Oil is a naturally occurring substance. It is believed to have formed from decaying plant and animal material that has become incorporated in the sediments of shallow seas and later overlaid by a succession of strata. Over time these organic residues are converted by heat and pressure into petroleum, migrating upwards, sometimes over extensive areas, either to reach the surface or be occasionally trapped in what are to become oil reservoirs. The important point here is that only a small proportion of the oil produced in the rocks is trapped; most of it has found its way to the surface. Oil has been part of the natural environment for millions of years.
What is meant by “clean”?
It is well known that biogenic and petrogenic hydrocarbons are ubiquitous in the marine environment (Myers and Gunnerson, 1976) and it would be unrealistic to define clean as a complete absence of hydrocarbons or a complete absence of petrogenic hydrocarbons. Baker et al. (1990) argues that in defining “clean” the size of the ecosystem is an important consideration. It should not be microscopic, but large enough to include the major plant and animal communities. Any definition should not necessarily require a return to some pre-existing background level, or the complete removal of hydrocarbons from the environment. Thus a working definition might be:
Clean, in the context of an oil spill, may be defined as the return to a level of petroleum hydrocarbons that has no detectable impact on the function of an ecosystem.
What is meant by “recovery”?
Recovery processes may take many forms depending on the nature of the oil spill damage under consideration. Concern over damage to human resources such as fisheries or recreational amenities often takes precedence over damage to the ecosystem because of commercial interests. Human resources are usually quick to recover and, with the exception of some shellfisheries, human uses of a spill-impacted area generally resume as soon as the bulk oil is removed. In many cases, the availability of human services (e.g., amenity beaches) is not closely related to biological recovery and is usually more rapid than biological recovery.
Biological recovery of an ecosystem damaged by an oil spill begins as soon as the toxicity or other damaging properties of the oil have declined to a level that is tolerable to the most robust colonizing organisms (Baker et al., 1990). However, the state to which an environment returns after damage is usually unpredictable. Recolonization will depend on the time of year, the availability of recolonizing forms, biological interactions, and climatic and other factors. Marine ecosystems are in a state of continual dynamic flux. Figure 1 shows an example of an offshore benthic community that has been monitored for many years, and has shown considerable annual fluctuation in numbers of individuals as well as long-term trends, none of which can be related to any known anthropogenic ...