[M-IS21_28PM1] Biogeochemistry

[MIS21-16] How does anthropogenic nitrogen input affect the nutrient dynamics and food web structures?

*Nobuhito OHTE¹, Hiroyuki TOGASHI², Naoko TOKUCHI³, Mayumi YOSHIMURA⁴, Yoshikazu KATO⁵, Naoto F. ISHIKAWA⁶, Michio KONDO⁷, Ichiro TAYASU⁵ (1.Graduate School of Agricultural and Life Sciences, University of Tokyo, 2.Field Science Education and Research Center, Kyoto University, 3.Tohoku National Fisheries Research Institute, Fisheries Research, 4.Japan Agency for Marine-Earth Science and Technology, 5.Faculty of Science and Technology, Ryukoku University, 6.Center for Ecological Research, Kyoto University, 7.Kansai Research Center, Forestry and Forest Products Research)

Keywords:river ecosystem, nitrogen input, stable isotope, food web

In last five decades, impacts of anthropogenic nutrient inputs on river ecosystems have continuously been a major concern for the governments and residents of the catchments in Japan. Major sources of anthropogenic nitrogen (N) include leachate from forest ecosystem, surplus fertilizers and sewage. Impacts of anthropogenic N inputs on nutrient dynamics and food web structures were investigated 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 ‰) to the downstream (~7 ‰) 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. Differences in δ¹⁵N between primary producers and predators were ~6-7 ‰ similarly at all sampling points. 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.