Impacts of oil spills

A great deal of research has been done on the effects of oil pollution on the marine environment and there are many books and articles dealing with this subject. Only a few major aspects can be dealt with here but key references are given at the end of the chapter.

Apart from fouling of beaches, which if severe can lead to the destruction of much of the intertidal population, the major threat from oil pollution is to

Table 10.4 Sources of inputs of petroleum hydrocarbons into the world's oceans and estimates of yearly inputs. Data taken from various sources between 1973 and 1981 based on GESAMR 1993.

Source Range of estimates

(thousands of tonnes)

Urban run-off and discharges 2500-1080

Operational discharges from tankers 1080-600

Tanker accidents 400-300

Non-tanker accidents 750-200

Atmospheric deposition 600-300

Natural seeps 600-200

Coastal refineries 200-60

Other coastal effluents 150-50

Offshore production losses 80-50

seabirds and mammals (see Table 10.5). Oil readily penetrates and mats the plumage of seabirds, making flight impossible and leading to loss of buoyancy and heat insulation. Attempts to preen lead to ingestion of oil and gut irritation. At the present time many hundreds of thousands of seabirds are destroyed annually by oil fouling. The species at greatest risk are those which live mainly on and in the water, e.g. puffins, guillemots, razorbills, shags, cormorants, ducks and divers. Seals and sealions may also suffer as oil fouling of their fur reduces heat insulation. Oil in their eyes causes irritation or blindness.

The impact of oil on marine organisms depends on characteristics of the oil spill such as its toxicity and viscosity, the amount of oil and the time for which it is in contact with the organism. Marine organisms and different life stages of organisms also vary in their sensitivity to oil. For example, on seashores, brown seaweeds are protected by their slimy covering of mucilage such that the oil easily washes off. Barnacles and sea anemones can also survive covered in oil for several days. Grazers such as sea snails and limpets are much more susceptible. Eggs and larval stages of fish, crustaceans and some other groups tend to be more susceptible than adults.

Toxicity

Crude oils and oil products differ widely in their chemical composition and therefore in their toxicity. The direct toxicity of oil to organisms is attributable mainly to the light aromatic components. Because these fractions usually evaporate fairly quickly, oil which reaches the shore soon after spillage is likely to be far more poisonous to the intertidal population than if it had been afloat for a longer time. The greatest toxic effects in the field have been caused by spills of light oil, especially when these have been confined in a small area. In addition to its direct lethal effects, oil may cause death by inducing a state of narcosis in

Table 10.5 Sensitivity of marine animals and plants to oil pollution (modified from IPIECA, 1991).

Group

Comments

Mammals

Birds

Fish

Invertebrates

Planktonic organisms

Larger algae

Marsh plants

Mangroves

Whales, dolphins, seals and sealions have rarely been significantly affected. Sea otters are more vulnerable because of their way of life and fur structure.

Birds using the air-water interface are at risk, particularly auks and divers. Badly oiled birds usually die.

Eggs and larvae in shallow bays may suffer heavy mortalities under slicks, particularly if dispersants are used. Large kills of adult fish are rare. Adult fish in farm pens may be killed or stressed such that they succumb to disease.

Invertebrates including molluscs, crustaceans, worms of various kinds, sea urchins and corals, may suffer heavy casualties if coated with fresh crude oil. On the shore, barnacles are more resistant than limpets and snails such as winkles.

Serious effects on plankton have not been observed in the open sea. This is probably because high reproductive rates and immigration from outside the affected area counteract short-term reductions in numbers caused by the oil.

Large algae such as kelps and brown wracks have a mucilaginous coating which often prevents oil sticking to them. Oil sticks better to dry algae such as those high on the shore and these may be broken by waves due to the weight of oil.

Some species of plant are more susceptible than others. Perennials with robust underground stems and rootstocks tend to be more resistant than annuals and shallow-rooted plants. If, however, perennials such as the grass Spartina are killed, the first plants to recolonize the area are likely to be annuals such as glasswort (Salicornia). This is because such annuals produce large numbers of tidally dispersed seeds.

The term 'mangrove' applies to several species of tree and bush. They have a variety of forms of aerial 'breathing' root which adapts them for living in fine, poorly oxygenated mud. They are very sensitive to oil, partly because oil films on the breathing roots inhibit the supply of oxygen to the underground root systems.

which animals become dislodged from their substrates. Though some may recover and re-establish themselves, others succumb through being washed into the strandline where they cannot survive.

The heavier fraction which remains after weathering appears not to be particularly poisonous, and, in general, well-weathered crude oil has little effect. However, if oil comes ashore in great quantity, intertidal populations may be killed by smothering with clinging, tarry material.

In the past, many shore animals were killed by the use of detergents sprayed onto the oil to emulsify and disperse it. The present generation of detergents are much less harmful when used properly and in appropriate circumstances.

Tainting of seafoods such as fish, shellfish and crustaceans may be caused by absorption of hydrocarbons into the tissues. This imparts an unpleasant oily taste and there is concern over the dangers to humans from eating polluted seafood since some oil-derived hydrocarbons may be carcinogenic (IPIECA, 1997).

Fate of spilt oil

When oil is first spilt, there is an initial evaporation of the lighter fractions. In the Amoco Cadiz incident, the spilt oil was mainly light crude and approximately 30 per cent of it evaporated. Much of the remaining oil formed a stable water-in-oil emulsion known as 'chocolate mousse'. This tends to float and is often the form in which oil ends up on the shore. On some species these emulsions are more adherent and harmful than unemulsified oil.

Oil remaining on the sea or shore after evaporation gradually disappears, mainly as a result of biodegradation by micro-organisms. This is assisted by mechanical breakup of lumps and patches by wave action; for example waves may re-float oil from the shore and break it up into small droplets. This increases the surface area and aeration of the oil and thus assists biodegradation. In the case of the Braer spill off Shetland in 1993, the wave action was so severe that most of the oil was dispersed through the water column and very little came ashore. Although most biodegradation is via microbial action, some is ingested by larger animals and at least partially broken down.

Some oil may be partially broken down by chemical degradation catalysed by exposure to sunlight. Remaining products may then be more easily biodegraded. The rate and extent of chemical degradation are affected by light intensity and duration, aeration and oil thickness.

Well-weathered oil at sea may form 'tar balls' which may continue to be washed ashore for many months after the spill, causing a great nuisance to beach users.

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