how separate water and dietary exposures relate to combined exposures. However, Hook and 
Fisher (2001) have demonstrated reproduction of freshwater cladocerans and marine copepods to 
be reduced at much lower silver concentrations when exposure was via food equilibrated to 
various water concentrations, as opposed to directly to water at those same concentrations. Other 
unpublished work by Hook and Fisher (2000) has indicated the same is true for mercury, zinc, 
and cadmium. However, in similar experiments, Kim et al. (2000) have reported that 
reproduction of cladocerans was more severely affected by waterborne cadmium than by dietary 
cadmium. 
The repercussions of this issue in regulatory programs are large and persistent. The significance 
of dietary exposure has not only been a major point of contention in the Superfiind assessment of 
the CFR, but has infiltrated debates on a number of regulatory issues. These include the 
adequacy of ambient WQC for metals, to advisability of assessing waterborne metals on the basis 
of dissolved (rather than total recoverable) metal, and the adequacy of EPA’s proposed ESGs 
(formerly Sediment Quality Criteria) for metals. None of these programs currently considers 
dietary exposure to metals as part of assessing risk. Yet, at present, the technical debate is mired 
in conflicting and insufficient data with no clear resolution of the biological significance of the 
dietary pathway, much less a way of quantifying the risk for incorporation into environmental 
regulation. This project will address this fundamental uncertainty in the toxicity model for 
metals and will contribute to APG 4. 
The general objective of this project will be to assess the importance of considering routes of 
exposure other than water in aquatic risk assessments and criteria development for cationic 
metals. Initial efforts will address the importance of dietary exposure relative to water and will 
include 1) review and synthesis of past and ongoing work of other investigators to better define 
the state of knowledge regarding the importance of dietary exposure, and 2) targeted experiments 
which ccxifirm critical work and address uncertainties. 
Scientific Approach 
1. Dietary Metals Effects on Fish. 
This will include a series of experiments in which juvenile fish (rainbow trout, fathead minnows, 
channel catfish) will be fed diets of live, intact invertebrates that have been enriched with metal 
in a variety of ways, including through waterborne exposure of the prey to metals and through 
rearing prey in metal-contaminated sediment, both field-collected and artificially spiked. The 
oligochaete, Lumbriculus variegatus, has a number of attractive features as a prey species, 
including ease of mass culture and tolerance of sediment contamination, and we anticipate using 
it as a primary prey model. However, we will also conduct limited experiments with other 
invertebrates, such as the midge, since some researchers have speculated that the metal 
interaction with chitin may be involved in the observed effects. By using sediments collected 
from field locations such as the CFR and the Keweenaw Waterway (Michigan), these 
experiments can also address responses to real-world mixtures of metals in addition to 
laboratory-prepared mixtures. Exposures in initial experiments will be solely through the diet, 
minimizing exposure via water by maintaining high flows of uncontaminated water through 
exposure tanks and by not allowing excess prey to remain in the tanks. Concentrations both in 
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