The trophic structure of a macroinvertebrate community was investigated using carbon and nitrogen stable isotopes analysis (δ13C and δ15N) at three longitudinal sites along the main channel of the Hirose River, Japan, and at an outlet site of the tributary reservoir. Along the main channel, primal food supply gradually changed from allochthonous to autochthonous source. The percentage of autochthonous origin of the macroinvertebrate community calculated by a carbon-mixing model (=38–83%) increased with that of suspended fine particulate organic matter (FPOM = 21–67%), a main resource for filtering-collectors that dominated the communities. The lowest percentage of autochthonous in the lake outlet community (=21%) was a result of the notably low percentage autochthonous of FPOM due to the decomposition of allochthonous coarse particulate organic matter (e.g., leaf) strained in the reservoir. Predators had higher mean δ15N than non-predators in three sites. The average enrichments of δ15N signature per trophic level for all sites were 3.82‰ from producers to first consumers and 1.24‰ from first to second consumers. Our data suggest the strong potential for stable isotope analysis to enhance our understanding of the trophic structure of stream macroinvertebrate community.

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