Abstract
On Midori River Tidal Flat and Arao Tidal Flat, which are major sandy tidal flats in Ariake Bay, Kyushu, Japan, I examined various heavy metal contents of the surface sediment to reconfirm that, among the heavy metals, only manganese deposited on the sediment at extremely high levels. On these two tidal flats, 790 to 2,230 mg kg−1 dry sediment of total manganese was detected in the surface sediment. The manganese deposited (mainly as manganese dioxide) in the sediment tends to be reduced by bacterial activities, released as manganese ion to the interstitial water of the sediment, and weakly bound electronically in an exchangeable form with organic or inorganic matter in the sediment. I also examined the manganese ion concentration of the interstitial water and the content of the exchangeable form of manganese of the sediment. High concentrations of manganese ions, as affect the physiology of the aquatic organisms, were detected from the interstitial water of the sediment on Midori River Tidal Flat (5.8 to 9.9 mg L−1), while high contents of the exchangeable form of manganese (approximately 28 mg kg−1 dry sediment) were found from the sediment on Arao Tidal Flat. It seems probable that the manganese ions reduced from the manganese in the sediment acts as one of the major limiting factors on the occurrence of the clam on these two tidal flats.
