A criticism of laboratory toxicity tests is that the artificial environment, especially for vertebrates, can create stresses different from what the organisms experience in their habitat. Furthermore, the modes of exposure in the field will be mixed, and this may be difficult to simulate experimentally. In situ measurements overcome these problems, although introducing others. It is harder to get reproducible results since there is no control over the culturing and handling of the organisms. A negative control cannot be assured to differ only in its exposure to a toxicant. Therefore, a greater number of samples may be needed to establish statistical significance than for laboratory experiments. On the other hand, whatever unknown variables there may be, at least one can be fairly certain that they are representative of a natural setting. Finally, although in situ measurements may indicate that a disturbance has occurred, the cause cannot always be attributed to toxic exposure.
Biochemical tests on organisms in their habitats that can indicate toxic effects are called biomarkers. Biomarkers are of particular interest for compounds that do not bioac-cumulate, and that are thus difficult to measure in tissues directly. Any of the biochemical effects described above could be used. Examples include activity of enzymes such as cytochrome P450 (a positive indicator of toxicity) or acetylcholinesterase (a negative indication). Increased metallothionein is a sensitive biomarker for metal exposure. Despite their potential, they have not been widely found useful in environmental applications. Some have proven useful in clinical application for cancer detection.
Indicator species are species that are naturally present and that are sensitive to the toxin. They may be used in one of two ways: (1) to indicate the health of an ecosystem by their presence or absence; or (2) to bioaccumulate toxins, making the toxins more easily detectible than in the environment. For example, bivalves are often used as indicator species for bacterial pollution in coastal and estuarine ecosystems since, as filter feeders, they tend to accumulate them.
Sentinel organisms are organisms deliberately introduced for later recovery and testing. The classic case is the canary in the coal mine. Mussels have been used as sentinels by suspending them in the water in plastic trays. Organisms that are sessile or otherwise easy to recover are required for this type of study. Sentinels can provide early warning to take necessary action in time. For example, sentinels may be used in wells at the boundary of contaminated sites.
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