有明海の赤潮頻発に端を発する生態系異変のメカニズム

抄録

Ariake Bay, located in the western coast of Kyushu, Japan, has been praised as “fertile sea” due to its extremely high production of various sea foods. However, since the late 1990s, red tide has occurred about twice as frequently as before, although the total nutrient discharge from the lands surrounding the bay has even decreased slightly since the 1970s. The increase of organic load on the sea floor by the frequent occurrence of red tide has caused seasonal hypoxia in the bottom water, and brought mass mortality of benthic animals in a wide area of the inner parts of the bay. These phenomena have occurred commonly in the enclosed coastal seas that have been suffering from eutrophication throughout the world. In this review, I collected various information on the occurrence of red tide, nutrient discharge, water conditions when red tide occurred, characteristics of the tidal current, etc., in Ariake Bay from the previous studies, and tried to clarify the mechanism how the red tide have occurred frequently, which is the most responsible for the recent marked decline of the benthic ecosystem in the bay. I focus on the impacts of the closure of the dike in a reclamation project conducted in the inner part of Isahaya Bay (an inner bay of Ariake Bay located in its western side of the middle part of the bay) in April 1997 not only on the tidal current in Isahaya Bay, but also the anticlockwise residual current that originally dominates in Ariake Bay.