[M-IS21_28PM1] 生物地球化学

[MIS21-16] 河川への人為起源窒素の負荷が水棲生物の食物網構造に与える影響

*大手 信人¹、冨樫 博幸²、徳地 直子³、吉村 真由美⁴、加藤 義和⁵、石川 尚人⁶、近藤 倫生⁷、陀安 一郎⁵ (1.東京大学大学院農学生命科学研究科森林科学専攻、2.京都大学フィールド科学教育研究センター、3.独立行政法人水産総合研究センター東北区水産研究所、4.独立行政法人海洋研究開発機構、5.龍谷大学理工学部、6.京都大学生態学研究センター、7.森林総合研究所関西支所)

キーワード：河川生態系, 窒素負荷, 安定同位体比, 食物網構造

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.