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We analyzed the variation of fish species richness and trophic structure along an upstream-downstream gradient and identified the factors associated with the pattern observed. The fish community composition varied along a headwater-downstream gradient. Species richness and trophic structure decreased significantly with increasing altitude and increased with stream order and distance from source. Headwater communities showed a simple structure. Oncorhynchus mykiss was the only fish captured or noticeably dominant at high altitudes. Thus, headwater can be classified as "trout zones". From these zones to downstream areas fish communities changed because of the addition of other fish species. This phenomenon probably occurs because of an increase in habitat diversity.Key words: fish species richness, rivers, altitude, distance from source, order. RESUMO Padrões de riqueza de espécies ícticas em rios da região central da ArgentinaO presente trabalho teve por objetivo analisar a variação da riqueza de espécies ícticas e da estrutura trófica das comunidades do gradiente nascente-desembocadura, nos rios da região central da Argentina, e identificar os fatores associados ao padrão observado. A composição da comunidade íctica variou desde as cabeceiras até a desembocadura dos sistemas lóticos considerados. A riqueza e a estrutura trófica se relacionaram significativamente com a altitude, a ordem e a distância da origem. As comunidades de cabeceira apresentaram estrutura simples. Oncorhynchus mykiss foi a única espécie capturada ou claramente dominante nas maiores altitudes. Assim, as zonas de nascentes podem ser classificadas como "zonas de trutas". Desde estas zonas até a desembocadura, as comunidades de peixes mudaram devido à adição de novas espécies. Esse fenômeno provavelmente se deve ao incremento da diversidade de habitat e à maior estabilidade ambiental que caracteriza as zonas baixas dos cursos.Palavras-chave: riqueza de espécies ícticas, rios, altitude, distância da origem, ordem.

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Toxicity tests using adult specimens of Jenynsia multidentata were carried out during 96 hours in order to determine the lethal concentration (LC50) of endosulfan. Histological alterations were determined in gills and liver. Gill damage was quantified as secondary lamellae thickness. The 96 hr LC50 values were significantly different between males (0.719 microg x L(-1)) and females (1.317 microg x L(-1)). The sex difference was attributed to the dimorphism in the lipid content in females (2.16%) and males (1.79%). Histological alterations in gills included hypertrophy and lifting of the epithelium of the secondary lamellae and aneurisms. These alterations caused a significant increase of the secondary lamellae thickness in treatment versus control fish. Finally, reversible histological alterations (such as hydropic degeneration and dilation of sinusoids) were observed in the liver of exposed fish as well as an irreversible change such as necrosis at the highest concentrations.

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