In March 1989, the oil tanker Exxon Valdez ran aground on Blight Reef in northern Prince William Sound in Alaska and released approximately 42 million liters of crude oil into the sea and caused an ecological catastrophe. The oil spills of the massive amount contaminated at least 1990 km of the pristine shoreline and significantly raised wildlife mortality.
After the oil spill, short-term effects of wildlife were comprehensively studied. Marine mammals and seabirds were significantly affected due to their routine contact with the sea surface. The spilled oil on the sea surface puncture the fur and feather isolation coats present in these animals, and may lead to hypothermia, as well as smothering, drowning, or ingestion of hazardous hydrocarbons. Thousands of animals were killed in the first days after the oil spill including 250 000 seabirds, between 1000 and 2800 sea otters, and 300 harbor seals. Destruction of billions of salmon and herring eggs were also reported. Due to thorough cleanup efforts, visual evidence of the oil spill hardly remained in areas just a year later.
In the long term, the consequences ofthe oil spill have been reported and reductions in population have been seen in various animals. The chronic exposure enhanced death rate in the following years in fish, sea otters, and ducks. After the spill, fish embryos and larvae were chronically exposed to partially weathered oil in dispersed forms that accelerated dissolution of multiringed polycyclic aromatic hydrocarbons (PAHs) which are toxic to pink salmon eggs at as low as 1 ppb. Many sea-birds showed evidence of persistent exposure to residual oil after the spill, and some populations did not return to their pre-oil-spill population size. Some marine birds that forage on littoral benthic invertebrates showed induction of the CYP1A detoxification enzyme in the late 1990s. Otters born after the oil spill experienced the exposure to contaminated sediment and ingestion of contaminated benthic invertebrates. At heavily oiled areas, sea otters have remained at half the estimated pre-spill numbers with no recovery initiated by 2000, whereas population just doubled in the period 1995-98 in un-oiled areas. Higher levels of the CYP1A have been observed in individuals from heavily oiled areas in 1996-98.
Cascades ofindirect effects were also present after the oil spill, where indirect interactions lengthened the recovery process on rocky shorelines for a decade or more. For example, dramatic initial loss of cover habitat, the rockweed, triggered a cascade of indirect effects, losses of important grazers and promoted blooms of unwanted ephemeral green algae and opportunistic barnacles.
A large oil spill that happened in the Arctic shows the contributions of delayed, chronic, and indirect effects of oil contamination in the marine environment. This suggests the importance of the inclusion of long-term effects of a petroleum activity when ecological impacts of such events need to be assessed.
Break of a Holding Lagoon of a Zinc Mine at Aznalcollar
In April 1998, a holding lagoon ofthe Los Frailes zinc mine containing acid waste from the processing of pyrite ore collapsed and released 5 million m3 of highly polluting sludge, rich in zinc (0.8% dry weight) and lead (1.2% dry weight) and arsenic (0.6% dry weight), among other metals and acidic water, with a pH of 2, into the Rio Guadiamar, SW Spain. The Rio Guadiamar ecosystem encompasses the World Heritage Site of Dofiana National and Natural Parks, one of the most important bird-breeding and overwintering area in Western Europe. Following the accident, sludge and acid water moved into the Guadiamar River and reached 45 km further south to the edges of the Dofiana National Park. The area affected by the sludge encompassed 4400 ha of crops, pasture land, and woodland.
The acid water lowered the pH of the wetland from 8.5 to 4.5 and caused massive metal contamination of the wetland, killing most of the fish and invertebrates in its path. In open water, Zn levels were recorded up to 270 mg l _ , Pb up to 2.5 mgl _ , and As up to 0.011 mgl _ at the border of the Natural National Park, which has important white stork and black kite colonies. Zinc levels in solutions for the contaminated reaches were well above the LC50s for aquatic life. The levels of cadmium and lead were within the range of concentrations for the LC50s for a wide range of species for all trophic levels. These imply that the cumulative effects of the high levels of metals could have a large-scale impact on local ecosystems. The sediment contamination levels of cadmium, lead, and zinc found around the parks greatly exceeded the levels recorded previously, and was in cases higher than levels observed around mining areas.
This accidental spill resulted in catastrophic damage. Considerable fish and invertebrate populations disappeared, and habitat for waterfowl was destroyed. For aquatic life, burial, blows, gill blocking, and drastic changes in water properties were the main causes of animal death, with 37 t of dead fish being collected in the month following the accident. In the mud-polluted watercourses, all living crabs and shellfish disappeared. After the spill, the metal levels in the blood, liver, and eggs of birds in Donana appeared to be elevated in relation to unconta-minated areas. The chemical analyses of bone and liver samples of waterfowl for As, Pb, Cu, Zn, and Se showed that metal concentrations were elevated in certain individuals, but they did not reach levels widely considered to be toxic. Due to the lack of detailed historical monitoring information in relation to metal levels, developing an accurate understanding of the impact of the spill on the ecosystem would be difficult. Long-term monitoring of the habitats and species combined with contaminant levels in the area will provide valuable information for understanding ecotoxicological impacts on ecosystems.
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