The physicochemical speciation of dissolved iron in the Bering Sea, Alaska
Limnol. Oceanogr., 52(5), 2007, 1800-1808 | DOI: 10.4319/lo.2007.52.5.1800
ABSTRACT: 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 L1 ligand class and a weaker L2 ligand class. Conditional stability constants for both ligand classes were comparable between surface and subsurface samples, with mean log KcondFeL1,Fe0 = 11.5 ± 0.3 and mean log KcondFeL2,Fe0 = 10.3 ± 0.3 in surface samples, and mean log KcondFeL1,Fe0 = 11.4 ± 0.2 with a weaker ligand and mean log KcondFeL2,Fe0 of 10.2 ± 0.2 in subsurface samples. The concentrations of dissolved Fe were strongly correlated with ambient stronger L1 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 L1 ligand concentrations in samples with intermediate dissolved Fe, and this is a seemingly ubiquitous feature of dissolved Fe cycling in the marine environment.