LAUSR

Several studies have shown that the origin of carbon fuelling food webs in streams depends on riparian cover type. In forested stream sites allochthonous resources fuel food webs, whereas autochthonous resources support biomass in grassland (open-canopy) stream sites. However, some studies suggest that autochthonous carbon (of highest quality) is preferentially assimilated regardless of riparian cover and that the food-chain length (FCL) may be larger in grassland than in forested sites. We used stable isotopes of carbon and nitrogen in adjacent grassland and forested reaches to compare the contribution of autochthonous vs. allochthonous resources to the biomass of the whole macroinvertebrate assemblage and to the most abundant taxa. Moreover, we compared the FCL between forested and grassland sites by estimating the trophic position of brown trout, Salmo trutta. Autochthonous support to macroinvertebrate biomass was higher in grassland than in forested sites, often changing from a dominantly autochthonous to an allochthonous-generated biomass from grassland to forested. This held true for the whole macroinvertebrate assemblage and for specific species. FCL remained similar between reach types. Our study suggests that autochthonous resources are assimilated to a higher extent when their availability increases with canopy openness but allochthonous carbon sustain macroinvertebrate biomass in forested reaches.