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. Organic ligands are a key factor determining theavailability of dissolved iron (DFe) in the high-nutrient low-chlorophyll(HNLC) areas of the Southern Ocean. In this study, organic speciation of Feis investigated along a natural gradient of the western Antarctic Peninsula,from an ice-covered shelf to the open ocean. An electrochemical approach,competitive ligand exchange – adsorptive cathodic stripping voltammetry(CLE-AdCSV), was applied. Our results indicated that organic ligands in thesurface water on the shelf are associated with ice-algal exudates, possiblycombined with melting of sea ice. Organic ligands in the deeper shelf waterare supplied via the resuspension of slope or shelf sediments. Further offshore,organic ligands are most likely related to the development of phytoplanktonblooms in open ocean waters. On the shelf, total ligand concentrations([Lt]) were between 1.2 and 6.4 nM eq. Fe. The organicligands offshore ranged between 1.0 and 3.0 nM eq. Fe. The southern boundaryof the Antarctic Circumpolar Current (SB ACC) separated the organic ligandson the shelf from bloom-associated ligands offshore. Overall, organic ligandconcentrations always exceeded DFe concentrations (excess ligandconcentration, [L′] = 0.8–5.0 nM eq. Fe). The [L′] made up to 80 % of[Lt], suggesting that any additional Fe input can be stabilized in thedissolved form via organic complexation. The denser modified CircumpolarDeep Water (mCDW) on the shelf showed the highest complexation capacity ofFe (αFe'L; the product of [L′] and conditional bindingstrength of ligands, KFe'Lcond). Since Fe is also supplied by shelfsediments and glacial discharge, the high complexation capacity overthe shelf can keep Fe dissolved and available for local primary productivitylater in the season upon sea-ice melting.

Fe-binding organic ligands in coastal and frontal regions of the western Antarctic Peninsula