Division of substrate groups and gammaridean amphipod assemblages in sublittoral Ariake sound [Japan]: A comparison between 1997 and 2002

The artificial closure of the innermost Isahaya Bay, which is an embayment situated in the inner Ariake Sound, western Kyushu, Japan, was conducted in April 1997. In June 1997 and June 2002, benthic survey was made using a Smith-McIntyre grab with a 0.05-square m sampling area at a total of 88 stations established over the entire sublittoral area of Ariake Sound. Of the macrobenthic community, the species composition, individual density, and distribution of the gammaridean amphipod assemblage were described for the two years in relation to the substrate grain-size compositions. For the substrates, no noticeable changes were observed in the distribution of median diameter and silt-clay content of sediments and in the spatial arrangement of the three sediment groups classified based on both a group-averaged clustering and a non-metric multidimensional scaling. For the amphipod assemblage, a total of 12,434 individuals of 94 species belonging to 24 families were collected in 1997, with the most dominant species being Photis longicaudata (Family Isaeidae). In 2002, those values were 37,649 individuals, 89 species, and 27 families, with the most dominant species being Corophium sp. A (Family Corophiidae). In 1997, the 11 most dominant species comprised 72% of the total number of individuals, while in 2002, the six species and 11 species accounted for 71% and 86%, respectively. Of these 11 species in 2002, seven species (Corophium sp. A, Photis reinhardi, Gammaropsis utinomii, C. crassicorne, Ericthonius pugnax, Corophium sp. B, Lembos clavatus) have remarkably increased their total numbers of individuals from 1997 by a factor of 5.7 to 152.0. Except for L. clavatus, these species belong to Corophiidae or Isaeidae. The 11 most dominant species in each year mainly occupied the largest sediment group of Ariake Sound distributed along its longitudinal axis and outside Isahaya Bay, with poorly-sorted medium to coarse sand. In this area, the above-mentioned six dominant species of Corophiidae or Isaeidae have expanded their distribution ranges in 2002.