Summary of the potential effects of oil on the environment

There are a number of ways that oil may be introduced into the marine environment, including the operational, accidental and illegal discharges from shipping (and to a lesser extent boating), tanker accidents resulting in major oil spills, dumping of industrial wastes, sewage and industrial discharges and atmospheric deposition. For ports and harbours located within urbanised areas, all of these sources are likely to occur. For many estuaries, inlets or bays chronic inputs (for example sewage and industrial effluents) are the most important source of oils. It follows from this that within the port environment, port or shipping related activities might not be the only cause, or the major cause, of any oil contamination that may exist.

Over 80% of reported oil spills occur within port and harbour areas, however the majority are small in size and result from normal operations such as loading and bunkering (MPCU 1997). Other inputs may occur from the transport of oil in tankers, including the accidental or illegal discharges of tank washings and oil-contaminated ballast water. However, oil pollution is not only a concern of ports with oil terminals or commercial traffic, but small ports, harbours and marinas can also contribute to the amounts of oils entering the marine environment. Inputs from recreational craft are generally recognised as being insignificant in comparison to the inputs from commercial shipping (BMIF 1997), but can contribute to the potential effects of oil pollution in marine SACs. For example, sources of oil contamination in marinas include, spills of fuel and lubricating oils, exhaust emissions, wood treatment solutions, and run-off from marina parking lots (Voudrais & Smith 1986). These are common sources that also arise from shipping and maintenance activities in ports and harbours.

It is difficult to assess the effect of oil in the marine environment because of the large variation in sources, quantities, and nature of the oil, also the physical, chemical and biological conditions of the environments involved. The majority of research relating to the effects of oil on the marine environment relate to major oil spill events, usually from shipping accidents and groundings, the environmental effects of which are well known by all, particularly the associations with oiled birds and mammals. However, very little literature describes the effects of chronic discharges from run-off or numerous small discharges of oil, which are common in port and harbour areas. A summary of some of the potential effects of oil on the environment is shown below. As well as causing environmental damage, oil pollution can be very costly to clean up.

Summary of the potential effects of oil on the environment

• Marine animals and plants tend to be tolerant of low level concentrations of oil in sediments from chronic or small discharges, however this is not always the case.
• Exposure to major and minor oil spills can lead to the mass mortality of benthic communities, fish, marine mammals and birds, and the severe damage of saltmarsh.
• Conversely, the effects of major oil spills on marine habitats and species can often be temporary and non-fatal (for example Zostera beds were exposed to oil after the Sea Empress incident with little or no observable effects).
• Saltmarsh vegetation often recovers well after a single spill, however chronic pollution may cause the long-term loss of saltmarsh vegetation (Toft et al 1994). Different saltmarsh species show different tolerance to oil, with the result that repeated spillages may alter the community structure and allow tolerant species to become dominant (Field Studies Council Oil Pollution Research Unit 1994).
• Contamination of sediments with oil may modify chemical, physical and biological processes (Berge, Lichtenthaler & Oreld 1987). Contaminants can be trapped in the sediments and later released as a result of disturbance, such as erosion.
• In sediments, as it is organic, oil will be broken-down relatively quickly by micro-organisms which may result in the localised removal of oxygen from the sediments and surrounding water with possible effects on marine life.
• The persistent toxic constituents of oil, such as heavy metals, can become stored in the sediments and taken up into the food chain. Therefore, following large oil spills, even where animals recover in diversity and density, they may continue to suffer physiological and behavioural disorders which can result in reduction of growth and reproduction, and in the worse cases, death. For example, liver lesions in flatfish are associated with high concentrations of oil in sediments (Weston 1990).
• The breakdown of oil tends to be slowest in intertidal areas, which leads to the highest concentrations and longest residence times (Keizer et al 1978).

The containment, dispersal or clean-up of oil spills can greatly minimise the extent of the effects on the environment. The use of dispersants assists in the breakdown of oil, removing it from the water surface and preventing its spread, therefore timely use in the right locations may prevent oil spills reaching the intertidal and may avoid or reduce impacts on birds. However, they promote the penetration of oil into the sediments, potentially affecting shallow fishing grounds and other sensitive intertidal habitats. In cases where oil cannot be prevented from covering intertidal habitats it may sometimes be better left untreated and allowed to be removed by tidal action, as the clean up operations are often more damaging than the effects of the oil alone (Howard, Baker & Hiscock 1989). For example, considerable damage was caused by vehicles driving over eelgrass Zostera beds during clean-up operations following the Sea Empress spill (SEEEC 1996). All environmentally sensitive areas should be identified in the risk assessment.

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