Abnormal water temperature downstream dam construction and its effects on river biota in some Japanese rivers.

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Effects of dam construction on biota in its lower reaches were studied in some Japanese rivers such as the Tenryu,(Shizuoka Pref.), the Hida (Gifu Pref) and the Kuma (Kumamoto Pref) Rivers.For species composition of primary producer, effects of low temperature water originating from upper dam impoundment on distribution of freshwater Rhodophyta was detected. Biomass of attached algae in riverbed was increased in summer just downstream dam construction, though maximum algal biomass was recorded in winter in reference rivers without such constructions. On the other hand, no significant difference of primary production rate was detected between rivers with dams and without such constructions.Density of aquatic insects, except for Chironomidae (diptera), down reaches dam impoundments was low through the year in contrast with reference rivers. Both concentration and loading of particulate organic matte originated from dam impoundment were rapidly decreased longitudinally along the stream. Based on the observations of dominance of Trichoptera with net and of the content of their guts, consumption by aquatic insects contributed to decrease of organic matter.The species composition of algae in guts of P1aoglossus altivelis which feed on attached algae showed significant difference between rivers with dam and that without such constructions. Filamentous Cyanophyta especially Homeothrix janthina in former and diatoms in latter were detected respectively Index of fatness (weight/(body lengths)^3 X 1,000) of P. altivelis in reference rivers were larger than that in a river with dam constructions.