Determination of iron and iron-binding ligands in seawater with flow injection analysis and voltammetry

Recent years have seen an increased occurrence of large icebergs in the Southern Ocean originating from Antarctic ice shelves. These free-drifting icebergs may serve as an important source of Fe to surrounding waters. I measured the concentrations and speciation of dissolved Fe via flow injection-chemiluminescence and cathodic stripping voltammetry (CSV) in the waters surrounding several icebergs during cruises to the Scotia and Weddell Seas in June 2008 and March 2009. Surface dissolved Fe (dFe) concentrations varied from 0.70 to 2.65 nM and were elevated up to 60% at some stations <1 km from the nearest iceberg. Significantly higher Fe(II) (up to 727 pM, 50% of dissolved Fe) was observed at the face of iceberg, possibly due to direct and indirect Fe(II) sources associated with the iceberg. Electrochemical measurements revealed that over 99% of dissolved Fe was bound to organic ligand, and concentrations of total Fe-binding ligands ranged from 0.98 nM to 3.07 nM, with conditional stability constants (LogK'FeL) ranging from 11.5 to 13.1. During the fall cruise, the highest concentrations of ligands were observed within 1 km of icebergs, presumably due to enhanced grazing activity near the iceberg. In contrast, no enrichment of Fe-binding ligands was seen during the 2008 cruise, presumably due to low biological activities in the winter.;Grazing experiments were also conducted using the marine diatom Thalassiosira pseudonana as prey and the heterotrophic dinoflagellate Oxyrrhis marina as grazer to investigate the potential production of Fe-binding ligands via grazing processes. The concentration of dissolved Fe increased from 0.87 nM to 7.67 nM by the end of the grazing period (36 h), suggesting intracellular Fe of phytoplankton cells was released during the grazing process. Organic Fe-binding ligands were detected after 12 and 24 h with concentrations of 4.34 and 5.62 nM, repectively. The conditional stability constant of the ligands (LogK'FeL) was around 11.2, comparable with typical porphyrin-like intracellular ligand such as phaeophytin. These results will help us to better understand the source of Fe-binding ligands in the ocean.