| We investigated the biomass and community structure of benthic macroinvertebrates in the lower reach of Kurobe dam、 where bedrock exposures on streambeds have recently been increasing、 and in the upper reach without bedrock exposure、 to better understand the impact of channel degradation on a macroinvertebrate community in a mountain gravel-bed river. Based on quantitative samplings at 4 stations (a total of 6 sampling reaches) in the upper and lower reaches、 major contrasts in the macroinvertebrate community among habitats (riffles、 runs、 and pools) were evident、 with the macroinvertebrate biomass being greatest in riffles (4.6-fold greater than in runs and 8.6-fold greater than in pools). The macroinvertebrate community differed between the upper and lower reaches、 though the differences were not as large as those among habitats. Further samplings of riffles in the lower reach revealed that the macroinvertebrate biomass was greater in large-cobble than in small-cobble (with fewer sedentary taxa) and boulder riffles (with fewer taxa encountering underneath the cobbles). Macroinvertebrates were usually scarce in bedrock riffles except for blackflies that inhabited the substrate surface、 while community compositions at small gravel clusters (gravel patches) in bedrock riffles were similar to those in gravel-bed riffles. Based on the difference in the amount and composition of habitats and bed-substrates between the upper and lower reaches of the dam、 we estimated that the channel degradation potentially reduced the macroinvertebrate biomass up to 1/6 at an entire reach through alterations in habitats and bed characteristics. |
