Limnol. Oceanogr., 44(2), 1999, 403–414

A large fraction of the trace metal pool in estuarine and coastal waters is complexed with colloidal-sized macromolecular organic matter. Bioaccumulation and bioavailability studies of trace metals have generally utilized freeionic dissolved metal ions. Consequently, little is known about the uptake and depuration kinetics of the colloidally bound species. Chemical characteristics such as the relative hydrophobicity of these metal–colloid complexes may alter their bioavailability by interfering with or enhancing transport across membrane lipid bilayers. We used radiotracers to compare the bioaccumulation and sites of accumulation of colloidally complexed and free-ionic forms of Ag, Cd, Ba, Fe, Sn, Zn, Co, Hg, and Mn in juvenile brown shrimp Penaeus aztecus. In this paper, we present the results from 14-d uptake and bioaccumulation studies in which shrimp could take up radiotracers only from the water. Our results indicated that the colloidally complexed forms were bioavailable to shrimp. Uptake kinetics appeared to be similar in the two treatments for most metals, although by the end of the uptake study, the wholebody activities of Ba, Sn, and Zn were significantly higher in shrimp exposed to free-ionic metals. While the hepatopancreas appeared to be the site of highest accumulation on a mass-specific basis for both treatments, the total activity level of radiotracer differed between the two treatments. In the colloidally complexed treatment, the majority of total tracer activity was associated with the hepatopancreas while shrimp exposed to free-ionic metals exhibited the highest proportion of total activity in the abdomen. The mechanisms underlying these differences remain to be tested. Heavy metals are some of the most toxic, persistent, and widespread contaminants in estuarine systems. Dissolved or suspended metals can degrade water quality and become available to plankton, nekton, and benthic filter and deposit feeders. Aquatic organisms can acquire trace elements from food, from suspended particles, or directly from the water (Luoma 1989; Luoma et al. 1992; Carvalho and Fowler 1993). Uptake of dissolved metals from solution through permeable surfaces into the bodies of marine invertebrates is generally considered to be a passive process (Williams 1981), while metals associated with macroscopic particles are taken up via active ingestion. The solution speciation of trace metals is thought to be the primary determinant of their bioavailability to, and bioaccumulation in, aquatic organisms (Zamuda and Sunda 1982; Bruland et al. 1991; Luoma et al. 1992). Heavy metal bioaccumulation has also been shown to be a strong function of salinity (Phillips 1977; Part et al. 1985); pH (Hart and 1 Corresponding author.