Abstract
Dam construction has negative impacts on river ecosystems, but we do not understand well how and why it influences riverine macroinvertebrate assemblages. The purpose of this study was to determine the influences of the Oishi Dam (Sekikawa Village, Niigata Prefecture, Japan) on downstream macroinertebrate assemblages with particular focus on net-spinning caddisflies. I conducted a monthly field survey at one site upstream and three sites downstream of the dam in Oishi River from April to December in 1994. The upstream macroinvertebrate assemblage was dominated by Ephemeroptera and Plecoptera, whereas the downstream assemblages by Trichoptera, especially net-spinning caddisflies Stenopsyche. Net-spinning caddisfly species were more abundant at the downstream sites than at the upstream site. Sestonic organic matter concentration in river water increased in summer at the downstream site that received much water directly from the reservoir, probably due to a large amount of drifting phytoplankton. Early instar larvae of Stenopsyche showed remarkably high density in summer at this site. Phytoplankton originated from the reservoir might enhance downstream population of Stenopsyche.
