Page 31 - The Toxicology of Fishes

Bioavailability of Chemical Contaminants in Aquatic Systems 11

bioavailability fall into three categories. First, processes external to the fish determine the concentration
and speciation of the chemical to which a fish is exposed. A detailed discussion of fate and transport of
chemicals is not in the scope of this chapter, but this topic will receive some attention here because
evaluating bioavailability requires some understanding of the nature and origin of the chemical species.
Second, the fish can absorb the chemical by various routes and mechanisms that are functions of both
chemical speciation and organism physiology. It cannot generally be said that a chemical species is either
bioavailable or not; rather, bioavailability is relative and will reflect how much a chemical species con-
tributes, directly or indirectly, to this absorption. Third, once absorbed, the chemical will be modified and
distributed within the fish, thus determining the nature and amount of chemical at the site of toxic action.
This chapter is organized into two sections. The first section addresses some general principles of
chemical bioavailability with regard to chemical fate in aquatic environments and chemical uptake via
different routes of exposure. Additional information regarding chemical uptake and disposition in fish
is provided in Chapter 3. This first section also discusses various measures that are used in evaluating
bioavailability. The second section presents several case studies regarding the bioavailability of a wide
variety of chemicals. These examples are not presented to give a comprehensive review of bioavailability
for specific chemicals but rather to illustrate a range of issues and processes that can be important in
defining and assessing bioavailability for any chemical.

Assessing Bioavailability: Principles, Processes, and Measures
Chemical Behavior in the Aquatic Environment
Chemicals may enter the aquatic environment via a variety of point and non-point sources, including
direct discharges, soil and pavement runoff, and atmospheric deposition (Figure 2.1). Those chemicals
that enter aquatic environments in large quantities, that are relatively persistent, or that are very potent
typically are of most concern. Some types of anthropogenic chemicals are unlikely to enter the aquatic
environment because of how they are used. In other cases, chemicals may not remain in aqueous solution;
for example, relatively volatile chemicals usually are not persistent in aquatic systems. In other instances,
degradation processes, such as microbial metabolism, aqueous hydrolysis, or photolysis, markedly
decrease concentrations of chemicals before or after they enter aquatic systems, thus reducing the
probability of significant exposure of aquatic organisms. Approaches exist (based on production volume,
use pattern, chemical structure, toxicity, etc.) for predicting the potential hazard of newly manufactured
chemicals in aquatic environments. Although consideration of these types of predictive models is beyond
the scope of this text, they are generally the first step in determining the need for further application of
prospective risk assessment methodologies.
A number of factors related to the structure and properties of a chemical in an aquatic environment
will dictate its fate, distribution, and, ultimately, bioavailability to aquatic organisms. Aromatic rings
and halogenated substituents, for example, tend to be associated with persistent organic contaminants
such as polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polychlorinated
dibenzofurans (PCDFs), and dibenzo-p-dioxins (PCDDs), as well as organochlorine pesticides such as
DDT (and DDE, DDD), dieldrin, and toxaphene. Chemical elements that are neither created nor destroyed
but are redistributed in the environment by the activities of humans also are persistent contaminants.
Some of the more toxic elements in this class are metals and metalloids such as cadmium, copper,
mercury, nickel, zinc, lead, silver, and arsenic. There are also several relatively persistent and toxic
organometals such as methylmercury and tributyltin. Even chemicals that are not persistent can be of
concern if large quantities are released to aquatic systems or the chemicals are very potent. An example
of the former situation would be ammonia, which, under appropriate conditions, can be converted via
nitrification to nontoxic products but is a chemical of toxicological significance to fish because of large
inputs to aquatic systems. An interesting example of a relatively labile compound that nonetheless remains
of concern because of its potency is the synthetic estrogen ethynylestradiol. This component of birth
control pills is found only in small concentrations in certain types of municipal effluents, but because
of its great affinity for the estrogen receptor it has been associated with an increased incidence of
feminized male fish in exposed populations (Desbrow et al., 1998).