We investigated the effect of interannual variation of hydrosedimentological regime and connectivity on the zooplankton biodiversity in the Upper Paraná River floodplain. Zooplankton samplings were undertaken between 2000 and 2007, in different environments of the floodplain, including connected and isolated floodplain lakes, backwaters, rivers and channels. The zooplankton included 541 species. Rotifers showed the highest species richness and abundance. Among the zooplankton species, 71 represent new occurrence records for the floodplain. The species accumulation curve showed a continuous increase in gamma diversity, demonstrating the importance of long-term research for accurate knowledge of biodiversity in heterogeneous and dynamic ecosystems, such as the floodplains. Interannual beta diversity among studied years indicated a lesser alteration in community composition in 2001, when a long limnophase period was observed. In most of the environments, the highest species richness values were related to the greatest flooding amplitudes. Flooding amplitude, which is associated with connectivity, favors faunal exchange amongst the environments and between the pelagic and littoral zones. This explains the occurrence of both planktonic and non-planktonic species within the community. On the other hand, mean zooplankton abundance values were higher when a long isolation period occurred. Differences between the potamophase and limnophase amplitude associated with connectivity among the environments were the most important factors for the structure and dynamics of the zooplankton community in the Upper Paraná River floodplain.

We evaluate the influence of abiotic and biotic factors on the community structure of rotifers across a regional hydrological cycle in lotic and lentic environments of the upper River Parana Depth, transparency, temperature, pH, electrical conductivity, dissolved oxygen, chlorophyll a and densities of rotifers were measured at two stations in Lake Guarana (littoral and open water regions) and at one station in the River Baia (open water region) Highest densities of rotifers were found at the lake littoral Canonical correlation analysis related environmental variables with the densities of the most abundant rotifers The strongest relationship was with chlorophyll a, dissolved oxygen, hydrological level and water temperature Diversity of rotifers at each station was mainly explained by fluctuations in hydrological level Results of grouping analysis suggested the formation of groups according to phases of the hydrological cycle

The effect of fish larvae on the diel vertical migration of the zooplankton community was investigated in two tropical lakes, Finado Raimundo and Pintado lakes, Mato Grosso do Sul State, Brazil. Nocturnal and diurnal samplings were conducted in the limnetic region of each lake for 10 consecutive months from April 2008 to January 2009. The zooplankton community presented a wide range of responses to the predation pressure exerted by fish larvae in both environments, while fish larvae showed a typical pattern of normal diel vertical migration. Our results also demonstrated that the diel vertical migration is an important behaviour to avoid predation, since it reduces the spatial overlap between prey and potential predator, thus supporting the hypothesis that vertical migration is a defence mechanism against predation.Keywords: diel vertical migration, spatial overlap, fish larvae, zooplankton, ecological interactions. Migração vertical diária e sobreposição espacial entre larvas de peixes e zooplâncton em dois lagos tropicais, BrasilResumo O efeito de larvas de peixes sobre a distribuição vertical dia-noite da comunidade zooplanctônica foi investigada em duas lagoas tropicais, lagoa Finado Raimundo e Pintado, Mato Grosso do Sul, Brasil. Foram realizadas amostragens noturnas e diurnas na região limnética de cada lagoa durante um período de 10 meses consecutivos, entre abril de 2008 a janeiro de 2009. A comunidade zooplanctônica apresentou uma variada gama de respostas à pressão de predação exercida pelas larvas de peixes nos dois ambientes, enquanto que as larvas de peixes exibiram um padrão típico de migração vertical diária normal. Os resultados mostraram ainda, que a migração vertical diária é um comportamento importante para evitar a predação, uma vez que diminuiu a sobreposição espacial entre presa e seu predador em potencial, dando suporte a hipótese de que a migração vertical é um mecanismo de defesa contra a predação.Palavras-chave: migração vertical diária, sobreposição espacial, larvas de peixes, zooplâncton, interações ecológicas.

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