流入濁質がダム貯水池水環境に及ぼす影響

抄録

In the present study, we investigated influence of turbid water on water environment in the Asahireservoir, Nara prefecture, Japan, by some field surveys and experiments.
Chemical contents in new sediments collected with sediment traps were analyzed throughout the year. As aresult, carbon, nitrogen and phosphorus contents in sediments during storm run off period were 30-40%lower than those during dry period. In addition, it was proved that nitrogen in the sediments was decomposedinto dissolved nitrogen, but phosphorus was not decomposed practically. Oxygen consumption rate in bottomsediment in December, 1997 after storm events was in the range from 0.27 to 2.07 mgO₂ m^-2 h^-1 which was lessthan one-fifteens of the rate in December, 1996. This was thought to be caused by new sediments coveringlake bottom. Turbid waters also transformed an aspect of phytoplanktons in the reservoir, namely, dominantspecies changed in turns into Cryptophyceae Rhodomonas sp. and Cryptomonas sp., Chlorophyceae Eudrinaelegans and Pandrina morum, and Bacillarisphyceae Melosira spp. after the rise in turbidity. Thesephytoplanktons except Melosira spp. have two flagellum, and can stay in water surface by swimming withthem. Therefore, it was inferred that the rise in turbidity decreasing underwater light intensity caused thetransition of dominant species to the flagellates.
It is no doubt that inflow of turbid waters shallow lakes and reservoirs, and promote eutrophication in the longterm. In the short term, however, it may also have anti-eutrophication effect such as decrease of nutrientsloading from lake sediment. In any case, turbid waters flowing into lakes and reservoirs lead their waterenvironment into unstable state, and impact on water quality, benthic environment and aspect ofphytoplanktons in a short period.