Abstract
From August 2006 to December 2007, the concentrations of silicate (Si(OH)4), phosphate (PO4), and dissolved cadmium (Cd) were monitored in the surface water at three stations in the coastal zone off Miyara River, Ishigaki Island and the mouth of the Miyara River. The distances of offshore stations from the shoreline were approximately 1800, 2700, and 3900 m, respectively. From the results of the surface-water temperature and salinity, offshore sites were classified into three seasonal water mass groups, namely, from December to May, June to October, and November. From December to May, the surface-water temperature was relatively low, and the surface-water salinity was relatively high. On the other hand, from June to October, the water temperature increased, and the salinity scattered in a wide range towards the low salinity zone due to the influence of relatively large precipitation in this season. In November, the T-S diagram intermediated between those two groups. The concentrations of Si(OH)4, PO4, and Cd at the offshore sites were extremely small (the maximum concentrations observed were 7μM, 0.13μM, and 32 pM, respectively) and showed little seasonal variations, suggesting that the influences such as eutrophication and heavy metal pollution upon the adjacent coral reef ecosystem could not be recognized in the present day.
