Sentinel Species - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 176

7.2.2.2.2 Origins of Sentinel Species
Berthet et al. (in Amiard-Triquet et al. 2011) have reviewed the sources of intraspecific
variability of tolerance in sentinel species exposed to contaminants, which may lead to
differences in survival of organisms. They discussed the use of wild populations versus
“laboratory rats,” of populations taken in stable environments versus fluctuating ones and
of naive populations versus exposed ones. Concerning the first point, it is often considered
that the genetic impoverishment of laboratory populations is an important bias in ecotoxi-
cological tests. According to the origin of the strain (a relatively clean or polluted site), the
clones used in ecotoxicological bioassays may vary in their tolerance to xenobiotics and
other contaminants, and the results obtained may thus lead to under- or overestimation of
the risk for a natural population. This can be taken into account by the application of safety
factors. On the second point, Berthet et al. (in Amiard-Triquet et al. 2011) indicate that in
laboratory studies, test organisms are almost always kept in optimal conditions of tem-
perature, oxygen, salinity, food, etc., in order to optimize performance in the control, and
to isolate the effects of the tested chemical. On the contrary, in their natural environment,
organisms are rarely in optimal conditions, and have to cope with environmental stresses.
Thus, for a long time, euryoecious species, such as estuarine organisms, have been consid-
ered to be more tolerant to xenobiotics than stenoecious ones, but this assertion has still
not been verified (Berthet et al. and Hummel et al. in Amiard-Triquet et al. 2011), and that a
substantial part of uncertainty in toxicity tests could be explained by natural factors.
The mechanisms that allow organisms to cope with the presence of various contaminants
in their environment have been recently reviewed by Johnston (in Amiard-Triquet et al. 2011)
and Rainbow and Luoma (in Amiard-Triquet et al. 2011) for metal tolerance, Regoli et al. (in
Amiard-Triquet et al. 2011) for antioxidant defenses, Roméo and Wirgin (in Amiard-Triquet
et al. 2011) for biotransformation of organic contaminants, and Mouneyrac and Roméo (in
Amiard-Triquet et al. 2011) for stress proteins. They may be individual physiological acclima-
tions after exposure to a contaminant (mechanisms of defense); in this case, the tolerance may
be lost if exposures are not maintained, or genetic adaptations (heritable) when a selective
force applied by a contaminant is maintained across multiple generations.
In all cases, both inter- and intraspecific variabilities of sensitivity of organisms to pol-
lutants have to be taken into account in order to correctly interpret the results of toxicity
tests, and to avoid, as much as possible, any under- or overestimation of risk. Because
the sensitivity of a given species depends on the pollutant involved, the ecotoxicological
endpoint measured, and the type of test used, several species and endpoints have to be
investigated in risk assessment, in order to reach a sound extrapolation at the level of the
whole ecosystem.
7.2.3 Choice of Physical Compartment
In the aquatic environments, ecotoxicological research has, over a long period, mainly con-
cerned species that are representative of the water column. On the other hand, although
sediment quantitatively represents the most contaminated environment, both by organic
and inorganic pollutants (Gagnon and Fisher 1997; Bernes and Naylor 1998), the endo-
fauna and endoflora were relatively little taken into account during this period. Moreover,
Chapman and Wang (2001) underline that most methods of evaluating the toxicities of
sediment are especially applicable to freshwater or marine sediments, but few take into
account the specific characteristics of estuaries. These authors attempted to produce an
inventory of the species used in tests of the overall toxicity of estuarine sediments. They
noted a relative phylogenetic poverty, with a wide dominance of crustaceans, in particular