Once an oil spill occurs, primary responders need to immediately decide upon a course of action. Decisions to be made may involve ecological considerations and there is often a tradeoff between potentially lethal effects on different species and the potential impacts on natural resources. Ecological information such as the status of a population (endangered, threatened, or common species), prevalent life stage of the species, and the time of the year (spawning, nesting, or migration) will also affect decision making. An understanding of the ecological consequences and toxicological impacts between variable habitats and species will help agencies predict impacts on the ecosystem and make better response decisions. Although each oil spill is unique and it is difficult to extrapolate toxicity data from the laboratory to the field, a thorough toxico-logical assessment prior to a spill will facilitate more effective decision making. Ideally, toxicological tests (such as bioassays) should cover both acute and chronic effects and incorporate the most sensitive life stages of as many pertinent species as possible. The oil type, condition (fresh vs. weathered), and exposure regimen (continuous, pulsed, static renewal, etc.) should be taken into consideration. The most common toxicological endpoints include acute effects such as mortality, narcosis, and necrosis, and chronic effects such as impacts on development, behavior, and reproduction.
Bioavailability dictates how much of a toxicant is available for absorption by an organism - it greatly influences overall toxicity. Routes of exposure will determine the oil distribution as well as toxicity, and possible routes include the skin surface, gills, other exposed membranes, and gastrointestinal tract (via diet). Both dissolved hydrocarbon and suspended parti-culate material phases can be taken up by the organism and lead to potentially toxic effects. Dissolved hydrocarbons distributed in the body via blood circulation can interfere with physiological functions, while suspended particulate matter can physically impact an organism by coating body surfaces or gills, impairing respiratory gas exchange.
When considering oil toxicity, the adverse health impacts from both dissolved and particulate oil should be considered. In general, uptake of oil via diet is comparatively lower than from water. However, filter-feeding zooplankton and invertebrates ingest a large amount of oil by filtering the droplets. Species at higher levels of the aquatic food web (such as marine mammals or birds) are at much greater risk of exposure and toxic effects posed by biomagnification, since they feed on organisms which may accumulate the toxic constituents of oil.
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