Sentinel Species - Ecological Biomarkers: Indicators of Ecotoxicological Effects - page 170

7.2.1.2.3 Filtering Activities
Filter-feeding species, in particular bivalve mollusks or sponges, are well known for their
action on the cycling of organic matter. Indeed, by their activity of filtration of the seston,
they accelerate the sedimentation of particularly organic matter into biodeposits via feces
and/or pseudofeces, as well as the pollutants associated with the organic material (Dobson
and Mackie 1998; Björk et al. 2000). Thus, filter-feeders are responsible for an important
part of the transfer of pollutants between the water column and the sediment, with, as
a consequence, a modification of their retention times in coastal zones (Björk et al. 2000),
a decontamination of the aqueous phase and in parallel an enrichment of the sediment,
as well as an increase in the bioavailability of the pollutants to deposit feeders including
sediment ingestors (Turner and Millward 2002). Cho et al. (2004) have investigated, under
laboratory conditions, the extent of biodeposition and bioconcentration of PCBs by zebra
mussels ( Dreissena polymorpha ) collected from the Hudson River. They concluded that,
since zebra mussels filter a wide range of particles, they can increase not only the residence
time of PCBs in the Hudson River through redeposition to sediments, but also the trans-
mission of PCBs to animals up the food chain, through exposure to benthic organisms at
a lower trophic level.
By taking into account these various keystone species, the functioning of the ecosystem
will doubtless be protected, but this will not resolve the problem of the conservation of its
bio d ive r s it y.
7.2.2 Conservation of Biodiversity
In this respect, two questions in particular arise: the protection of the most sensitive spe-
cies, and the extrapolation of the data acquired on keystone species to all the species con-
stituting the ecosystem. A commonly held proposition is that the protection of the most
sensitive species, or of their most sensitive biological stages, in an ecosystem ensures the
protection of the whole community.
7.2.2.1 Interspecific Variability
The validation of a sentinel species is based on the possibility of extrapolating the results
obtained on such a species to all the species of the ecosystem. However, even between very
closely related taxa, sensitivity to xenobiotics and other contaminants can vary by sev-
eral orders of magnitude, depending on numerous factors, such as the taxonomic group
or, within the same group, a different natural history. Species that are relatively sensi-
tive or, contrastingly, insensitive to a toxic contaminant are recognized in numerous taxo-
nomic groups, from microorganisms to invertebrates and vertebrates, from microalgae to
higher plants, as indeed are contaminant-tolerant populations within the same species. In
a recent topic, Tolerance to Environmental Contaminants (Amiard-Triquet et al. 2011), numer-
ous examples of interspecific and intraspecific variability are described. Several chapters
are devoted to particular taxa including bacteria (Imfeld et al. in Amiard-Triquet et al.
2011), higher plants (Frérot et al. in Amiard-Triquet et al. 2011), and insects (Ghanim and
Ishaaya in Amiard-Triquet et al. 2011), whereas Johnston (in Amiard-Triquet et al. 2011)
and Berthet et al. (in Amiard-Triquet et al. 2011) highlighted and illustrated the fact that
species belonging to most of the major taxonomic groups, and organisms living in differ-
ent media (terrestrial, freshwater, estuarine, and marine organisms), show examples of the
development of tolerance to chemicals. Some examples chosen from different taxa further
illustrate these points.