Dissolved Fe (DFe) and Fe-binding dissolved organic ligands were analysed during two GEOTRACES cruises in the Mediterranean Sea in May and August 2013 - data Version 5

Method: Competing ligand exchange adsorptive cathodic stripping voltammetry (CLE-aCSV) was performed using two systems consisting of a μAutolab potentionstat (Metrohm Autolab B.V.), a 663 VA stand with a Hg drop electrode (Metrohm) and a 778 sample processor with ancillary pumps and dosimats (Metrohm), all controlled using a consumer laptop running Nova 1.9 (Metrohm Autolab B.V.). The characteristics of Fe-binding dissolved organic ligands, that is both the ligand concentration [Lt] (in nano-equivalents of molar Fe, nEq of M Fe) and the conditional binding constant K′ (M−1) with respect to [Fe3+], commonly expressed as log K′ are determined using 2- (2-Thiazolylazo)-p-cresol (TAC) as an added measuring ligand (Croot and Johansson, 2000). TAC was used with a final concentration of 10 μM, and the complex (TAC)2-Fe was measured after equilibration (> 6 h). The increments of Fe concentrations used in the titration were 0 (2×), 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.5, 2, 2.5, 3, 4, 6, and 8 (2×) nM. Using a non-linear regression of the Langmuir isotherm, the electrical signal recorded in nA (nano-Ampere) was converted into a concentration in nM, and the ligand concentration [Lt] and the binding strength K were estimated (Gerringa et al., 2014). Using [Lt] and K, the concentration of Fe bound to a natural Febinding ligand [FeL], the concentration of inorganic Fe [Fe′] and the concentration of natural unbound ligand [L′] were calculated under the assumption of chemical equilibrium. Since K is log transformed, the standard error (SE) is asymmetric to lower and upper values; therefore two SE values are obtained, one to lower (down) and to upper (up) values.