Organic complexation and speciation of iron in the South and Equatorial Atlantic

Our investigation into the organic complexation and speciation of dissolved Fe in water column samples taken during the 1996 Intergovernmental Oceanographic Commission Baseline Expedition to the South and Equatorial Atlantic Ocean confirms the ubiquity of Fe-complexing ligands. In most samples Fe is greater than 99% organically complexed by a ligand with a conditional stability constant (KFeL,Fe′′) between 1012 and 1014. The deep-water masses exhibit no systematic differences with respect to ligand concentration or percent organic complexation. Greater than 2% of total dissolved Fe was present as inorganic Fe in some surface waters (1 m). The Amazon River does not appear to be a major source of these Fe-complexing ligands to the open ocean. Therefore, even in this area of relatively high atmospheric deposition, the majority of dissolved Fe is organically complexed by a ligand of unknown origin, and phytoplankton could potentially be Fe-limited if they only respond only to inorganic Fe.