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

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 accumulation 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 tide has been occurring. The correlation between red tide occurrence and dam construction in Japan was researched using the data of many government organizations. 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 of 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 flowed from dams contain nutrients that stimulate the growth of algae.