Environmental+Toxicology+Tox+2000+notes (2)

The value of the use of oil dispersants to clean up

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Unformatted text preview: be developed, assessed and invoked. The value of the use of oil dispersants to clean up oil spills is a point in que stion. The dispersant has obvious cosmetic value and may save a few mammals and/or birds but — would another treatment have had a less toxic impact on aquatic organisms. An example of this is the cleanup after the Exxon Valdez oil spill in Alaska in 1989. We should ask ourselves if we have a deep enough understanding of ecotoxicology to be able to assess the treatments that we currently apply to the ecosystem, often in the hope that they, like the leeches and humours of old, will be effective. Organism Community/ecosystem Loss of Lifeless function Diversity Recolonization Extinction er Sucession Succession Low Avoid Physiological adaptation y Opportunists ov Sicken Habitat conditioning Re c pa ct Population Im Intensity of effect High Immigration None Low Time/concentration High Figure 3-33 The relationship between impact of a toxicant and the response of the ecosystem In ecosystems, impact and recovery are closely related — the one is the reverse of the other. As with the response of a single organism to in creasing concentration of a toxicant, release of a toxic substance in an ecosystem may have a number of consequences. These start at the lowest level with a metabolic response in the organism to the presence of the toxicant (Figure 3-3). This may be the only response and may, in fact, lead to an increase in the number of (or activity of) the organisms. This response is known as hormesis and is commonly seen at small exposures (Calabrese 2005). At greater concentrations of tox ic substances, organisms may respond with avoidance behavior and, at still greater concentrations, they will sicken and eventually die out. In the extreme, this leads to the total disappearance of the ecosystem where even microorganisms do not survive and the system is completely lifeless. This situation is rare — microorganisms are known to survive in very unusual and extreme environments ranging from hot springs to the cold of the arctic. Unless the release of the toxic substances into the environment continues, this situation will eventually reverse itself. Even the most persistent and toxic organic substance will eventually break down and/or toxic sediments will be covered and buried with fresh uncontaminated sediments. The time -scale will naturally differ and depends on the persistence of the toxic substances as well as the ability of the ecosystem to respond. As recovery occurs, one would see a return to normal — almost a reversal of the process that resulted in the impact in the first place (). In contrast to human health protection, individual organisms in the ecosystem are regarded as transitory and, because they are Table 3-10 usually part of a food chain, are Examples of assessment endpoints in relation to taxonomic group Taxonomic group of organisms Basis for assessment endpoint Functional responses Aquatic bacteria, fungi, and microbiota X Algae X Aquatic macrophytes X Aquatic invertebrates X Structural responses If keystone or food organisms Aquatic vertebrates X Aquatic vertebrate keystone or food organisms X Endangered aquatic organisms X Terrestrial (soil) bacteria, fungi, algae, and other microbiota X Terrestrial plants X Terrestrial invertebrates X If keystone or food organisms Terrestrial vertebrates X Terrestrial vertebrate keystone or food organisms X Endangered organisms X terrestrial individually unessen...
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