Project Title N4, Risks of Hea^y Metals to Aquatic Organisms from Multiple Exposure 
Routes 
Project Coordination and Resources (4.0 FTEs: MED-4.0) 
Objectives 
A fundamental uncertainty with the use of typical laboratory toxicity tests in assessing risks of 
nonbioaccumulative toxicants to aquatic organisms is the failure to account for other routes of 
chemical exposure which may occur in natural systems. Of particular concern is exposure to 
chemicals via food or by incidental ingestion of contaminated non-food solids. For many 
nonbioaccumulative toxicants this failure is arguably of little consequence, but for some 
chemicals risk might be significantly underestimated using only water exposures. Conventional 
wisdom in aquatic toxicology was that water is the primary exposure route for metals (e.g., 
copper, cadmium, nickel, lead, zinc) to fish and other aquatic organisms (with the exception of 
metals such as mercury which form significant amounts of bioaccumulative oiganometallic 
species). Although exposure to metals via the diet was known to produce some level of 
bioaccumulation, it was not considered to significantly increase risk relative to water-only 
exposures, unless the diet was highly contaminated relative to that in equilibrium with water. 
Thus, environmental criteria and other toxicity assessments for metals have focused on 
waterborne toxicity, but there has been considerable concern whether this is adequate. 
Beginning in the early-1990's a series of dietary toxicity studies were conducted (Woodward et 
al. 1994, 1995; Farag et al. 1994) that involved feeding young rainbow trout diets prepared from 
invertebrates collected from metal-contaminated rivers, primarily the Clark Fork River (CFR) in 
Montana. The Clark Fork watershed is highly contaminated with several metals, with copper 
being generally considered to be the metal of greatest concern. Results of these studies showed 
that fish fed a diet of pellets prepared from metal-enriched invertebrates showed reduced growth 
relative to fish fed similar diets prepared from invertebrates fi'om reference areas, or less 
contaminated portions of the CFR. A more recent study from the same laboratory (Farag et al. 
1999) reports comparable findings for invertebrates from the Coeur d’Alene watershed in Idaho, 
where the primary metals of concern are lead and zinc. The authors of these studies conclude 
that the metals in these diets are the cause of the toxicity to rainbow trout. However, these 
conclusions conflict with previous studies which have not shown such toxicity from dietary 
metals. Additionally, Mount et al. (1994) conducted a laboratory study which fed a live diet of 
brine shrimp (Artemia) that were cultured at high metal concentrations to produce nauplii that 
were high in metal content. These studies did not indicate that dietary metals caused the degree 
of effects noted in studies with the field-collected invertebrate diet. The discrepancies among 
these studies is attributed by some to differences in the form of the metal in the diet. 
For invertebrates, similar uncertainties and conflicting results exist regarding the importance of 
the dietary route of exposure. A variety of studies have demonstrated intake of metals via 
ingestion in various molluscs and crustaceans, but the importance of this uptake relative to 
uptake via water and its significance to toxic response generally has not been well determined. In 
particular, there are questions about differences in the efficacy of metals taken up via different 
routes, about the water exposure a particular dietary exposure should be compared to, and about 
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