ARTICLE IN PRESS + MODEL Marine Chemistry xx (2006) xxx–xxx www.elsevier.com/locate/marchem An investigation into the exchange of iron and zinc between soluble, colloidal, and particulate size-fractions in shelf waters using low-abundance isotopes as tracers in shipboard incubation experiments ⁎ Matthew P. Hurst a, , Kenneth W. Bruland b a Department of Chemistry and Biochemistry, University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA b Department of Ocean Sciences, University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064, USA Received 6 September 2005; received in revised form 20 June 2006; accepted 10 July 2006 Abstract A vertical mixing event was simulated in shipboard incubation experiments on the mid-continental shelf of the eastern Bering Sea to investigate Fe and Zn cycling between the soluble (b0.03 μm or 200 kDa), colloidal (0.03–0.2 μm), and particulate (0.2–10 μm, N10 μm) size-fractions. The particulate Fe and Zn were further separated into chemically labile (25% acetic acid-leachable) and refractory pools. The experiment employed 57Fe (+0.90 nM) and 68Zn (+0.99 nM) as stable, low-abundance isotope amendments to the soluble fraction, and the exchange of Fe and Zn between the different physico-chemical fractions was measured using high resolution-inductively coupled plasma- mass spectrometry (HR-ICP-MS). More than 50% of the added 57Fe partitioned to the colloidal fraction within 45 min of adding the tracer. Both the 57Fe and 56Fe colloidal fraction were removed from the dissolved phase at a faster rate than the soluble Fe fraction. In contrast, the colloidal 66Zn and 68Zn concentrations remained constant over the 5-day experiment, suggesting a unique removal mechanism for colloidal Fe. The net removal of dissolved 57Fewasobservedtobe3to4timesmorerapidthandissolved56Fe, which can be attributed to the regeneration of particulate Fe. Using a simple first-order model, it was determined that the net removal of 2.0 nM of dissolved Fe during the experiment was a consequence of dynamic cycling, whereby 2.9 nM of particulate Fe was regenerated and contributed to an overall removal of 4.9 nM of Fe from the dissolved phase. The amended 68Zn tracer resided in the soluble fraction and was assimilated by the diatom biomass (N10 μm size-fraction) at the same rate as 66Zn. This similarity in rates suggests that nearly all of the net removal of Zn was due to assimilation and that regeneration did not play a significant role in Zn cycling within the incubation experiment. This research demonstrates the advantage of using low-abundance isotopes as tracers and the importance of particulate and colloidal Fe in the overall biogeochemical cycling of Fe in ocean surface waters. © 2006 Elsevier B.V. All rights reserved. Keywords: Iron; Zinc; Seawater; Inductively coupled plasma-mass spectrometry; Stable isotopes 1. Introduction The development of sensitive analytical methods and ⁎ Corresponding author. Present address. Department of Chemistry, Humboldt State University, 1 Harpst Street, Arcata, CA 95521, USA. trace metal clean techniques has allowed progress in Tel.: +1 707 826 5720; fax: +1 707 826 3279. defining the role of trace metals as an important factor E-mail address: [email protected] (M.P. Hurst). influencing primary productivity in marine systems (Morel 0304-4203/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.marchem.2006.07.001 MARCHE-02388; No of Pages 16 ARTICLE IN PRESS 2 M.P. Hurst, K.W. Bruland / Marine Chemistry xx (2006) xxx–xxx et al., 2003). These advancements have led to the under- the experimental setup. Adequate methodology has yet to standing that bioactive trace metals, such as Fe and Zn, may be developed to accurately assess the bioavailability of the approach concentrations in the marine environment where particulate pool. To continue the progress towards a more they can become the limiting nutrient (Martin and Fitzwater, complete understanding of the biogeochemical cycling of 1988; Morel et al., 1994; Coale et al., 1996). Other studies Fe and Zn, it is necessary to examine the exchange and have demonstrated that variations in Fe and Zn concentra- partitioning of trace metals between the different physico- tions can cause shifts in the structure of phytoplankton chemical fractions within the dissolved and particulate communities in ocean surface waters (Hutchins et al., 1998; phases. Crawford et al., 2003; Leblanc et al., 2005). This investigation uses stable, low-abundance isotopes Much of the research on the bioavailability of Fe and of Fe and Zn as tracers in an incubation experiment that Zn has focused on the dissolved phase and several mech- simulated a vertical mixing event on the mid-continental anisms for Fe and Zn sequestration by marine phyto- shelf waters in the eastern Bering Sea, whereby the mixing plankton have been suggested (Hutchins et al., 1999; of subsurface waters containing macronutrients and Fe Ellwood and van den Berg, 2000; Barbeau et al., 2001; with nutrient-depleted surface waters induced a diatom Shaked et al., 2005; Lohan et al., 2005). There is consensus bloom. By adding the tracers to the dissolved phase, that the dissolved speciation of Fe and Zn is dictated by the changes in the isotopic ratio within different physico- presence of biogenic ligands in ocean surface waters, and chemical fractions over time were used to determine the these ligands chelate N99.9% of dissolved Fe (Rue and net removal of dissolved trace metals by the phytoplankton Bruland, 1995; Boye and van den Berg, 2000) and 98% of biomass and the regeneration rate of particulate trace dissolved Zn (Bruland, 1989; Ellwood and van den Berg, metals into the dissolved phase. Previous studies have used 2000). However, the dissolved phase is operationally stable, low-abundance isotopes as tracers to examine defined as the fraction of sample that passes through a 0.2 adsorption/desorption processes of particulate trace metals or 0.4 μm filter, but in actuality, this fraction contains a (Cu, Zn, Ni) in the estuarine environment (Gee and continuum of colloidal and soluble species within which Bruland, 2002) and bioaccumulation processes of Hg in trace metals may partition. Data collected from laboratory lakes (Pickhardt et al., 2002). The present investigation incubation experiments (Nishioka and Takeda, 2000; focuses on the contributions of particulate, colloidal, and Chen et al., 2003) and from field work in both open and soluble trace metals to the dynamics of trace metal cycling coastal ocean regimes (Nishioka et al., 2001a,b; Wu et al., in a shelf water system. The partitioning of the various 2001) suggest that colloids play an unique role in Fe isotopes in the different physico-chemical fractions reveals bioavailability, where the colloidal Fe fraction appears to the importance of chemically labile and refractory be preferentially removed by phytoplankton. These inves- particulate trace metals and colloidal trace metals in the tigations have demonstrated the need to distinguish bet- overall cycling and recycling of Fe and Zn within such a ween colloidal and soluble forms of Fe, and more research system. is needed to elucidate the mechanism(s) linking colloidal Fe and biological uptake, if any. 2. Material and methods Furthermore, very few studies have investigated the interactions between particulate trace metals and the dis- 2.1. Sample collection and handling solved phase, or the bioavailability of particulate trace metals with respect to phytoplankton. This is particularly The surface sample was collected using a clean surface important for Fe, which resides largely in the particulate pump system that included an all PTFE Teflon™ dia- pool within surface waters (Wells and Mayer, 1991; Sunda, phragm pump (Bruiser™, Osmonics, Minnetonka, MN) 2001). Previous research has demonstrated that a portion of and PFA Teflon™ tubing (Bruland et al., 2005). The the particulate trace metals is bioavailable to marine phy- sample inlet was mounted to a PVC fish system and toplankton in coastal shelf waters (Wells and Mayer, 1991; lowered to 3 m below the sea surface. The speed of the Wells et al., 2000), which suggests that understanding the ship was about 5 knots; thus, ensuring that the intake regeneration of particulate trace metals is an absolute would not be influenced by the ship's wake. The unfiltered necessity if the total amount of bioavailable trace metals is sample was delivered directly to an acid-cleaned and to be estimated. Regeneration of biogenic Fe, Mn, and Zn seawater-conditioned 55 L carboy in a Class 100 clean amongst grazers and planktonic prey has been investigated area; the carboy was filled to the halfway point. The 40 m with the use of radiotracers (Hutchins and Bruland, 1994; subsurface water sample was collected using a Teflon- Hutchins and Bruland, 1995), but the technique was unable coated, 30 L GO-Flo bottle (General Oceanics, Miami, to study the cycling of nonradioactive trace metals within FL) lowered on a Kevlar hydroline (Bruland et al., 1979). ARTICLE IN PRESS M.P. Hurst, K.W. Bruland / Marine Chemistry xx (2006) xxx–xxx 3 The subsurface sample was transferred through Teflon™ of 57Fe did not significantly change the amount of total Fe tubing to fill the carboy. The mixed sample was aliquoted in the sample (2% of total). to 2.5 L polycarbonate incubation bottles, which were After adding the isotopes to the individual samples, acid-cleaned with 3 M HCl (trace metal grade, Fisher the bottles were sealed by wrapping Parafilm and then Scientific, Pittsburgh, PA) for 2 weeks at room temper- electrical tape around the bottle opening. They were ature, filled with dilute 0.05 M HCl, and stored until use. placed in a Plexiglas incubator located on the top deck Prior to the experiment, the incubation bottles were con- of the ship, which was equipped with a flow-through ditioned by rinsing with clean surface seawater aboard seawater system that kept the incubator at surface water ship.