Impacts of anthropogenic nitrogen (N) inputs on nutrient dynamics and food web structures using stable N isotope techniques in the Arida river catchment, Japan. Riverine survey utilizing 5 regular sampling points showed that δ¹⁵N of nitrate (NO₃^-) increased from forested upstream (~2 per mil) to the downstream (~7 per mil) due to the sewage loads and fertilizer effluents from agricultural area. Correspondingly the δ¹⁵N of benthic algae and aquatic insects increased toward the downstream. This indicates that primary producers of each reach strongly relied on the local N sources and it was utilized effectively in their food web. Simulation using a GIS based mixing model considering the spatial distributions of human population density and fertilizer effluents revealed that strongest impacts of N inputs was originated from organic fertilizers applied to orchards in the middle to lower parts of catchment. Food web structural analysis using food network unfolding technique based on observed δ¹⁵N suggested that the structure of nutrient pyramid did not differ significantly along the riverine positions, while the members of species in each trophic revel changed and the impact of anthropogenic N input was visible along the river.