A traíra Hoplias aff. malabaricus (Bloch, 1794) na planície de inundação do alto rio Paraná : influência sobre as assembléias de peixes e aspectos da auto-ecologia. | The lungfish Hoplias aff. malabaricus (Bloch, 1794) in the Upper Paraná River floodplain : influence over the assemblies of fish and aspects of self-ecology.

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, in which phytoplankton changes in Paraná River were verified, in function of the formation of Porto Primavera Reservoir and the climatic alterations that occurred during the period studied (1993-1994 and 2000-2005), especially those related to El Niño and La Niña phenomena. In the second chapter entitled "Interannual variation of phytoplankton in a river-flooding lake system (Upper Paraná River, Brazil)", we verified that the alterations of phytoplankton structure in Patos Lagoon and Ivinhema and Paraná rivers were associated to the fluctuations of hydrosedimentological regime of Ivinhema and Paraná rivers, probably, influenced by the phases La Niña (2000-2001) and El Niño (2003-2004) Southern Oscillation cycle (ENSO).

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 número 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

PhD