内海性浅海域の生物とその生態的機能を探る 有明海の砂質干潟における植物プランクトン食ギルドのなかの二枚貝資源―環境収容力制限仮説の提示

Abstract

On extensive intertidal sandflats in Ariake Sound, the yield of the Manila clam, Ruditapes philippinarum, considerably increased in the 1970s but has remarkably decreased since the early 1980s. A survey conducted in 2004 of the individual density and distribution of the large-sized members of the macrobenthic community on a sandflat located at the mouth of the Shirakawa River in the central part of the Sound demonstrated four dominant species: two thalassinidean shrimps (Upogebia major and Nihonotrypaea japonica), which inhabit deep-reaching burrows, and two bivalves (Mactra veneriformis and R. philippinarum). In the late 1970s the Manila clam population prevailed in high densities over the entire area, while its distribution was limited to the lowest zone in 2004. By contrast, the other three species have increased their densities and distribution ranges and may have outcompeted the Manila clam through competition for habitat space. In particular, the strong bioturbating activities of the two shrimps probably havenegatively affected the abundance of the Manila clam. The carbon and nitrogen stable isotope compositions of the four species and their potential food sources were analysed in 2003, specifying phytoplankton (mainly diatoms) as the exclusive food source and suggesting that the four species belong to the same feeding guild. The wet biomass of the populations of N. japonica, U. major, M. veneriformis, and R. philippinarum (the whole body for shrimps and the soft tissues for bivalves) over the sandflat was estimated at 315 t, 110 t, 400 t, and 237 t, respectively. In conclusion, it is hypothesized that the amount of phytoplankton determines the carrying capacity for the benthic community on the sandflat, both quantities having attained their maxima in the 1970s, and (2) the subsequent increased competition for space has caused a further decline in the biomass of the Manila clam population.