Changes in distributional patterns in water quality, bottom sediments, and community structures of bivalves, gammaridean amphipods, and polychaetes in the Ariake Sea after dike construction for the reclamation of Isahaya Bay, western Kyushu, Japan

In the inner part of the Ariake Sea, Kyushu, Japan, several environmental changes caused by hypoxia and red tides have been observed after the dike construction for the reclamation of Isahaya Bay in 1997. We examined the changes in water quality, bottom sediments, and community structures of major benthic macrofauna (bivalves, gammaridean amphipods, and polychaetes) using samples obtained from 88 fixed stations covering the whole area of the Ariake Sea between June 1997(two months after the complete closure of the inner part of the Isahaya Bay by the dike construction) and June 2002(two months after the temporary opening of the water gates in the dike). Our results showed that the distribution of benthic macrofauna was altered by spatial patterns of mud content and median grain size of bottom sediments. Hierarchical cluster analysis identified seven macrobenthic animal assemblages distributed in the innermost and central-east parts (groups I to III), inner-south to central-west parts (groups V to VII), and in the mouth of the Ariake Sea (group IV) in 1997. However, in 2002, group VI (characterized by abundance of Photis reinhardi, Gammaropsis utinomii, Corophium sp. A, and Modiolus comptus), expanded its distribution into the central-west and inner-south parts of the Ariake Sea, where mud content of bottom sediment changed to 0–1% and hypoxia had occurred in the bottom waters from 1997 to 2002. These results suggest that the changes in macrobenthic community structures are altered by changes in bottom sediments and the occurrence of hypoxia in the Ariake Sea.