Abstract
Macrobenthic fauna at 2 sites on tributaries of the Koise River system was compared to assess the effects of river water toxicity revealed by the shrimp-test on the fauna. Net-stones (4 stones in a net bag) were placed on the riverbed at each sampling site. Macrobenthos collected at the downstream site, where high water toxicity was found, were composed exclusively of several dominant species: 2 species of Trichoptera (both insecticide-resistant), Dipteran Antocha bifida and Chironomidae (several unidentified species), and insecticide-susceptible Baetis thermicus (Ephemeroptera). The continuous existence of B. thermicus in the toxic environment might be due to natural night drift or passive drift independent of the physiological diurnal cycle; the insects were inert and had a slightly opaque body color. At the upstream site, where the shrimp mortality in the river water was comparatively low, the number of macrobenthic taxa was twice that at the downstream site. Sampling using net-stones on the sandy riverbed was efficient for assessing water toxicity to macrobenthos.
