| Anthropogenic modifications have disrupted aquatic ecosystems by modification of the flow regime and transport of sediment. The impacts of dams are believed to be very large, but, despite the abundance of dams in monsoonal Asia, great scientific uncertainty still exists about the effect of dams on macroinvertebrates. Therefore, we studied macroinvertebrate assemblages at three reaches (downstream of a dam, downstream of a confluence and on a tributary) of the Yahagi River, central Japan, to confirm the impact of long-term impoundment on the relationships between macroinvertebrate assemblages and biotic and abiotic environmental factors and also the role of a major tributary in community shift. Four Surber samples and associated physical measurements (depth, velocity and substrate composition) were taken from four study sites (riffles) at each study reach during the period from 10 to 21 February 2004. Drifting materials (zooplankton, POM and bedload sediment) and periphyton were also sampled. Significant differences were found in the macroinvertebrate fauna of these different reaches. Faunal distributions downstream of the dam were severely altered, with high macroinvertebrate abundance and low taxa richness in contrast to those in the tributary. The two-way indicator species analysis (TWINSPAN) classification clearly distinguished these samples from the others. Based on nonmetric multidimensional scaling (NMS) ordination and correlation analysis (environmental variables vs. NMS axis score), these differences in community structure reflected changes in substrate composition and quantity and quality of suspended and benthic FPOMs. Our observations suggest that while dam-derived zooplankton and reduced sediment transport had a great impact on the fauna, the tributary inflow, with a catchment size of less than 20% of that above the dam, acted as a major source of sediment input and facilitated macroinvertebrate community recovery. |
