This study described the use of food resources and the distribution of fish trophic guilds along the longitudinal gradient of a coastal Atlantic stream, southern Brazil. Allochthonous resources (terrestrial insects) predominated in the headwaters, whereas autochthonous food items (algae, fish) and detritus predominated in the mouth section. Detritivorous, aquaticinvertivorous, and terrestrial-and aquatic-insectivorous species occurred throughout the gradient, while omnivorous and piscivorous in the headwaters and middle stretches, respectively, and herbivorous in the middle and mouth. Detritivores and aquatic-insectivores were significantly more specialized than the other guilds, however, there was no evidence of a longitudinal increase in trophic specialization. Density and biomass of aquatic-invertivores and aquatic-insectivores decreased, whereas that of detritivores increased longitudinally. The distribution of trophic guilds was significantly associated with the stream section, where aquatic and terrestrial insectivorous were more frequent in rocky and flowed stretches from the headwater and detritivores in deeper environments with finer particles of substrate from the mouth. This suggests that fish assemblages in coastal streams with a steep longitudinal gradient may follow patterns in the use of food resources according with the food availability along the river, as predicted by the River Continuum Concept.Este estudo descreve o uso dos recursos alimentares e a distribuição das guildas tróficas de peixes ao longo do gradiente longitudinal de um riacho da encosta Atlântica, Sul do Brasil. Recursos alóctones (insetos terrestres) foram consumidos com maior frequência na cabeceira, entretanto, recursos de origem autóctone (algas, peixes) e detritos foram mais frequentemente registrados na foz. Espécies detritívoras, invertívoras aquáticas, insetívoras terrestres e aquáticas ocorreram ao longo de todo gradiente, enquanto que as onívoras e piscívoras na cabeceira e meio, respectivamente, e herbívoras no meio e foz. Detritívoras e insetívoras aquáticas foram significativamente mais especializadas em relação às demais guildas; entretanto, não houve nenhuma evidência de aumento longitudinal da especialização trófica. A densidade e a biomassa de invertívoras e insetívoras aquáticas diminuíram, enquanto que das detritívoras aumentaram no sentido cabeceira-foz. A distribuição das guildas tróficas foi significativamente associada com a secção do riacho, onde insetívoras aquáticas e terrestres foram mais frequentes em trechos rochosos e de elevado fluxo da cabeceira, enquanto que detritívoras em ambientes mais profundos com partículas finas de substrato da foz. Tais resultados sugerem que assembleias de peixes de riachos tropicais de encosta, os quais apresentam acentuado gradiente longitudinal, podem seguir padrões de uso dos recursos alimentares de acordo com a disponibilidade alimentar ao longo do rio, como previstos no Conceito de Continuidade de Rios.
We investigated the potential effects of an invasive alien Poaceae (Urochloa arrecta) on the abundance, richness, and composition of native macrophytes and fish. Samples were collected in patches of macrophytes dominated by the invasive species and in others dominated by a native macrophyte (Eichhornia azurea). We assessed the potential effects of these two species on macrophyte and fish abundance and richness, as well as in fish assemblage composition. The effects of both macrophytes did not differ when they were in similar, low biomasses. Consistent with our expectations, however, the abundance of native macrophytes and fish decreased with increasing U. arrecta biomass, and both assemblages were eliminated at the extreme high levels of biomass attained by this species. In contrast, E. azurea did not affect the assemblages because this native species never reached levels of biomass as high as those recorded for the invasive species. Competition for space and light most likely determines the elimination of macrophytes in patches that are extensively colonized by U. arrecta, whereas fish assemblages do not occupy these patches due most likely to the limited space available for movement and foraging. Thus, U. arrecta is a source of concern for biodiversity conservation because it has negative influences on both assemblages when it is well developed, which is true in the later stages of the invasion process.
This study evaluated the effect of seasonality on diet composition and overlap in fish species inhabiting the littoral zone of a small lake. We analyzed whether the way the species exploit food resources may help explain their coexistence. Fish samples were collected in a lake in the Upper Paraná River floodplain in September (dry period) and December (rainy period) of 2010. The diet of six species was evaluated through stomach content analysis, which showed that the most consumed resources were microcrustaceans, aquatic insects, aquatic higher plant and algae. Diet composition was significantly different between periods for all species studied. Diet overlap values were low in both periods, but trophic segregation was even higher in the rainy period, indicating that with greater abundance of food resources, species could exploit preferential resources. Trophic segregation, evidenced by the low overlap values in both periods, indicates the absence of interspecific competition, which may explain the coexistence of these species in the littoral zone of this small lake.
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