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 degreesS at 20 degreesE. Dissolved iron concentrat ions were low at 0.1-0.6 nM, with average concentrations of 0.25 +/- 0.13 n M. Organic iron complexing ligands were found to occur in excess of the dis solved 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 colu mn (top 200 m) suggesting in situ production by microorganisms, and less at the surface consistent with photochemical breakdown. Our data are consiste nt with the presence of stable organic iron-complexing ligands in deep glob al ocean waters at a background level of similar to0.7 nM. It has been sugg ested that this might help stabilise iron at levels of similar to0.7 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 o f iron (by scavenging or biological uptake) to well below the concentration of these ligands. Scavenging reactions are probably inhibited by such liga nd competition, so it is likely that biological uptake is the chief cause f or the further removal of iron to these low levels in waters that suffer fr om very low iron inputs. (C) 2001 Elsevier Science Ltd. All rights reserved .