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Samples inside and outside the Arctic Ocean's TransPolar Drift (TPD) have been analyzed for Fe-binding organic ligands (L-t) with Competitive Ligand Exchange Adsorptive Stripping Voltammetry (CLE-AdCSV) using salicylaldoxime (SA). This analysis is compared to prior analyses with CLE-AdCSV using 2-(2-thiazolylazo)-p-cresol (TAC). The TPD's strong terrestrial influence is used to compare the performance of both CLE-AdCSV methods in representing the nature of natural organic ligands. These measurements are compared against direct voltammetric determination of humic substances (HS) and spectral properties of dissolved organic matter. The relationship between the two CLE-AdCSV derived [L-t] versus HS in the TPD has a comparable slope, with a 40% offset toward higher values obtained with SA. Higher [L-t] values inside the TPD, most probably due to HS, explain high dissolved Fe concentrations transported over the Arctic Ocean by the TPD. Outside of the TPD in the surface Arctic Ocean HS occur as well but at lower concentrations. Here changes in HS relate to changes in dissolved Fe concentration and to [L-t] obtained with SA, whereas [L-t] obtained with TAC remain constant. Moreover, with decreasing HS the offset between the methods using TAC and SA decreases. We surmise that in the presence of HS, the TAC method detects HS only either at higher concentrations or of specific composition. On the other hand, the SA method might overestimate [L-t], as an offset with the TAC method that remains constant where HS are not detected. Regardless, HS are the dominant type of Fe-binding organic ligand in the surface of the Arctic Ocean.

Plain Language Summary The Arctic Ocean is surrounded by land and is subject to strong influences of surrounding rivers. Climate change-induced increases in organic material from these rivers have already been reported, making the role of river runoff for the biogeochemistry of the open Arctic Ocean an urgent question. Water with a riverine component is carried across the Arctic Ocean via a stream of surface seawater and sea ice known as the Transpolar Drift. A component of organic material brought into the Arctic Ocean surface is humic substances, organic breakdown products from plant material known for their ability to bind iron. Iron is an essential trace element for primary production in the oceans and does not dissolve well in seawater without being bound by a dissolvable substance, of which the humic substances are an example among many. This study aims to gain insight into the role of humic substances in the binding of iron in the Arctic Ocean using multiple electrochemical methods while also discussing the suitability of these methods in the ongoing effort to characterize the diverse pool of iron binding organic substances.