Two or three replicates of benthic macroinvertebrate community were taken from rifles at each station using a Surber sampler (the area: 25 cm?25 cm or 50 cm?50 cm) with a net (38 mesh). The macroinvertebrates were kept in the plastic vessels containing 80%
ethanol on the site and then identified to the lowest possible taxonomic level using a stereoscopic microscope according to Tsuda (1979), Kawai (1985), Ueno (1986)
and Merritt and Cummins (1996). The ranges of individual number (?) per 0.25m2 in each family identified were classified into six groups to reduce the influence of excessive numbers in any predominant taxa on multivariate analysis, and then the abundance scores from 0 to 5 were distributed into each range as shown in Table 2. In order to summarize the information on benthic macroinvertebrate community structure at seven stations, PCA was conducted based on the z-standardized value of the average abundance scores in each season.