Size-fractionated Iron Concentrations and Fe(III) Hydroxide Solubilities in Various Coastal Waters

The distribution of iron in two size fractions (<0•025 and <0•45μm) was studied in various coastal waters from northern Japan and in oceanic water, and it was found that estuarine waters and surface waters in Funka Bay (Hokkaido) had the highest colloidal iron concentrations (0•025–0•45-μm fraction, 7–19 nM), with a large portion (85–93%) in the <0•45μm fraction. The high colloidal iron concentrations in estuarine and bay surface waters were probably due to the rapid formation or contribution of colloidal iron through riverine and atmospheric inputs, respectively. Dissolved organic substances, such as humic substances and other organic Fe(III) chelators, possibly contributed to the formation and stabilization of the colloidal iron fraction in seawater. In addition, ultraviolet (UV)-irradiation treatment of coastal and estuarine waters reduced the iron concentrations in all size fractions (<0•025μm, 0•025–0•45μm, <0•45 μm) and the Fe(III) hydroxide solubilities, resulting from the decomposition of natural organic iron chelators. These results suggest that natural organic Fe(III) chelators control the colloidal and dissolved iron concentrations in seawater.