Trophic ecology of low-order streams in southern Brazil: relationships between primary and secondary consumers

O conceito de rio continuo (RCC) afirma que ocorrem altera????es das caracter??sticas f??sicas e qu??micas ao longo do gradiente longitudinal de arroios, o que induz ao acr??scimo de esp??cies de diferentes h??bitos alimentares. Neste sentido o objetivo deste estudo ?? a caracteriza????o tr??fica de esp??cies de peixes de cabeceira de arroios subtropicais e a identifica????o do fluxo de energia entre macroinvertebrados e peixes. Espera-se que a biomassa de peixes detrit??voros aumente conforme o gradiente longitudinal, devido ao aumento da produ????o primaria neste sentido. O estudo foi desenvolvido em 32 s??tios de 1?? a 4?? ordem pertencentes a oito microbacias da Bacia do Rio dos Sinos. As coletas de ictiofauna foram realizadas utilizando pesca el??trica em uma ??rea de 100m2. Macroinvertebrados foram coletados com amostrador Surber, para avaliar a disponibilidade de recurso e biomassa de consumidores prim??rios. A riqueza e a diversidade da comunidade ??ctica foram calculadas. Foi avaliada a dieta das esp??cies de peixes mais abundantes e calculados os ??ndices de import??ncia relativa e de amplitude de nicho. Para verificar a transfer??ncia de energia entre consumidores prim??rios e secund??rios foi quantificada a biomassa seca de peixes e macroinvertebrados. Os resultados demonstram que a diversidade, a riqueza e a biomassa aumentaram conforme o gradiente longitudinal que arroios de cabeceira, corroborando as previs??es do RCC. Tamb??m constatou-se que a comunidade ??ctica ?? composta por duas guildas alimentares (inset??voros e detrit??voros). Estes ??ltimos, por sua vez, elevaram a biomassa ??ctica e estabeleceram duas vias de fluxo de energia atuando como consumidores prim??rios e secund??rios.The River Continuum Concept (RCC) suggests that changes in physico-chemical factors across the stream longitudinal gradient leads to the adding of species of different feeding habits. In this sense, the aim of the study is the trophic characterization of fish headwater species from subtropical streams and to identify energy flow between macroinvertebrate and fishes. It is expected that detritivorous fish standing crop increase across stream longitudinal gradient due to increase in primary production in the same direction. This study was carried out in 32 streams from 1st to 4thorder in eight microbasins in the Sinos River basin. Ichthyofauna was sampled with electric fishing in 100m2 sections. Macroinvertebrates were sampled with Surber sampler to assess resource availability and primary consumer standing crop. Richness and diversity of fish communities were also calculated. The diet of the most abundant fish species was assessed as well as relative importance and niche amplitude indexes. The results displayed that fish diversity, richness and standing crop increased across stream longitudinal gradient corroborating RCC postulates. It was also indicated that fish communities was composed by two feeding groups (insetivorous and detritivorous). The detritivorous fishes have in turn increased fish standing crop and established two directions of energy flow which act as both primary and secondary consumers