Trophic structure of three streams with contrasting riparian vegetation and geomorphology

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The relative influence of riparian vegetation and geomorphology on trophic structure was examined at three streams in adjacent catchments of the French Massif Central mountains. The study sites differed mainly by the degree of anthropogenic alteration of riparian and watershed vegetation, and by valley geomorphology. Fishes, benthic macroinvertebrates (grouped into functional feeding groups: FFG), macrophytes and periphyton were sampled seasonally between July 1991 and April 1992. At the riffle-pool (10 0 m) scale, instream morphological units appear to control the spatial partitioning of trophic resources and their consumers. For example, depositional zones supported a specialized trophic structure based on coarse organic matter consumption, whereas in erosional zones (subdivided into lotic and lentic units) periphyton and fine organic matter were the main food source. At the reach-segment scale (10 1 to 10 2 m), valley morphology was the primary factor controlling the stream ecosystem, but anthropogenic alteration of riparian vegetation seems to override geomorphological controls on the trophic structure. For example, the open-canopied Triouzoune Creek (gentle plateau valley), characterized by scrapers (>60% by density) and a greater epilithic algal biomass, had a autotrophic-based structure. At this site, 20 % of total fish biomass was represented by brown trout (Salmo trutta) dominated by fish 50% by density) and fish communities dominated by brown trout with high densities of age 1+ individuals. Overall, the results emphasize the importance of riparian vegetation on the trophic structure of streams, especially when anthropogenic alteration of riparian vegetation is severe. We conclude that there is a need to better integrate riparian vegetation is severe. We conclude that there is a need to better integrate riparian vegetation into European stream management practices in order to maintain the vitality of these systems over the long-term.