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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 Kcond FeL1,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. [ABSTRACT FROM AUTHOR]

Abstract: This study examines the distribution of leachable particulate iron (Fe) in the Columbia River, estuary, and near-field plume. Surface samples were collected during late spring and summer of 2004–2006 as part of four River Influence on Shelf Ecosystems (RISE) cruises. Tidal amplitude and river flow are the primary factors influencing the estuary leachable particulate Fe concentrations, with greater values during high flow and/or spring tides. Near the mouth of the estuary, leachable particulate Fe [defined as the particulate Fe solubilized with a 25% acetic acid (pH 2) leach containing a weak reducing agent to reduce Fe oxyhydroxides and a short heating step to access intracellular Fe] averaged 770 nM during either spring tide or high flow, compared to 320 nM during neap tide, low flow conditions. In the near-field Columbia River plume, elevated leachable particulate Fe concentrations occur during spring tides and/or higher river flow, with resuspended shelf sediment as an additional source to the plume during periods of coastal upwelling and spring tides. Near-field plume concentrations of leachable particulate Fe (at a salinity of 20) averaged 660 nM during either spring tide or high flow, compared to 300 nM during neap tide, low flow conditions. Regardless of tidal amplitude and river flow, leachable particulate Fe concentrations in both the river/estuary and near-field plume are consistently one to two orders of magnitude greater than dissolved Fe concentrations. The Columbia River is an important source of reactive Fe to the productive coastal waters off Oregon and Washington, and leachable particulate Fe is available for solubilization following biological drawdown of the dissolved phase. Elevated leachable Fe concentrations allow coastal waters influenced by the Columbia River plume to remain Fe-replete and support phytoplankton production during the spring and summer seasons. [Copyright &y& Elsevier]