Organic complexation of iron in the Southern Ocean

The chemical speciation of iron was determined in the Southern Ocean along a transect from 48 to 70°S at 20°E. Dissolved iron concentrations were low at 0.1-0.6 nM, with average concentrations of 0.25 ± 0.13 nM. Organic iron complexing ligands were found to occur in excess of the dissolved iron concentration at 0.72 ± 0.23 nM (equivalent to an excess of 0.5 nM), with a complex stability of log K′_FeL = 22.1 ± 0.5 (on the basis of Fe³⁺ and L′). Ligand concentrations were higher in the upper water column (top 200 m) suggesting in situ production by microorganisms, and less at the surface consistent with photochemical breakdown. Our data are consistent with the presence of stable organic iron-complexing ligands in deep global ocean waters at a background level of ∼0.7 nM. It has been suggested that this might help stabilise iron at levels of ∼0.7 nM in deep ocean waters. However, much lower iron concentrations in the waters of the Southern Ocean suggest that these ligands do not prevent the removal of iron (by scavenging or biological uptake) to well below the concentration of these ligands. Scavenging reactions are probably inhibited by such ligand competition, so it is likely that biological uptake is the chief cause for the further removal of iron to these low levels in waters that suffer from very low iron inputs.