Heavy Metal Contamination in the Sediment and its Effect on the Occurrence of the Most Dominant Bivalve, Ruditapes philippinarum, on the Tidal Flats of Ariake Bay in Kumamoto Prefecture, the West Coast of Kyushu, Japan

抄録

Abstract: A short neck clam, Ruditapes philippinarum, is one of the most dominant fauna in the benthic communities on the sand tidal flats across the Japanese coast. On the tidal flats facing to Ariake Bay in Kumamoto prefecture, more than 60, 000tons of the clam were harvested per year in the 1970s. However, the dense populations collapsed on the tidal flats in the 1980s to 1990s. The results of the recent studies on the clam showed that its population could recover only at the sand covers on the tidal flats, which were created with the sand collected from the sea floor of the offshore areas, while the young juveniles of the clam still suffered from extremely high mortality on the sediment of the sand flats. We focussed on the concentrations of heavy metals(Mn, Zn, Cu, Cd, Pb)in the sediments as contaminants imposing a negative impact on the physiology of the clam. The sediments on the tidal flats where the clam populations collapsed contained 1, 700 to 2, 900 μg/g of manganese, although the concentrations of the other four heavy metals of the sediments were lower than 63 μg/g.The results of this study indicated that the dense patches of the clam were not established on the sediments containing(at least)more than 1, 000 μg/g of manganese. It is very likely that the elevation of the concentration of manganese in the sediment is responsible for the collapse of the Ruditapes populations on the tidal flats.