The diet and trophic groups of an assemblage of aquatic insects were studied in a tropical stream. Genera of the orders Ephemeroptera, Odonata, Plecoptera, Lepidoptera, and Hemiptera showed feeding specialization. Others, such as Trichoptera, Coleoptera, and Diptera, showed great diet variation with genera of different trophic groups. Seasonal variation of insect diet, evident only for some genera of the orders Trichoptera, Lepidoptera, Coleoptera, and Diptera, was due to the differences observed in community composition and to generalist habits of these genera. However, the seasonal comparison of trophic groups showed no significant statistical differences. The great importance of organic matter, a non-limited resource, in the diet of Ribeirão do Atalho aquatic insects may be the explanation for the trophic stability in this community organization.Key words: aquatic insects, diet, trophic groups, tropical stream. RESUMO Dieta e grupos tróficos de uma comunidade de insetos aquáticos em um riacho tropicalA dieta e o grupo trófico de uma assembléia de insetos aquáticos foi estudada em um riacho tropical. Os gêneros de algumas ordens de insetos, como Lepidoptera, Ephemeroptera, Plecoptera, Odonata e Hemiptera, apresentaram especialização alimentar. Por outro lado, as ordens Trichoptera, Coleoptera e Diptera mostraram grande variação na dieta, com gêneros de diferentes grupos tróficos. Alguns gêneros das ordens Trichoptera, Lepidoptera, Coleoptera e Diptera apresentaram variação sazonal na dieta relacionada a mudanças na composição da comunidade e à dieta generalista desses gêneros. Entretanto, a comparação sazonal de grupos tróficos mostrou diferenças não significativas estatisticamente. A grande importância da matéria orgânica, um recurso não limitado, na dieta dos insetos aquáticos no Ribeirão do Atalho pode ser a explicação para a estabilidade da organização trófica dessa comunidade.Palavras-chave: insetos aquáticos, dieta, grupos tróficos, riacho tropical.

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This study investigated the structure and properties of a tropical stream food web in a small spatial scale, characterizing its planktonic, epiphytic and benthic compartments. The study was carried out in the Potreirinho Creek, a second-order stream located in the south-east of Brazil. Some attributes of the three subwebs and of the conglomerate food web, composed by the trophic links of the three compartments plus the fish species, were determined. Among compartments, the food webs showed considerable variation in structure. The epiphytic food web was consistently more complex than the planktonic and benthic webs. The values of number of species, number of links and maximum food chain length were significantly higher in the epiphytic compartment than in the other two. Otherwise, the connectance was significantly lower in epiphyton. The significant differences of most food web parameters were determined by the increase in the number of trophic species, represented mainly by basal and intermediate species. High species richness, detritus-based system and high degree of omnivory characterized the stream food web studied. The aquatic macrophytes probably provide a substratum more stable and structurally complex than the sediment. We suggest that the greater species richness and trophic complexity in the epiphytic subweb might be due to the higher degree of habitat complexity supported by macrophyte substrate. Despite differences observed in the structure of the three subwebs, they are highly connected by trophic interactions, mainly by fishes. The high degree of fish omnivory associated with their movements at different spatial scales suggests that these animals have a significant role in the food web dynamic of Potreirinho Creek. This interface between macrophytes and the interconnections resultant from fish foraging, diluted the compartmentalization of the Potreirinho food web.

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