Abstract
Based on quantitative survey data of 108 rivers and 724 freshwater sites collected during a national census of river environments, we determined the frequency distribution of total invertebrate biomass (total biomass) of riffles. We also examined the spatial pattern of total biomass by classifying sites according to channel gradient, bed substrate, and region. The median value of total biomass (in wet-weight) among all sites was 19.56 g m-2 for spring-winter and 10.04 g m-2 for summer-autumn. These values were similar to those previously reported for Japanese rivers, but were several times higher than those reported for rivers from other countries. The total biomass was lowest for sites with gentlest gradient classes (<1/3200) and larger for sites with steeper channel classes of up to 1/800-1/400. The total biomass was also lowest for sites with the finest substrate classes (silt, sand: <2 mm) and was larger for sites with coarser substrate classes up to small cobbles (5-10 cm). Total biomass was also larger for sites in southwestern regions (especially, Chugoku, Shikoku, Kyushu) than in northeastern regions (especially, Tohoku, Kanto, Hokuriku) with 2.7-6.6 fold differences in median values between these regions. Many taxa showed a high preference (i.e., disproportionately higher occurrence and abundance) for the slope and substrate classes with larger total biomass, and there was also a positive correlation between their own density and total biomass. These patterns were most conspicuous for net-spinning caddisfly taxa, which are likely to contribute significantly to the total biomass. According to habits of these taxa, bed characteristics such as amount of interstices and bed stability are likely to be factors controlling total biomass. A positive correlation between total biomass and number of genera suggests that many genera coexist in riffles with high total biomass, rather than these riffles being occupied by a few genera.
