Abstract
The trophic structures of benthic faunal communities in a river are expected to respond to the relative amounts of autochthonous and allochthonous energy sources in the environment. Using isotopic analysis of δ13C, we investigated the percentage of autochthonous energy of benthic fauna at 3 sites longitudinally sited along the Hirose River, and at a site at the outlet of the Okura reservoir in a tributary. The relative trophic level of each taxon was also estimated from δ15N, and a biomass pyramid for each community was produced. As a result, the percentage of autochthonous energy of benthic faunal community formed a longitudinal distribution along the river, showing a high peak midstream with lower values both upperstream and downstream. This profile was similar to a longitudinal pattern of that of Fine Particulate Organic Matters (FPOM) in flowing water, a main resource for filtering-collectors that dominate these communities. At the Okura reservoir outlet, the percentage of autochthonous energy of the community was the lowest among the four sites, reflecting a low percentage of autochthonous energy in FROM due to the decomposition of allochthonous organic matter in standing water. The biomass pyramid for each community showed an energy peak in the middle trophic level.
