Organic iron(III) speciation in surface transects across a frontal zone: The Chatham Rise, New Zealand

Iron (Fe) is a critical nutrient in marine systems and the organic complexation of Fe is a central factor of the marine biogeochemistry of Fe. In the present study, total dissolved Fe and its organic speciation were measured in filtered seawater samples (<0.2 ?m) collected along three surface transects across the subtropical (ST) front, east of New Zealand, in austral spring (October 2000). Total dissolved Fe concentrations were low (∼0.1 nm) in the subantarctic (SA) waters. The highest Fe concentration (∼0.8 nm) was observed at the mixing boundary north of the Subtropical Convergence (STC) and then decreased relatively quickly both southward and northward. Cathodic stripping voltammetry was used to determine Fe speciation. The dissolved Fe(iii) was fully complexed (>99.9%) by natural organic ligands, which were found to occur in excess of the dissolved Fe concentration at 1.29 ± 0.33 nm (equivalent to an excess over Fe of ∼1.0 nm), and with a complex stability of log K ′ FeL,F e 3+ - > K?_FeL,Fe3? ≤ 22.67 ± 0.22. The total ligand concentrations were consistently higher (∼0.5 nm) in the ST and STC waters than in the SA waters. Our Fe data imply that the regional currents may be an important vehicle for transporting the elevated Fe across the front.