62 巻 (2007) p. 17-33
In April, 1997, construction of a dike in the inner part of Ariake Sound to close off the innermost area of Isahaya Bay was completed. In June of both 1991 and 2002, benthic surveys were conducted using a Smith-McIntyre grab sampler at 88 stations over the entire sublittoral area of Ariake Sound. The structure of the gammaridean amphipod assemblage was compared between 1997 and 2002 with regard to any change in grain-size composition of the substrates and dissolved oxygen concentration of the bottom waters. No noticeable changes were observed in the distribution of the median particle diameter and silt-clay content, nor in the spatial arrangement of the sediment types of the substrates. Hypoxic water masses were observed in the bottom waters in 1997 in and around Isahaya Bay and in the southern part of Ariake Sound. Other studies also recorded hypoxic water masses in the former area in 1999 and 2001. For the amphipod assemblage, a total of 12, 434 individuals of 94 species belonging to 24 families was collected in 1997; the most dominant species was Photis longicaudata (family Isaeidae). In 2002, those values had changed to 37, 649 individuals, 89 species, and 27 families, with the most dominant species being Corophium sp. A (family Corophiidae). In both years, the amphipod assemblage was dominated by six families, including Isaeidae, Ampeliscidae, Corophiidae, Priscomilitaridae, Melitidae, and Aoridae, although the dominance order changed partially. The amphipod assemblage in both years inclusive was divided into eight groups using group-averaged clustering and non-metric multidimensional scaling. The group with the highest number of stations occupied the largest bottom area, composed of poorly-sorted medium to coarse sand along the longitudinal axis of Ariake Sound; the number of stations in this group increased from 41 in 1997 to 57 in 2002. Increase in individual density and expansion of distribution range of seven species of the superfamily Corophioidea (Corophiidae, Isaeidae, Priscomilitaridae), which occurred around the two hypoxic areas, contributed most to the range extension. For the above-mentioned six most dominant families, density of individuals and proportional composition of number of individuals in each of the two hypoxic areas and other, non-hypoxic areas were compared between the two years. In particular, both values remarkably increased for several members of the Corophiidae, including Corophium sp. A and C. crassicorne, in 2002; the exception was the value for density of individuals in the southern hypoxic area. The Shannon-Wiener species diversity index values were lower in 2002 for both the two hypoxic areas and the combined non-hypoxic areas, and for the three main groups of the amphipod assemblage.