How does an oil spill affect coral reefs?

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Oil and the chemicals sometimes used to disperse oil can harm corals, causing death or limiting the coral's ability to grow, reproduce, and produce energy from photosynthesis (using its symbiotic algae).

Oil slicks moving onto coral reefs of Galeta, Panama at low tide during the Bahia las Minas refinery spill in April of 1986. Photo credit: NOAA

Impacts of oil spills to coral reefs are difficult to predict because each spill presents a unique set of physical, chemical, and biological conditions. How corals are exposed to oil—and the composition of the oil at the time of impact—bears directly on how serious the impact will be.

There are three primary modes of exposure for coral reefs in oil spills:

• Direct oil contact is possible when surface oil is deposited on intertidal corals that live near the surface of the water and become exposed with the tides.
• Rough seas and a light, soluble oil can combine to mix the oil into the water below the surface, where it can impact corals. Corals are exposed to less oil beneath the water surface, but the lighter oil components that mix easily are often the most toxic.
• Subsurface oiling can occur when heavy oils weather, or mix with sediment material. This increases the density of the oil to the point where it may actually sink, potentially smothering corals.

Effects of Oil and Dispersants on Coral Reefs

Laboratory, field studies, and actual oil spill events often appear to show contradictory results for effects of oil and dispersants on coral reefs.

The old notion that coral reefs do not suffer acute toxicity effect from oil floating over them is probably incorrect. Direct contact with spilled oil can lead to coral death, but depends on coral species, growth form, life stage, and type/duration of oil exposure.

Longer exposure to lower levels of oil may kill corals, as well as shorter exposure to higher concentrations. Death may not be immediate, but rather take place long after the exposure has ended.

Instead of acute mortality, it is more likely that oil effects occur in sublethal forms, such as reduced photosynthesis, growth, or reproduction. Early developmental forms (like coral larvae) are particularly sensitive to toxic effects, and oil slicks can significantly reduce larval development and viability.

Coral communities may recover more rapidly from oil exposure alone than from mechanical damage. Recovery of coral reefs after oil exposure; however, may depend partly on the recovery of associated communities (e.g. nursery or foraging habitats, such as mangroves and seagrasses) that may be more seriously affected than the reef itself. Recovery time depends on the type and intensity of the disturbance and can range from several years to decades.

Past Oil Spills Impacts to Coral Reef Ecosystems

One extensively studied spill occurred at Bahia Las Minas, Panama in April, 1986. An estimated 60,000-100,000 barrels of medium-weight crude oil spilled into the waters of the bay, causing widespread lethal and sub-lethal effects to coral.

In contrast, in the Arabian (Persian) Gulf Spill in January 1991, the largest oil spill in history, an estimated 6.3 million barrels of oil were released. Given the magnitude of this release and the coral reef impacts noted at other tropical spills, there were dire expectations of severe impacts to reefs in Kuwait and Saudi Arabia. However, to date, the extent of coral reef damage directly attributable to the Gulf Spill has been remarkably minor.

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