Temporal changes in benthic amphipod assemblages in the central part of the Ariake Sea during the five years following the dike closure in Isahaya Bay, Japan

Temporal changes in benthic amphipod assemblages after the dike closure in Isahaya Bay were investigated using amphipod samples collected from the central part of the Ariake Sea between 1997 and 2002. Amphipod densities decreased every year from June 1997 (5,207 ind. m−2) to June 2001 (413 ind. m−2) but increased abruptly in June 2002 (22,944 ind. m−2). Non-metric multidimensional scaling (nMDS) ordination of sampling occasions showed that amphipod assemblages were divided into three groups (A: June 1997 and June 2002; B: November 1998, June 1999 and June 2000; C: November 2000, June 2001 and November 2001). Group A was dominated by species of Corophiidae and Photidae, and group B and C were dominated by species of Ampeliscidae. In terms of total amphipod density, the order was as follows: group A>group B>group C. These differences among groups A–C indicate that corophiid and photid species contribute significantly to the fluctuation of amphipod density and assemblages in the central part of the Ariake Sea. In fact, an explosion of the amphipod density that occurred in June 2002 was attributed to rapid increases in the populations of Crassicorophium sp. A, Crassicorophium crassicorne and Photis reinhardi. Judging from the changes in the bottom sediment grain size, we conclude that the grain-size reduction on the east side of the study area led to a rapid increase in the density of these opportunistic species.