The benthic fish assemblages as well as the influence of seasonal variation on the structure of these assemblages are still little known, especially in large tropical rivers. In this study, we evaluated the spatial and temporal variation and trophic structure of benthic fish of the lower Purus River, Amazonas State, Brazil. Through sampling with benthic trawls, we obtained information on spatial and temporal variation on the composition, abundance, richness and diversity of these assemblages. In addition, we measured environmental variables (depth, current velocity, temperature, dissolved oxygen, conductivity and pH) of river channel and we evaluated the responses of the fish to seasonal changes in the environment. Food availability was also sampled and we relate to dietary studies and trophic structure of benthic fishes. Our results show that species abundance distributions and species composition vary seasonally. These results suggest these differences may arise in part because of the asymmetrical use of the spatial habitat during the seasons when the water is rising and receding, hence maintaining higher levels of diversity. During the rising, when marginal areas are inundated, many fishes can migrate laterally from the channel toward these areas where they supposedly find better conditions for foraging, as well as refuge from predators. In the receding, fishes follow the water level movement back to the river channels. Among the environmental variables, the depth influenced the distribution of some species of benthic fish. Finally, diet data revealed the importance of autochthonous food resources to diet of fish benthic, especially aquatic insects, as well as trophic plasticity and omnivorous diet of species in response to seasonal availability of food items. The combination of these results indicates that the seasonal aquatic environment (várzeas) comprises an important habitat for several species of benthic fishes of main channel with an important role in maintaining benthic community diversity, once the input of resources to the channel, characterized by low autochthonous primary productivity can occur through the flood pulse or even along a river continues.
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