Variation in iron(III) solubility and iron concentration in the northwestern North Pacific Ocean

Vertical distributions of Fe(III) hydroxide solubilities (<0.025 mu m) and dissolved Fe (<0.2 mum) concentrations at 0-250 m depth were studied inside (HP) and outside (LP) a high-production (phytoplankton spring bloom) patch area in the northwestern North Pacific Ocean during May 1999. In the surface mixed layer, the Fe(III) solubility values at HP were much higher (2-4 nM) than those (0.3-0.9 nM) at LP and strongly correlated with chlorophyll a and nutrient concentrations. The high Fe(III) solubility observed in the surface mixed layer was probably due to a higher concentration or stronger affinity of natural organic Fe(III) chelators. In the surface waters, the dissolved Fe concentrations were generally lower than the Fe(III) solubility values, resulting from the active biological removal of dissolved Fe and excess concentration of Fe-binding organic ligands. The Fe(III) solubility minima (0.2-0.4 nM) were present in a narrow depth range (40-125 m) below the surface mixed layer at all stations. The subsequent Fe(III) solubility levels appeared to increase up to 0.6-0.8 nM with depth at 100-250 m, in association with the increase in nutrient concentrations. The strong linear correlations between Fe(III) solubility values and nutrient concentrations in middepth waters suggest that the formation of organic Fe(III) chelators may be related to microbial decomposition of sinking biogenic organic matter. In middepth waters, the dissolved Fe concentrations were generally higher than the Fe(III) solubility values, which suggests that the small colloidal iron phases may be present in the dissolved Fe (<0.2 mu m) fraction.