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 (<200 kDa or <0.03 μm) and small colloidal Fe (200 kDa–0.2 μm, 200 kDa–0.1 μm, 0.03–0.2 μm or 0.03–0.1 μm) were studied during September 1997, June and September 1998, and February 1999 along a transect extending from southern Vancouver Island to Ocean Station Papa (OSP). At OSP, vertical profiles of soluble Fe exhibited nutrient-like distributions. In June and September, 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 generally 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 μm) and it has temporal variation. Concentrations of soluble Fe (0.20–0.57 nM) were higher in February 1999 than those in June and September 1997–1998. Concentrations of small colloidal Fe from 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 fraction (<0.2 μm) and that there are temporal changes in soluble and small colloidal Fe concentration in OSP seawater. Meanwhile, vertical profiles of Fe at stations near Vancouver Island did not show nutrient-like distributions. High concentrations of small colloidal Fe and large labile particulate Fe (>0.2 μm, labile at pH 3.2) were observed in deep water at coastal stations both in September 1998 and February 1999. These results suggest that soluble and small colloidal Fe vary spatially and temporally. Particularly, in our observation, the soluble Fe has temporal changes in open ocean seawater while the small colloidal and large labile particulate Fe has spatial changes at coastal area. Thus, we need to consider the existence of small colloidal Fe in the dissolved Fe fraction (<0.2 μm) in seawater to better understand the Fe dynamics in the ocean.