The physicochemical speciation of dissolved iron in the Bering Sea, Alaska

The physicochemical speciation of dissolved iron ( Fe) across natural dissolved Fe gradients in the oceanic and shelf domains of the southeastern Bering Sea was examined in surface and subsurface samples using competitive ligand exchange - adsorptive cathodic stripping voltammetry with the added ligand salicylaldoxime. Two ligand classes were measured in all samples, a stronger L-1 ligand class and a weaker L-2 ligand class. Conditional stability constants for both ligand classes were comparable between surface and subsurface samples, with mean log K-FeL1, Fe'(cond) = 11.5 +/- 0.3 and mean log K-FeL2,Fe'(cond) = 10.3 +/- 0.3 in surface samples, and mean log K-FeL1, Fe'(cond) = 11.4 +/- 0.2 with a weaker ligand and mean K-FeL2, Fe'(cond) = of 10.2 +/- 0.2 in subsurface samples. The concentrations of dissolved Fe were strongly correlated with ambient stronger L-1 ligand concentrations for all samples with dissolved Fe concentrations greater than 0.2 nmol L-1. In samples with dissolved Fe concentrations less than 0.2 nmol L-1, large and variable excesses of L1 ligand concentrations were measured, coincident with observed Fe stress or limitation on the ambient phytoplankton. These observations suggest that the phytoplankton community is readily able to access dissolved Fe from the FeL1 complex, resulting in excess L1 in these waters. The available speciation data from other sources indicate that a significant correlation exists between dissolved Fe and L-1 ligand concentrations in samples with intermediate dissolved Fe, and this is a seemingly ubiquitous feature of dissolved Fe cycling in the marine environment.