諌早湾潮止め後の有明海における二枚貝群集の変化

Abstract

The inner part of Isahaya Bay was isolated from the rest of Ariake Bay by a reclamation dike in April, 1997. Thereafter, several changes in environmental conditions, such as a drastic increase in red tides and hypoxic water, were observed in Ariake Bay. We periodically monitored the number of species and mean densities of bivalves in two regions: (1) 20 fixed stations around the mouth of Isahaya Bay from June, 1997, to November, 2003; and (2) 24 fixed stations around the mouth of the Mizunashi River, Shimabara, located about 20 km south of the mouth of Isahaya Bay, from November, 1995, to December, 2001. In June, 1997, hypoxic water masses appeared in both the Isahaya and Shimabara regions, and most of the bivalve species distributed near the hypoxic regions decreased rapidly from 1997 to 1999. The number of species and mean density of bivalves reached a minimum in June, 2000, in the Isahaya region and in December, 1999, in the Shimabara region. Thereafter, since 2000, these values have been increasing in both regions. In the Shimabara region, most of the species dominant before 1999 recovered by December, 2001. In the Isahaya region, however, hypoxic water also recurred in June of both 1999 and 2001; there, only a few bivalve species increased rapidly at first and then subsequently decreased in abundance until November, 2003. In conclusion, if there is no amelioration of the environmental changes that have been occurring in Ariake Bay since 1997, we predict that a few opportunistic species of bivalve will continue to increase rapidly while other species disappear completely from the Isahaya region.