Effects of Toxic Chemicals and Other Pollutants on Aquatic Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 271

Some toxicants have negative effects on reproduction while having lit-
tle influence on the general health of the adult organism. These compounds
may cause complete extinction of a population, but the effects may be dif-
ficult to demonstrate with standard laboratory tests (i.e., the LD 50 is much
higher than environmental concentrations). An example of deleterious
effects on reproduction is the response of certain waterbirds to
dichlorodiphenyldichloroethane (DDE), which is a metabolite of dichloro-
diphenyltrichloroethane (DDT). DDE causes the birds to lay eggs with thin
shells, leading to reproductive failure (Laws, 1993) and extinction of local
populations. This effect almost led to the extinction of the bald eagle and
still threatens many migratory birds.
The chronic effects of toxins can be delayed. This is particularly the
case in mutagenic substances in which prolonged exposure increases the
chance of deleterious mutations. If these mutations lead to formation of
cancerous cells, a toxin is termed carcinogenic.
Several additional issues are important with regard to estimating the
influence of pollutants on aquatic organisms and humans. Extrapolating
effects to low concentrations of pollutants can be a problem. It has been
argued that there is a threshold below which contaminants are not harm-
ful. This is expected to be the case if an organism can repair a limited
amount of damage caused by a toxicant or can excrete it up to some lim-
ited rate, or if the compound does not interact with biological molecules
below some concentration. Such a threshold has not been established for
most toxic chemicals. A possible threshold is particularly important in reg-
ulating human carcinogens in the environment. If no threshold exists, very
low concentrations of materials can be predicted to cause a significant
number of deaths if a large number of people are exposed. If there is a
threshold, then exposure to levels below the threshold is not expected to
cause problems.
Low concentrations of toxicants may actually stimulate biological ac-
tivity (Calabrese and Baldwin, 1999). This further complicates regulation of
a toxin and estimation of long-term effects. Such effects mean extensive test-
ing of each suspected toxin is necessary before release into the environment.
Nontoxic factors can alter toxicity. For example, benign chemicals
and temperature can modify toxic effects. Obviously, it is difficult to pre-
dict toxicity of a compound when it is a function of several other vari-
able environmental factors. For example, it is known that zinc toxicity is
greater for fish in high temperatures and in low conductivity water (Fig.
14.3). Extrapolating laboratory results such as those from Fig. 14.3 to
field effects may yield inaccurate results; thus, a combination of field and
laboratory approaches may be best for toxic assessment (Blus and Henny,
1997).
If two toxicants are present it is difficult to know what their influence
will be on one another. In some cases they may alleviate the influence of
each chemical alone (antagonism), but in others the effects may be strictly
additive. In the worst-case scenario, the sum of the effects is greater than
simply adding the individual toxic effects (synergistic). Which of these in-
fluences will occur cannot be predicted a priori, and direct testing is gen-
erally necessary to establish an interactive effect.