Abstract
Nitrogen and phosphorus have been treated as important substances related to the eutrophication and the water quality such as red tide. However, silicon is one of substance to compose an organic matter (Redfield ratio; C:N:P:Si=106:16:1:16). Recently, it was reported that Si variation causes the change of the species composition of phytoplankton in estuary. This phenomenon effects on the ecosystem balance. One of the reasons that the Si supply from river to sea decreases is assumed to dam constructions. However, the mechanism of Si decrease in river is not clarified due to few observation data of Si in river basin. In this study, we observed the temporal and spatial variations of DSi (Dissolved Silicate) concentration of surface waters (stream, pond, spring, river) from 2000 to 2003 in the Kinokawa river basin. From this result, we can see that DSi of pond water varies seasonally. On the contrary, DSi of spring water and river water are almost constant all through the year. Average value of DSi of river water is lower than that of other waters. Moreover, as compared our data with the DSi concentration of river water observed in 1957 (before dam constructions), we find that DSi concentration decreased.
