duction in the environment, and the narcotic effects of hydrocarbons on nerve transmission are major biological factors in determining the ecologic impact of any release. Weathering processes may alter oil composition and thus its toxicity (Burns et al., 2000; Neff et al., 2000). With weathering, there is a subsequent loss of monoaromatic compounds, and the polycyclic aromatic hydrocarbons become more important contributors to the toxicity of weathered oils. Other factors that may contribute to alterations in toxicity include photodegradation and photoactivation (Mallakin et al., 1999; Boese et al., 1999).
Data gathered from several spills that occurred in the 1970s and 1980s demonstrated that the higher molecular weight aromatic compounds, such as the alkylated phenanthrenes and alkylated dibenzothiophenes, are among the most persistent compounds in both animal tissues and sediments (Capuzzo, 1987). Impairment of feeding mechanisms, growth rates, development rates, energetics, reproductive output, recruitment rates and increased susceptibility to disease and other histopathological disorders are some examples of the types of sublethal effects that may occur with exposure to petroleum hydrocarbons (Capuzzo, 1987). Early developmental stages can be especially vulnerable to hydrocarbon exposure, and recruitment failure in chronically contaminated habitats may be related to direct toxic effects of hydrocarbon-contaminated sediments (Krebs and Burns, 1977; Cabioch et al., 1980, Sanders et al., 1980; Elmgren et al., 1983).
Marine birds and mammals may be especially vulnerable to oil spills if their habitats or prey become contaminated. In addition to acute effects such as high mortality, chronic, low-level exposures to hydrocarbons may affect survival and reproductive performance of seabirds and some marine mammals. Sublethal effects of oil on seabirds include reduced reproductive success and physiological impairment, including increased vulnerability to stress (reviewed in Hunt, 1987; Fry and Addiego, 1987, 1988; Briggs et al., 1996). In contrast, in marine mammals, sublethal exposure to petroleum hydrocarbons has been shown to cause minimal damage to pinnipeds and cetaceans (e.g., Geraci, 1990; St. Aubin, 1990), although sea otters appear to be more sensitive (Geraci and Williams, 1990; Monson et al., 2000). Oil can also indirectly affect the survival or reproductive success of marine birds and mammals by affecting the distribution, abundance, or availability of prey.
Oil inputs from consumption activities vary widely in composition, persistence, loading rates by area and season, and effects. The single largest inputs of both petroleum hydrocarbons and PAH from this general source are land-based sources, which are composed of petroleum hydrocarbons that have already undergone considerable chemical and biological weathering during overland and riverine transport by the time they enter coastal waters. Further weathering rates will be slow. The hydrocarbons are mostly sorbed onto suspended sediments; thus their bioavailability is highly variable, depending on the feeding behavior of different organisms, sediment deposition patterns and rates, organic carbon content of the sediments, and the partitioning behavior of individual PAH. In contrast, although the input from the operation of recreational marine vessels in coastal waters is large, the bulk of the fuel is gasoline, which volatilizes from the surface water at rates that last on the order of several minutes to hours at 15ºC. The temporal and spatial discharge patterns are different from other sources, with most recreational boating being concentrated in the summer months and in coastal waters.
Chronic contamination by petroleum hydrocarbons from sources other than oil spills may be found in many coastal urban areas as a result of non-point source petroleum spillage, the burning of fossil fuels, and municipal wastewater discharges. The persistence of some compounds such as PAH in sediments, especially in urban areas with multiple