Oil pollution of beaches

The fate and effects of oil washed up on beaches depend not only on the variables such as oil type, mentioned above, but also on the energy level of the shore (degree of exposure to wave energy) and on substratum type.

In general, the more exposed the shore, the quicker the biological recovery time of the littoral benthos. This applies both to rocky and sedimentary beaches. Oil does not remain long on wave-battered rocky shores, and where vertical cliffs are present the oil may never reach them due to the action of reflected waves. In contrast, a gradually sloping boulder shore in a sheltered sea loch is likely to trap the oil which will get under the boulders and sink into any sediment beneath.

Once oil gets down into the substratum, it is protected from wave action and will take much longer to disappear. Freely draining sediment shores of coarse sand, gravel or stones allow easy penetration of oil. With time, the oil may become more viscous due to evaporation and weathering and cannot escape. Firm beaches

Figure 10.3 Schematic diagram of oil spill processes at sea and shorelines.

In GESAMP 1993 from Mackay (1985). Petroleum effects in the Arctic environment, Ed. Englehardt, Elsevier Applied Science.

Figure 10.3 Schematic diagram of oil spill processes at sea and shorelines.

In GESAMP 1993 from Mackay (1985). Petroleum effects in the Arctic environment, Ed. Englehardt, Elsevier Applied Science.

of waterlogged fine sand or mud resist penetration and most of the oil will be washed away on subsequent tides. However, where there are fine, productive sediments supporting rich populations of burrowing animals and rooted plants, the oil will penetrate deeply through burrows and down root systems. These are the types of conditions found in salt marsh and mangrove systems. Once the oil penetrates such shores, it may remain for many years in an unweathered state. Regeneration of mangrove systems has often been prevented by chronic oiling leaking from such sediments for many years after a spill. Oil that has penetrated shallow sublittoral sediments will similarly be protected to some extent from natural degradation, and may cause persistent fouling on subsequent release from wave-churned beaches long after the original pollution source has ceased.

The general topography of the area and the weather can also modify the effects of spilt oil. If oil drifts onto an irregular coast where there are headlands, inlets, rocks and islands, it will generally form a thinner covering than on a straight shoreline because of the greater distance over which it is spread. Oil on rocky shores tends to be carried upshore towards high water level, whereas on sandy beaches much more of it is spread over a range of shore levels.

Strong onshore winds may carry some of the pollutants inland in windborne spray. This was of particular concern following the Braer oil spill in 1993 in Shetland. Here oil was carried inland by gale-force winds, affecting sheep, cattle and humans.

The persistence of oil on a shore can therefore vary from a few months to many years (IPIECA report series).

Recovery of shore populations

The restoration of an intertidal population after destruction by oil pollution generally involves a sequence of stages in which different species are successively dominant. This sequence is now well recognized on British rocky shores.

An oil spill usually kills large numbers of grazers such as limpets. This means that provided no further oil comes ashore, there is usually a rapid settlement and growth of microalgae which turn the rocks green. Fast-growing green macroalgae, especially Enteromorpha and Ulva, follow. This is often known as the greening phase.

In the continued absence of grazers, sporelings of the larger, slower-growing normal intertidal brown algae settle and grow resulting in an abnormally dense cover of seaweeds. This is the fucoid phase.

Meanwhile, juvenile limpets and snails have settled out of the plankton in sheltered crevices and start to grow rapidly on the abundance of food. Patella spp. often become abnormally numerous and fast-growing. The effect of feeding by this increasing number of herbivores gradually reduces the plant population to normal levels, followed in turn by a decline in number of grazers. Space becomes available for resettlement by barnacles. This is the Patella phase.

The shore may look comparatively normal within two to three years. However, the fine balance between grazers and algae may take much longer to stabilize. In the case of the Torrey Canyon disaster, the shores around Cornwall took 10 years to return fully to normal mainly because large amounts of toxic detergent were used to clean the beaches.

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