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-μm, 0.025–0.22-μm, <0.22-μm and >0.22-μm fractions) and Fe(III) hydroxide solubilities were studied during a spring phytoplankton bloom (February to April, 1995) in Funka Bay, Japan. “Soluble Fe” (<0.025-μm fraction) concentrations were approximately 70% of the <0.22-μm Fe fraction with the exception of a few deeper samples. During the spring bloom, the vertical profiles of soluble Fe were variable (0.3–13 nM), probably resulting from temporal variations of atmospheric input, bottom sediment resuspension, decomposition of sinking organic matter and biological removal in this bay water. High “colloidal Fe” (0.025–0.22-μm 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 low 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 the 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 bloom. 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.