Diel variations in iron speciation in northern Australian shelf waters

Iron in northern Australian shelf waters was found predominantly in fine (< 0.2 μm) 8-hydroxyquinoline-reactive form or in larger (1.0 μm) particulate form. Partitioning into the larger size fraction dominated during a period of high turbidity following a cyclonic resuspension event. Only a small component of the iron pool reacts rapidly with the strong ferrous binding agent ferrozine. The concentration of the ferrozine-reactive component is strongly dependent on light intensity with maximum concentration observed at peak light intensity. The close correlation between ferrozine-reactive iron concentration and light intensity suggests a fine balance between Fe(III) reduction and Fe(II) oxidation with the steady state concentration observed being strongly influenced by light induced changes in redox kinetics. An observed lack of association between particulate iron concentrations and the concentration of ferrozine-reactive iron suggests that the soluble rather than the particulate iron pool is most influenced by light. The chromophore may be an Fe(III)-organic complex with the strong iron binding ligand that is now recognised to be present in seawater.