Effects of Marine Oil Spills

Oil spills can have a serious economic impact on coastal activities and on those who exploit the resources of the sea. In most cases such damage is temporary and is caused primarily by the physical properties of oil creating nuisance and hazardous conditions. The impact on marine life is compounded by toxicity and tainting effects resulting from the chemical composition of oil, as well as by the diversity and variability of biological systems and their sensitivity to oil pollution.

Impact of oil on coastal activities

The effects of a particular oil spill depend upon many factors, not least the properties of the oil.

Contamination of coastal amenity areas is a common feature of many spills leading to public disquiet and interference with recreational activities such as bathing, boating, angling and diving. Hotel and restaurant owners, and others who gain their livelihood from the tourist trade can also be affected. The disturbance to coastal areas and to recreational pursuits from a single spill is comparatively short-lived and any effect on tourism is largely a question of restoring public confidence once clean-up is completed.

Industries that rely on a clean supply of seawater for their normal operations can be adversely affected by oil spills. If substantial quantities of floating or sub-surface oil are drawn through intakes, contamination of the condenser tubes may result, requiring a reduction in output or total shutdown whilst cleaning is carried out.

Biological effects of oil

Simply, the effects of oil on marine life, are caused by either the physical nature of the oil (physical contamination and smothering) or by its chemical components (toxic effects and accumulation leading to tainting). Marine life may also be affected by clean-up operations or indirectly through physical damage to the habitats in which plants and animals live.

The main threat posed to living resources by the persistent residues of spilled oils and water-in-oil emulsions ("mousse") is one of physical smothering. The animals and plants most at risk are those that could come into contact with a contaminated sea surface. Marine mammals and reptiles; birds that feed by diving or form flocks on the sea; marine life on shorelines; and animals and plants in mariculture facilities.

The most toxic components in oil tend to be those lost rapidly through evaporation when oil is spilt. Because of this, lethal concentrations of toxic components leading to large scale mortalities of marine life are relatively rare, localised and short-lived. Sub-lethal effects that impair the ability of individual marine organisms to reproduce, grow, feed or perform other functions can be caused by prolonged exposure to a concentration of oil or oil components far lower than will cause death. Sedentary animals in shallow waters such as oysters, mussels and clams that routinely filter large volumes of seawater to extract food are especially likely to accumulate oil components. Whilst these components may not cause any immediate harm, their presence may render such animals unpalatable if they are consumed by man, due to the presence of an oily taste or smell. This is a temporary problem since the components causing the taint are lost (depurated) when normal conditions are restored.

The ability of plants and animals to survive contamination by oil varies. The effects of an oil spill on a population or habitat must be viewed in relation to the stresses caused by other pollutants or by any exploitation of the resource. In view of the natural variability of animal and plant populations, it is usually extremely difficult to assess the effects of an oil spill and to determine when a habitat has recovered to its pre-spill state. In recognition of this problem detailed pre-spill studies are sometimes undertaken to define the physical, chemical and biological characteristics of a habitat and the pattern of natural variability. A more fruitful approach is to identify which specific resources of value might be affected by an oil spill and to restrict the study to meeting defined and realistic aims, related to such resources.

Impact of oil on specific marine habitats

The following summarises the impact that oil spills can have on selected marine habitats. Within each habitat a wide range of environmental conditions prevail and often there is no clear division between one habitat and another.

Plankton is a term applied to floating plants and animals carried passively by water currents in the upper layers of the sea. Their sensitivity to oil pollution has been demonstrated experimentally. In the open sea, the rapid dilution of naturally dispersed oil and its soluble components, as well as the high natural mortality and patchy, irregular distribution of plankton, make significant effects unlikely.

In coastal areas some marine mammals and reptiles, such as turtles, may be particularly vulnerable to adverse effects from oil contamination because of their need to surface to breathe and to leave the water to breed. Adult fish living in nearshore waters and juveniles in shallow water nursery grounds may be at greater risk to exposure from dispersed or dissolved oil.

The risk of surface oil slicks affecting the sea bed in offshore waters is minimal. However, restrictions on the use of dispersants may be necessary near spawning grounds or in some sheltered, nearshore waters where the dilution capacity is poor.

The impact of oil on shorelines may be particularly great where large areas of rocks, sand and mud are uncovered at low tide. The amenity value of beaches and rocky shores may require the use of rapid and effective clean-up techniques, which may not be compatible with the survival of plants and animals.

Marsh vegetation shows greater sensitivity to fresh light crude or light refined products whilst weathered oils cause relatively little damage. Oiling of the lower portion of plants and their root systems can be lethal whereas even a severe coating on leaves may be of little consequence especially if it occurs outside the growing season. In tropical regions, mangrove forests are widely distributed and replace salt marshes on sheltered coasts and in estuaries. Mangrove trees have complex breathing roots above the surface of the organically rich and oxygen-depleted muds in which they live. Oil may block the openings of the air breathing roots of mangroves or interfere with the trees' salt balance, causing leaves to drop and the trees to die. The root systems can be damaged by fresh oil entering nearby animal burrows and the effect may persist for some time inhibiting recolonisation by mangrove seedlings. Protection of wetlands, by responding to an oil spill at sea, should be a high priority since physical removal of oil from a marsh or from within a mangrove forest is extremely difficult.

Living coral grows on the calcified remains of dead coral colonies which form overhangs, crevices and other irregularities inhabited by a rich variety of fish and other animals. If the living coral is destroyed the reef itself may be subject to wave erosion. The effects of oil on corals and their associated fauna are largely determined by the proportion of toxic components, the duration of oil exposure as well as the degree of other stresses. The waters over most reefs are shallow and turbulent, and few clean-up techniques can be recommended.

Birds which congregate in large numbers on the sea or shorelines to breed, feed or moult are particularly vulnerable to oil pollution. Although oil ingested by birds during preening may be lethal, the most common cause of death is from drowning, starvation and loss of body heat following damage to the plumage by oil.

Impact of oil on fisheries and mariculture

An oil spill can directly damage the boats and gear used for catching or cultivating marine species. Floating equipment and fixed traps extending above the sea surface are more likely to become contaminated by floating oil whereas submerged nets, pots, lines and bottom trawls are usually well protected, provided they are not lifted through an oily sea surface. Experience from major spills has shown that the possibility of long-term effects on wild fish stocks is remote because the normal over-production of eggs provides a reservoir to compensate for any localised losses.

Cultivated stocks are more at risk from an oil spill: natural avoidance mechanisms may be prevented in the case of captive species, and the oiling of cultivation equipment may provide a source for prolonged input of oil components and contamination of the organisms. The use of dispersants very close to mariculture facilities is ill-advised since tainting by the chemical or by the dispersed oil droplets may result.

An oil spill can cause loss of market confidence since the public may be unwilling to purchase marine products from the region irrespective of whether the seafood is actually tainted. Bans on the fishing and harvesting of marine products may be imposed following a spill, both to maintain market confidence and to protect fishing gear and catches from contamination.

(Further information is available in our Technical Information Paper on Fisheries (PDF 374 Kb))

Contingency planning

Because of the difficult decisions that will be required during an oil spill in order to mitigate damage and to resolve conflicts of interest, much can be done at the contingency planning stage to identify sensitive areas and to determine priorities for protection.

Points to remember

1. Persistent oils and mousse may seriously affect the visual appeal and use of coastal amenity areas; fresh crude and light refined products may constitute a fire and explosion hazard.
2. Oil spills can interfere with the normal working of power stations and desalination plants that require a continuous supply of clean seawater and with the safe operation of coastal industries and ports.
3. The effects on marine life are caused by the physical nature of the oil (physical contamination and smothering) and by its chemical composition (toxic effects and tainting).
4. In the context of marine ecology, the health of populations and associated plants and animals (communities) and the integrity of their habitats are more important than the status of any individual of a species.
5. The time taken for oil-damaged populations of plants and animals to recover is highly variable: the extent to which the biological recovery of a habitat can be accelerated is severely limited.
6. An oil spill can contaminate fishing equipment and mariculture facilities and cause loss of market confidence in marine products; stocks of adult fish are rarely, if ever, affected.
7. Marine life as well as natural and man-made structures can be damaged by clean-up techniques, such as the use of heavy equipment and high pressure hosing.
8. Following an assessment of the likely impact of an oil spill on each habitat or activity, attention should be given at the contingency planning stage to identifying areas to be protected, their order of priority and the techniques to be used.