Metal partitioning between colloidal and dissolved phases and its relation with bioavailability to American oysters Laodong Guo a, *, Peter H. Santschi b , Sammy M. Ray c a International Arctic Research Center, University of Alaska Fairbanks, Fairbanks, AK 99775, USA b Department of Oceanography, Texas A&M University, 5007 Ave. U, Galveston, TX 77551, USA c Department of Marine Biology, Texas A&M University, Galveston, TX 77551, USA Received 18 February 2001; received in revised form 15 December 2001; accepted 27 December 2001 Abstract Kinetics and the extent of metal partitioning between colloidal and dissolved phases and coagulation of metals associated with colloids were examined to determine their effects on the bioavailability of selected metals (Cd, Co, Hg, Ag, Fe, and Zn) to American oysters (Cras- sostrea virginica) using radiotracer and short term exposure experiments. After dispersion of radiolabeled colloids into low molecular weight (LMW, < 1 kDa) seawater, metal partitioning between dissolved ( < 1 kDa) and colloidal (1 kDa-0.2 mm) phases resulted in a consistent pattern, with a relatively constant percentage in the colloidal phase for each metal. On aver- age, about 90% of Hg and Fe,  60% of Ag and  40% of Zn, Co, and Cd were measured in the colloidal fraction during a short term exposure experiment, consistent with their parti- tioning in natural waters. Controlled laboratory experiments carried out in parallel using radioactively tagged colloids showed that coagulation of colloidal species, quantified as the fraction retained by a 0.2 mm filter, was insignificant for most metals under the conditions and time periods of the uptake experiments. The bioavailability of colloidally complexed metals, measured in terms of dry weight con- centration factor (DCF, ml g 1 ) and uptake rate constant (ml g 1 h 1 ), was somewhat depressed compared with their counterpart in the LMW treatment, but could be well pre- dicted from the results of the LMW treatment and metal partitioning. Both DCF values and uptake rate constants were higher in the LMW treatment than in the colloidal treat- ment. In addition, B-type metals, such as Ag, Hg, and Zn, all had higher values of DCF and uptake rate constants, regardless of treatments, except for Cd which had a lower DCF Marine Environmental Research 54 (2002) 49–64 www.elsevier.com/locate/marenvrev 0141-1136/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S0141-1136(02)00094-6 * Corresponding author. Tel.: +1-907-474-2794; fax: +1-907-474-2679. E-mail address: guol@iarc.uaf.edu (L. Guo).