Size‐fractionated iron concentrations in the water column of the western North Atlantic Ocean

We used an improved adsorptive cathodic stripping square‐wave voltametry method to determine vertical profiles of three operationally defined fractions of iron in the western North Atlantic Ocean. “Dissolved Fe” (<0.2‐µm fraction) exhibited a nutrient‐type distribution during our October 1991 cruise, with concentrations increasing from 0.2 nM in the surface water to 0.9 nM at 1,000 m. However, during our July 1992 cruise, the concentration of dissolved Fe was higher in the surface water (0.6 nM), decreased to a subsurface minimum of 0.2 nM at a depth of 50 m, then increased to 0.7–0.8 nM below 1,000 m. During the July cruise, the concentration of “colloidal Fe” (0.2–0.4‐µm fraction) was relatively constant at 0.15–0.25 nM from 10‐ to 500‐m depth, then decreased to below the detection limit at depths ≥ 750 m. “Particulate Fe” (>0.4‐µm fraction) increased from 0.1 nM in the surface water to 1.3 nM at depths below 1,000 m. Our results suggest the need to consider the temporal variation of iron and its size fractions in seawater if we are to understand the influence of iron on phytoplankton production in the ocean.