Marine Ecosystems under Toxic Pressure - Vulnerability of Coastal Ecosystems and Adaptation - page 33

Theoretical and empirical studies suggest that certain communities
show abrupt and nonlinear changes in their structure or functioning in
response to disturbances (Figure 1.8(d)) [CON 83, EST 95, MAY 77].
The concept of an ecological threshold has a direct relationship with
the concepts of resistance and resilience. The threshold for resistance
represents the concentrations of the contaminant that will lead to
sudden modifications of the community's structure. The threshold of
resilience represents “the point in time” where recuperation is initiated
and finally completed after the elimination of the stress factor. This
notion of a threshold is illustrated in several studies carried out on
different systems such as lakes, coral reefs and the pelagic system
[GRO 06, SCH 01].
1.5. Indirect effects and multiple stress factors
Generally speaking, marine ecosystems are simultaneously
exposed to a combination of stress factors (contaminants, temperature,
eutrophization, anoxia, etc.). Each stress factor can have an impact on
an individual or the community, and their combination may or may
not produce other additive effects [CAS 98]. Few studies have been
carried out on the effects of multiple stress factors on biological
communities [BRE 99], and even fewer have examined the
consequences in terms of indirect effects [CUP 02]. Resistance to
different types of contaminant is not necessarily correlated, which
renders the impact of concomitant stresses unpredictable at the level
of the community. For example, several studies [BRO 95, CUP 95,
VAN 95] suggest that insecticides increase the negative effects of
eutrophization by a “top-down” effect where the biomass of
periphyton is no longer controlled by grazers vulnerable to insecticide.
One of the challenges of ecotoxicology - ecology under toxic
stress - is to understand the toxic chemical pressure, amid the
complex mix of multiple disruptions to natural environments. In this
context, the importance of global climate change (GCC) and its
potential interaction with contaminants in the environment has
recently received increasing attention [CLE 09, NOY 09, SCH 07,
VAN 08]. Projections from the intergovernmental group of experts on
the climate Groupe d'experts Intergouvernemental sur l'Evolution du