On April 14, 1997, the inner part of Isahaya Bay was isolated from the Ariake Sea by a dike for reclamation. Thereafter, red tides and episodes of hypoxia began to occur frequently in the Ariake Sea. The government introduced seawater inside of the dike from April 24 to May 20, 2002, but the reclamation project was continued and finally completed on November 20, 2007. We periodically monitored the grain-size of bottom sediments and the benthic macrofauna using sediment samples collected from (1) 32–50 fixed stations around the inner part of the Ariake Sea in June or November every year from June, 1997, to June, 2007, and (2) 88 fixed stations throughout the entire Ariake Sea in June of 1997, 2002, and 2007. During the decade between 1997 and 2007, the median particle diameter of the bottom sediment decreased at many stations around Isahaya Bay and also in some stations around the mouth of the Ariake Sea. As for the benthic macrofauna, a total of 38,221 individuals were collected from the 88 fixed stations in June, 1997, and the most dominant taxa were polychaetes, gammaridean amphipods, and bivalves. In June of 2002, 74,273 individuals of benthic animals were collected from the same stations, but in June of 2007, just 12,386 individuals. Some opportunistic species such as Modiolus (M.) comptus and Corophium spp., those inhabit fine sediment around the mouth of Isahaya Bay, became abundant in June, 2002, but most of them disappeared by June, 2007 because of hypoxia and a change in the composition of the bottom sediments.
