We examined the larval population densities and biomass of a caddisfl y grazer, Micrasema quadriloba, and the abundance and community structures of periphyton at a segment scale (7.4 km with four study sites), along a second- to fourth-order Japanese mountain stream through- out the grazer’s life cycle. In the uppermost riffl e of the study segment (site 1), periphyton abundance was kept at low levels when the larvae occurred. The larval distribution spread downstream as larvae developed from fi rst instars in May to fi fth instars in January. We performed multiple regression analyses to test the effects of environmental variables and larval biomass on periphyton abundance in both the riffl e of site 1 and the study segment; the results revealed that the larval biomass was signifi cantly negatively correlated with periphyton abundance similarly in both the riffl e and the study segment. In addition, both the correla- tion and community analyses showed that the larval biomass was signifi cantly negatively correlated to the rela- tive abundance of large and/or fi lamentous microalgae, which appeared in the uppermost layer of the periphyton mat, and that larval biomass was signifi cantly positively correlated to the relative abundance of small diatoms, which strongly adhered to the substrate. Thus, the present study implied that the grazing of M. quadriloba larvae would regulate the abundance of periphyton in a riffl e and also regulate the abundance and community structure of periphyton at the segment scale with the expansion of their longitudinal distribution.
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