Abstract
The material cycling in the Yoshino River with many dams was investigated on the basis of measuring bio-elemental concentrations, carbon and nitrogen stable isotope ratios. In the main river, there were not large differences in water quality between the upstream and just the downstream of these dams. The nitrogen concentration increased in the middle to the downstream of the main river, where human activity in the river basin increased. δ15N of the river sediment also increased with the increase of the nitrogen concentration. These results indicated that the nitrogen loading by the human activity in the river basin was the main cause to increase the nitrogen concentration in river water. But, organic substance in the river sediment did not increase, and therefore the material load by the human activity did not accumulate in the river. However, judging from high δ15N, it seemed that the nitrogen load to the Yoshino River was close to an upper limit of its ecosystem capacity. On the other hand, in the downstream of the Dozan river where its main stream was dammed up by a series of dams, the concentration of organic substance, δ13C and δ15N were high, and therefore the progress in eutrophication was found in the river bed. The water resource exploited by these dams is usually sent to the next basin and the downstream of the Dozan River has only a small amount of water flow, and it may have promoted its eutrophication by increasing the concentration of organic matter and nitrogen.
