Soluble and colloidal iron in the olgotrophic North Atlantic and North Pacific

In the oligotrophic North Atlantic and North Pacific, ultrafiltration studi es show that concentrations of soluble iron and soluble iron-binding organi c ligands are much lower than previously presumed "dissolved" concentration s, which were operationally defined as that passing through a 0.4-micromete r pore filter. Our studies indicate that substantial portions of the previo usly presumed "dissolved" iron (and probably also iron-binding ligands) are present in colloidal size range. The soluble iron and iron-binding organic ligands are depleted at the surface and enriched at depth, similar to dist ributions of major nutrients. By contrast, colloidal iron shows a maximum a t the surface and a minimum in the upper nutricline. Our results suggest th at "dissolved" iron may be less bioavailable to phytoplankton than previous ly thought and that iron removal through colloid aggregation and settling s hould be considered in models of the oceanic iron cycle.