The Iodide and Iodate Distribution in the Seto Inland Sea, Japan

We report the vertical and horizontal distributions of inorganic iodine (iodide and iodate) and their related species (bromide nitrate and nitrite) in the Seto Inland Sea, which is a semi-enclosed coastal sea area of western Japan. In this study, ion chromatography with ultraviolet detection was employed to determine the iodide, iodate, bromide nitrate, and nitrite levels simultaneously in a single run. Iodide was higher at inshore sites than at offshore sites. Vertical profiles showed that iodide increased in the bottom layer of inshore sites of Osaka Bay and Hiroshima Bay, but were low in the bottom layer of the Kii Channel, the main channel connecting Osaka Bay with the Pacific Ocean. Iodates were low in the low-salinity inshore surface, but were high in the bottom layer of the Kii Channel. The riverine flux of iodine to the coastal marine environment was negligible. The vertical profiles of total inorganic iodine (iodide + iodate) looked uniform; however, plots of total inorganic iodine versus salinity demonstrated a net loss of total inorganic iodine in the low-salinity inshore surface. The iodine distributions in the Seto Inland Sea could be explained by three-end-member mixing, with one member being non-iodine river water, another high-salinity and high-iodate water of the open ocean, and the final high-iodide and low-iodate inland water with a salinity of around 32–33.