Iron organic complexation in the oligotrophic subtropical waters in the Tasman Sea

Abstract

As part of a larger program focused on understanding the biogeochemistry of the Tasman Sea, we participated in the Characterizing Oligotrophic Subtropical Waters (COST) voyage conducted in February 2005. Measurements of dissolved iron (Fe) and Fe organic speciation in the upper water column of two stations located south and north of the Tasman Front along 170°E in the Tasman Sea are presented. On-deck multifactorial incubation results confirm that both regions are typical of oligotrophic waters, in being dominated by picophytoplankton and cyanobacteria. The dissolved Fe concentrations were low (< 0.2 nM) in the surface waters and increased with depth below 150m. The chemical speciation of Fe was dominated by organic complexation at both stations and the ligand concentrations ranged between 0.4 and 1.0 nM, with complex stability logKFeL = 22.1'23.9. The Fe-complexing ligands in the upper 50m water column were consistently higher at the northern station compared to at the southern station. This difference in ligand concentrations contrasts with that of the concentrations of total dissolved Fe, which were slightly higher at the southern station. The vertical profiles of the ligands showed a maximum consistent with the potential biological sources for ligand production. Show more