Relationship between Red Tide Occurrences in Four Japanese Bays and Dam Construction

Abstract

Since about half a century ago, red tide has been occurring in many coastal places of Japan, such as Tokyo Bay, Ise Bay, Osaka Bay, and Ariake Sea. Red tide is algal accu mulation that could be a result of eutrophication in bays and lakes. At the same time, dams have been constructed in Japan on rivers that flow into the bays where red t ide has been occurring. The correlat ion between red tide occurrence and dam construction in Japan was researched using the data of many government organizat ions. The results indicate that the construction of dams influences the occurrences of red tide. When a dam is built on a river, there is a tendency for red tide to result in an estuary of that river a few years later. The number of red tide occurrences is related to the surface area of the dam: as the surface area o f a constructed dam increases, the number of red tide occurrences in a bay increases. Thus, the construction of dams seems to cause eutrophication in bays and lakes. Because it seemed that small particles flo wed fro m dams contain nutrients that stimu late the growth of algae.

Distribution of Sand Particles Along the Shoreline of Lake Biwa in Shiga Prefecture and Considerations from Lake Biwa and Seto Inland Sea, Japan

“…7 and 10, red tide occurred in the areas where particles smaller than MdΦ4 were deposited, such as in Osaka Bay, Harima-nada Sea, Hiuti-nada Sea, Hiroshima Bay, Oita Bay and Buzen Sea. These findings support my hypothesis that red tide is caused by mud carried out to sea by rivers on which dams are newly constructed 11,12 and demonstrate that heavier loading of fine particles (MdΦ< 4) is observed in the sea areas after the completion of a dam.…”

“…It was estimated that nutrients associated with the mud brought about eutrophication in these sea areas. Previous papers have shown that red tide tends to occur in areas influenced by the influx of mud carried by rivers on which a dam has been newly constructed 11,12 . The results obtained in this paper are consistent with the results of these studies.…”