Relation of hydrogeomorphology of Gravel Bar to Particulate Organic Matter Dynamics in Braided Alpine River

Abstract

タリアメント川における原生的洪水氾濫源の豊かな生物多様性を維持する一つの原因である, 網状河道砂州の粒状有機物の捕捉機能を定性的, 定量的に研究した。運河, 単断面河道, 網状河道などの調査地点で流下粒状有機物の濃度, 炭素-窒素の安定同位体比, CN比及び付着藻類のクロロフィルa値を分析した結果, 運河では粒状有機物の移動機能, 単断面河道では隣の陸地域からの供給機能, そして網状河道の砂州では捕捉機能が支配的であることが明確に示された。 また, 二次流の流速と流向が砂州地形の捕捉過程において重要であることが示された。

Channel geomorphology has ecologically important functions to remove organic particles from flowing water providing availability of organic matter to benthic organisms. The present study focuses on the roles of braided gravel-bar for riverine organic matter retention, aiming to elucidating the retention processes corresponding to the diverse hydrogeomorphological conditions in the Tagliamento River in north east Italy. Results of longitudinal changes in concentration, stable isotope analyses and C:N ratio of suspended particulate organic matter (POM) as well as in chlorophyll a of epilithon, which were collected at canal, narrow channel and braded bars, showed a distinct retention pattern and algal production of braided-bar. In addition, we found that secondary flow velocity and direction occurred near bar shoreline can be critical factors to affecting suspended POM retention. On the other hand, in an island bar unit, barhead riffle and bartail shore showed a selective retention capacity according to particle size. Overall the spatial diversity of POM quantity and quality along gravel-bar can lead to high heterogeneous habitats in braided channel.