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

These methods have begun to be accepted by regulatory

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Unformatted text preview: ubstances. Another useful reason for the use of field tests is the phenomena of homeostasis or self regulation. Most natural systems have con siderable flexibility and redundancy and are able to compensate in response to exogenous changes. Thus, most systems tend to return to a norm after being disrupted, even though this norm may itself be a progression or succession of events driven by seasonal or climatic forces. The second philosophy inherent in the use of field ass essments such as those carried out in enclosures and microcosms is that of the observational scientist and generalist. This philosophy, which was first related by the late Professor Reigler of McGill University, is opposite to the traditional and more reductionist approach based on single organism assays. This approach involves a number of steps which start at a general or ecosys tem level and normally end at that of the single organism or biochemical process. The first step is the definition of the syst em and an Preparing samples from limnocorrals Copepod from a microcosm Rotifer from a microcosm Effect of permethrin on cladocerans in a limnocorral Effect of permethrin on copepodites in a limnocorral Effect of permethrin on rotifers in a limnocorral assessment of its normal behavior and properties. This initial assessment is fol lowed by a search for cause and effect. This normally involves the per turbation of the system with the substance or process under study and an observation of resp onses in the system. Consistent and repeatable effects may then be linked with causation via an explana tory theory. This explanatory theory is the basis for the synthesis of hypotheses which may then be tested in the acceptable scientific manner. The approach is analogous to that of the clinical toxicol ogist working with whole animals. Relatively simplistic observations, such as behavior, growth rate, fur color, blood chemistry, and general condi tion of the animal allow certain inferences to be made which may then be tested in greater depth at the level of the organ, the cell and ultimately that of biochemical interaction. Photographs of field -based microcosms are shown on the next two pages. Constructed in-ground microcosms Constructed stream microcosms Limnocorral microcosms Treating limnocorral microcosms Sampling limnocorral microcosms Sampling limnocorral microcosms The results of such field tests can yield much useful information and some of these results are discussed below. Toxicity and recovery The acute toxic effects of pesticides under field conditions can be observed as a precip itous decline in numbers within 24 hours or a similar short time after treatment. The advantage of assessing this decline under field conditions is the large number of species which may be assayed at any one time. Added to this is the fact that the organisms are under climatological and predatory stress in the field. This may make them more susceptible to the effects of the toxicant than would perhaps be demonstrable in the laboratory and t...
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This document was uploaded on 04/07/2014.

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