SIZE-FRACTIONATED IRON CONCENTRATIONS IN THE WATER COLUMN OF THE WESTERN NORTH-ATLANTIC OCEAN

We used an improved adsorptive cathodic stripping square-wave voltamet ry method to determine vertical profiles of three operationally define d fractions of iron in the western North Atlantic Ocean. ''Dissolved F e'' (<0.2-mum 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 1999 cruise, the concentration of dissolved Fe was higher in the surface w ater (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 c ruise, the concentration of ''colloidal Fe'' (0.2-0.4-mum fraction) wa s relatively constant at 0.15-0.25 nM from 10- to 500-m depth, then de creased to below the detection limit at depths greater-than-or-equal-t o 750 m. ''Particulate Fe'' (>0.4-mum fraction) increased from 0.1 nM in the surface water to 1.3 nM at depths below 1,000 m. Our results su ggest the need to consider the temporal variation of iron and its size fractions in seawater if we are to understand the influence of iron o n phytoplankton production in the ocean.