The Exxon Valdez oil spill as an example

Some studies have addressed the ecosystem-level impacts of an oil spill. The most memorable and severe incident was the 1989 Exxon Valdez oil spill in northern Prince William Sound, Alaska. Forty-two million liters of North Slope Alaska crude oil were released into Prince William Sound, contaminating more than 1990 km of shoreline. It is estimated that more than 30% of the spilled oil evaporated into the atmosphere, leaving a viscous sticky fluid containing higher molecular weight hydrocarbons on the surface of the shoreline. It was estimated that up to 2800 sea otters, 300 harbor seals, and 250 000 seabirds succumbed to hypothermia, drowning, smothering, or digestion of oil in the days following the spill. Large numbers of plankton and benthic invertebrates were also killed from either smothering or the ingestion of toxic components. The acute mass mortality was severe, and the long-term ecosystem impacts continue to be studied.

Although debate continues as to whether or not the ecosystem has recovered, numerous studies suggest the persistence of oil, which continues to influence the region.

Research has shown that the residual oil, at sublethal concentrations, has changed the size and structure of marine populations through compromised health, growth, and reproduction. Chronic effects are closely related to those organisms interacting with oil-contaminated sediments (such as clams and mussels), as they accumulate significant amounts of oil. Since they have low metabolic rates, they tend to accumulate PAHs in their parent forms. Predators such as sea otters thus continue to be exposed to large amounts of PAHs from both contact with the sediment and the consumption of invertebrates. As a result, the detoxifying enzyme hepatic cytochrome P4501A (inducible by PAHs) has been reported to be at elevated levels in sea otters in polluted areas compared with others residing in unpolluted areas. As much as 11 years after the Exxon Valdez spill, the sea otter population had still not recovered, remaining at only 50% of its pre-spill level, while the populations of otters in unpolluted areas have increased.

While many natural factors can potentially affect migrating seabird populations, several studies have reported the chronic effects of the Exxon Valdez oil spill. When feeding on intertidal benthic invertebrates, seabirds were found to suffer from exposure to residual oils. For instance, during the winter seasons of 1995 and 1997, harlequin ducks showed a higher mortality of adult females in heavily contaminated areas, as opposed to those in unpolluted areas. The increase was correlated with both an increase of hepatic cytochrome P4501A and a decrease in body mass, demonstrating the impact of residual oil on a sensitive population and potentially explaining their population decline. Some 10 years after the Exxon Valdez oil spill, adult pigeon guillemots foraging for benthic invertebrates also showed elevated levels of hepatic cytochrome P4501A1, while there was no evidence of residual oil exposure to pigeon guillemot chicks (who consume fish). The population of Barrow's golden-eye also declined in oiled areas after the spill, and has still failed to fully recover.

The toxic effects observed in organisms that interact with sediments are also found in fish which lay eggs on gravel. Having embryos and larvae chronically exposed to residual oil during their developmental stages may explain the high mortality rates observed for pink salmon embryos years after the spill; weathered PAHs are much more toxic than their parent forms.

Other than mortality, effects on breeding, growth, and reproduction have been reported. Black oystercatchers, which prey on coastal mussel beds, showed increased mortality correlated with the degree of shoreline oil contamination in 1989 and 1990. In addition, a reduced breeding rate and abnormally small eggs were observed within a few months after the oil spill. Three years after the accident, delayed chick growth was also discovered. Both reduced growth rate and abnormal development were also reported for the pink salmon population; embryos and larvae exposed to oil displayed delayed growth, reproductive impairment, and higher mortality at later stages of life.

From the example of the Exxon Valdez oil spill, scientists realized that acute toxicity data alone are not sufficient for long-term risk assessment. In addition to mortality data, chronic effects such as impairment of growth and reproduction from sublethal exposures can impair a population. Moreover, since populations of different species are interdependent, both direct trophic effects and indirect interactions between species can change the balance of the ecosystem. Spill response agencies should consider the ecosystem, including delayed and chronic impacts, when decision making.

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