repetidas em duas categorias de hábitat (área aberta e coberta por macrófitas). Apesar de registradas em todas as assembléias e dependerem da categoria de hábitat, as reduções na riqueza de espécies foram significativamente mais pronunciadas em lagoas onde H. aff. malabaricus foi excluída. Nessas lagoas, o úmero de indivíduos também foi significativamente menor em relação àquelas nas quais o piscívoro foi adicionado ou mantido em densidade natural, enquanto para a eqüidade as tendências foram opostas. A análise da abundância relativa por classes de tamanho apontou uma redução drástica no número de indivíduos de pequeno porte (< 3 cm), que contribuiu para a ausência de alterações pronunciadas na biomassa entre os níveis de densidade do predador e das categorias de hábitat. As evidências apresentadas deram suporte à hipótese formulada e demonstraram que a compreensão dos mecanismos pelos quais predadores contribuem na persistência das demais espécies constitui-se num Desafio para pesquisas futuras, especialmente nos ecossistemas tropicais. 70 f