Environmental Context. Iron is an essential element for life in the world's oceans, and in some regions its concentration limits the growth of phytoplankton. The amount of iron(iii) which is available to an organism depends on the exact chemical form in which it exists, for example as dissolved ions or associated with organic compounds. There are widespread reports that marine iron(iii) is predominantly bound in extremely strong complexes. We show that such claims might be the result of an artefact of the measurement technique, CLE-AdSV. Ensuing ideas about the iron biogeochemistry in marine systems might require reconsideration as well.

Abstract. Iron(iii) speciation data, as determined by competitive ligand exchange?adsorptive stripping voltammetry (CLE-AdSV), is reconsidered in the light of the kinetic features of the measurement. The very large stability constants reported for iron(iii) in marine ecosystems are shown to be possibly due to an artefact of the technique, arising from the assumption that equilibrium is achieved between all iron(iii) species of relevance. Particular kinetic properties, related to the special nature of hydroxide as a metal complexant, have the consequence that CLE-AdSV measurements of iron(iii) in seawater generally correspond to the hydroxide complexes only. By the same token, dissolved hydroxide complexes are the key components of the bioavailable iron(iii) pool. The analysis presented herein opens opportunities to exploit CLE-AdSV for more rigorous investigation of the links between the speciation and the bioavailability of iron(iii).