Size-fractionated iron concentrations in the northeast Pacific Ocean: distribution of soluble and small colloidal iron

The spatial and temporal changes in vertical distribution of soluble Fe(< 2 00 kDa or < 0.03 mum) and small colloidal Fe (200 kDa-0.2 mum, 200 kDa-0.1 mum, 0.03-0.2 mum or 0.03-0.1 mum) were studied during September 1997, June and September 1998, and February 1999 along a transect extending from sout hern Vancouver Island to Ocean Station Papa (OSP). At OSP, vertical profile s of soluble Fe exhibited nutrient-like distributions. In June and Septembe r, concentrations of soluble Fe were low in the surface mixed layer (0.05-0 .07 nM) and increased below 300 m depth with temporal variation (0.17-0.42 nM, 300-600 m depth). The concentrations of small colloidal Fe were general ly low in the surface mixed layer (0.01-0.06 nM) with higher concentrations below 200 m depth. From 200 to 600 m depth, small colloidal Fe represented 13-50% of dissolved Fe (<0.2 <mu>m) and it has temporal variation. Concent rations of soluble Fe (0.20-0.57 nM) were higher in February 1999 than thos e in June and September 1997-1998. Concentrations of small colloidal Fe fro m 200 to 1000 m depth were also higher in February 1999 (0.17-0.37 nM) than those in June and September 1997-1998. These results suggest that, in OSP, a substantial portion of small colloidal Fe exists in the dissolved Fe fra ction (< 0.2 <mu>m) and that there are temporal changes in soluble and smal l colloidal Fe concentration in OSP seawater. Meanwhile, vertical profiles of Fe at stations near Vancouver Island did not show nutrient-like distribu tions. High concentrations of small colloidal Fe and large labile particula te Fe (> 0.2 mum, labile at pH 3.2) were observed in deep water at coastal stations both in September 1998 and February 1999. These results suggest th at soluble and small colloidal Fe vary spatially and temporally. Particular ly, in our observation, the soluble Fe has temporal changes in open ocean s eawater while the small colloidal and large labile particulate Fe has spati al changes at coastal area. Thus, we need to consider the existence of smal l colloidal Fe in the dissolved Fe fraction (< 0.2 <mu>m) in seawater to be tter understand the Fe dynamics in the ocean. (C) 2001 Elsevier Science B.V . All rights reserved.