Page 33 - The Toxicology of Fishes

Bioavailability of Chemical Contaminants in Aquatic Systems 13

FIGURE 2.2 Movement of chemicals through aquatic food webs. Numbers denote trophic level of organism (e.g., 1.0,
primary producers; 2.0, strict herbivores).

An additional type of interaction that can be important for the speciation of ionized chemicals is
competition with other ions in speciation reactions. This is particularly important for divalent cationic
metals, such as copper (see case study below), whose speciation and partitioning can be greatly influenced
by structurally similar but relatively nontoxic cations such as calcium and magnesium.
An understanding of chemical bioavailability requires understanding not only the chemical reactions
within an environmental compartment but also how the chemical moves among compartments. An
important route of exposure for cationic metals, for example, is via the gill in fish, yet the major repository
of some metals in many aquatic systems is the sediment. Therefore, to completely assess the potential
for a cationic metal to produce toxicity requires an understanding of bioavailability as it relates to both
water and sediment. In fact, the dynamic relationship between the water column and sediments, which
serve both as a source and sink for contaminants, is so critical that most state-of-the-art fate and effects
modeling at the watershed level explicitly considers interactions between the two compartments.
The interplay between the water column and sediments becomes particularly important for food webs
that determine contaminant exposure in fish diets (Figure 2.2). Contaminants in these food webs can
originate from the water column via absorption from solution by phytoplankton and other suspended
particles, which are consumed by filter feeding animals, which in turn support a series of predators. The
contaminants might also originate from sediments, where various invertebrates accumulate chemicals
from pore water or ingested sediment particles, thus providing another food base for predators. The
resulting dietary exposures to fish will have both water column and benthic components, the relative
importance of which will vary depending on the distribution of contaminant between the water column
and sediment, chemical speciation within the water and sediment, the nature of the food web, and the
position of the fish within the food web.

Accumulation via Gills and Skin
Fish gills serve a variety of physiological functions, including respiratory gas exchange, osmoregulation,
nitrogen excretion, and control of acid–base balance (Hoar and Randall, 1984). Because of these
functions, fish gills have the following features important for the exchange of toxic chemicals between
a fish and its environment: