Interannual variation from 1997 to 2015 in bottom sediments and community structures of macrobenthic fauna in the Ariake Sea, western Kyushu, Japan

Abstract

: In order to evaluate the effect of 35.5 km 2 of land reclamation in Isahaya Bay in the inner part of the Ariake Sea, Japan, we analyzed the interannual variation in the bottom sediments and community structures of macrobenthic fauna across the whole area of the Ariake Sea. Sediment samples were collected from 82 fixed subtidal stations consisting of four parts （ inner, central-north, central-south, and mouth parts ） in June 1997 （ two months after the complete closure of the inner Isa haya Bay by the 7-km dike ） , June 2002 （ two months after the temporary recovery of limited tidal currents within the inner Isahaya Bay by the opening of the water gates in the dike ） , June 2007, and June 2015. The median particle diameter of the bottom sediments decreased at many stations around the central to mouth parts of the Ariake Sea from 2002 to 2007, and it increased from 2007 to 2015. A total of 29,885 macrobenthic invertebrates （ mean density 7,289 ind./m 2 ） were collected from the 82 fixed subtidal stations in June 1997, and the most dominant taxa were polychaetes, gammaridean amphipods, ophiuroids, and bivalves. In June 2002, 62,314 individuals （ mean density 15,199 ind./m 2 ） were collected from the same sta tions. However, the number and density of the macrobenthos declined to 11,992 （ 2,925 ind./m 2 ） in June 2007 and 17,669 （ 4,310 ind./m 2 ） in June 2015. A similar pattern of the variation （ i.e., an increase from 1997 to 2002, and a decrease from 2002 to 2007 and 2015 ） was commonly observed in the densities of each dominant taxon, and for all macrobenthos in each of the four parts of the Ariake Sea, although the variation was more remarkable in the inner and central-north parts than in the others. Our results suggest that the macrobenthic community has declined from 2002 to 2007 and 2015 not only around Isahaya Bay （ in the inner part of the Ariake Sea ） , but across the whole area.