Techniques for determination of trace metals in small samples of size-fractionated particulate matter: phytoplankton metals off central California

The oceanic biogeochemical cycles of many trace elements are dominated by t heir association with the growth, death, consumption and sinking of phytopl ankton. The trace element content of marine phytoplankton reflects nutritio nal status, species composition, surface area to volume ratios, and interac tions with bioactive and toxic elements in the ambient seawater. Despite th e ecological and environmental importance of trace element assimilation by autotrophs, there are few modern measurements of trace elements in phytopla nkton assemblages from the natural environment. Here we introduce a new met hod for collection and analysis of size-fractionated particulate samples fr om practical seawater volumes. We pay particular attention to accurate dete rmination of trace element filter blanks which are typically the limiting f actor for analysis of such samples. Metals were determined at very low dete ction limits by high resolution inductively coupled plasma mass spectrometr y (HR-ICP-MS) for 11 elements (Ag, Al, Cd, Co, Cr, Cu, Fe, Mn, U, Zn and P, which is used as a biomass normalizer) in three types of polymer filters ( 0.45, 5.0, and 53 mu m pore size) and a quartz fiber filter (0.8 mu m pore size). To place these new determinations in a practical context, results ar e presented for a vertical profile of samples filtered from 1-4 1 of coasta l seawater (0.3-1.0 mg total solid dry weight) at a station off central Cal ifornia. The results demonstrate that the blanks of the evaluated filter ty pes, precleaned appropriately, are sufficiently low to allow accurate deter mination of the trace metal content of three size-classes of phytoplankton. At the Pacific station, measured phytoplankton Zn content (as Zn/P) agrees with values predicted from single-species culture studies growing at seawa ter Zn concentrations expected for coastal waters. The new method has utili ty as a generally applicable and simple size fractionation technique, and a llows determination of natural and pollutant elements in small samples of p hytoplankton and particles in coastal, estuarine and offshore marine regime s. (C) 1999 Elsevier Science B.V. All rights reserved.