Variation of size-fractionated Fe concentrations and Fe(III) hydroxide solubilities during a spring phytoplankton bloom in Funka Bay (Japan)

Vertical distributions of size-fractionated Fe concentrations (< 0.025-mu m , 0.025-0.22-mu m, < 0.22-mu m and > 0.22-mu m fractions) and Fe(III) hydro xide solubilities were studied during a spring phytoplankton bloom (Februar y to April, 1995) in Funka Bay, Japan, "Soluble Fe" (< 0.025-mu m fraction) concentrations were approximately 70% of the < 0.22-mu m Fe fraction with the exception of a few deeper samples. During the spring bloom, the vertica l profiles of soluble Fe were variable (0.3-13 nM), probably resulting from temporal variations of atmospheric input, bottom sediment resuspension, de composition of sinking organic matter and biological removal in this bay wa ter. High "colloidal Fe" (0.025-0.22-mu m fraction) concentrations (5-8 nM) were found in subsurface water near the shelf sediment interface (50-80-m depths, bottom depth of 92 m) after the spring bloom. The vertical changes of Fe(III) hydroxide solubilities appeared to have a small variation with l ow values (0.2-0.3 nM) throughout the water column during intense vertical mixing before the bloom, and to increase slightly during the beginning of t he bloom (0.3-0.4 nM). The highest Fe(III) solubilities (0.8-0.86 nM) were found in the surface waters (2-10-m depths) during and after the peak of bl oom. The high Fe(III) solubilities may be due to higher concentrations, or a stronger affinity of natural organic Fe(III) chelators that were possibly released by phytoplankton or bacteria. (C) 2000 Published by Elsevier Scie nce B.V.