Whitehead (1983) describes crude oil as a complex mixture of many compounds comprising mainly compounds of hydrogen and carbon. The composition of crude oil is very variable and all types will contain different quantities of the many different hydrocarbons together with quantities of sulphur, oxygen, nitrogen and traces of metals. Crude oil is variously described as light, medium or heavy, sweet or sour (containing hydrogen sulphide), paraffinic (light), asphaltic (heavy) or mixed (aromatic) base oil, dependent largely on the dominant proportion of hydrocarbons.

The hydrocarbons present in crude oil can range from aliphatic (straight chain) compounds to more complex aromatic (containing a benzene ring) and polynuclear aromatic (containing two or more benzene rings) compounds. Hydrocarbons can be present in the full range of physical states from gaseous through liquids to solids. Heavy crude oils have a greater proportion of long chain aliphatic hydrocarbons than light crude oils where short chain aliphatic compounds predominate. Aromatic compounds predominate in medium (mixed base) crude oils.

Refining of crude oil produces a range of products, ranging from lubricating oils and waxes, asphalts and heavy fuel oils through aviation fuel, diesel fuel and heating oils to gasoline and liquid petroleum gases. Other processes can be used to create a wide range of petrochemicals, including familiar hydrocarbons, such as propylene, acetylene, benzene, toluene and naphthalene.

GESAMP (1993) estimated that 2.35 million tonnes of oil per year entered the marine environment from all sources. At least 15% comes from natural oil seeps. Anthropogenic sources include chronic discharges from storage facilities and refineries, discharges from tankers and other shipping along major routes and accidental events, such as oil spills and ruptures of pipelines. Sources also include river-borne discharges, diffuse discharges from industrialised municipal areas, offshore oil production (e.g. drilling, transport, refining and burning of oil and petrochemicals) and the atmosphere. Locally, an important source is the exhaust from outboard engines.

Elliott and Griffiths (1987) classify sources of hydrocarbons to the Forth estuary as petrogenic, pyrogenic and biogenic (see table below) and this classification is probably applicable to oils and petrochemicals in the marine environment.

Petrogenic sources tend to be point sources of oil and petrochemicals, whereas pyrogenic and biogenic inputs are considered as diffuse sources .

In the UK, 14 of the 155 estuaries studied as part of the Estuaries Review (Davidson et al 1991) had oil refineries on them in 1989, including the Forth, Humber, Thames and Mersey. Nineteen of the 155 had import/export jetties and single point moorings and were therefore at risk from tanker related spillages. Oil spills in estuaries vary in size and their impact depends on the amount and type of oil involved. Elliott and Griffiths (1987) reported six spillages of heavy oil or diesel in the Forth estuary between 1970 and 1978, with only minor incidents up to 1987. In 1983, 6,000 tonnes of crude oil were released in to the Humber estuary at Immingham (the Sivand oil spill) (NRA, 1993). The Mersey estuary received 150 tonnes of crude oil from a fractured pipeline in 1989 and in 1996 the >Sea Empress= released 72,000 tonnes of crude oil in Milford Haven.

Oils and petrochemicals also form part of municipal discharges as a result of road run-off, domestic usage and the licensed discharge of small quantities to sewer. Oils and petrochemicals are therefore found in sewage and storm water discharges, either directly into the estuary or into the freshwater tributaries.

Pyrogenic sources of petrochemicals enter estuaries either directly or indirectly from atmospheric deposition and, although this is technically a diffuse source, deposition is likely to be greater in the footprint of point sources to the atmosphere, such as coal-fired power stations. The burning of fossil fuels is a major source of polynuclear aromatic hydrocarbons (PAHs) into the environment (see Section B46).

Biogenic sources of hydrocarbons can be generated in estuaries where sediment deposits accumulate and microbial and chemical conditions are appropriate.

The concentration of oil in the environment and in the biota is measured in a number of different ways, including total hydrocarbons, total aliphatic hydrocarbons, total aromatic hydrocarbons, persistent oils and grease. The use of these different parameters hinders spatial comparisons.

Various reviews on the fate and weathering of petroleum spills in marine waters have been conducted (Jordan and Payne 1980; Marine Technology Society 1984; Lange 1984; Kuiper and van der Brink (1987). The persistence of oil depends on the type of oil; the season, the geomorphology of the coast and the degree of exposure (GESAMP 1993). SEEEC (1998) review the fate of oil in relation to the Sea Empress oil spill.

Oil and petrochemicals exert impacts on the environment through both physical and chemical (toxic) means. Long-chain aliphatic hydrocarbons are effectively solids and exert their effects by physical means, coating surfaces and smothering organisms. Short-chain, low boiling point compounds, unsaturated compounds and aromatic hydrocarbons exert their effects by primarily chemical (toxic) means. The overall impact of a discharge of oil or petrochemical on the environment is dependent on the distribution and composition of the petroleum hydrocarbons, especially their weathering, persistence and consequently their bioavailability (GESAMP 1993).

Effects differ in open waters and in enclosed systems. In open waters, the action of waves and currents can decrease concentrations of contaminants rapidly. In enclosed systems, such as estuaries, the potential for dispersion is not so great and, on shorelines, a number of factors determine its persistence: properties of the oil, porosity of sediments, presence of animal burrows, wave action and type of vegetation (GESAMP 1993). The table below summarises the vulnerability of different coastal habitats and ranks them in order of vulnerability.

SEEEC (1998) review the effects of the Sea Empress oil spill on marine communities.

A number of projects assessing the environmental impact of the Sea Empress spill was commissioned by the Sea Empress Environmental Evaluation Committee on:

• fate of oil
• marine impacts
• shoreline impacts
• maritime vegetation and agriculture
• mammals
• birds
• reviewing the effectiveness of clean-up operations.

A list of all the reports is included within the SEEEC Report (SEEC 1998).

An exhaustive literature review on the toxicity of oils and petrochemicals to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (e.g. GESAMP 1993, Elliott and Griffiths 1987). The most sensitive groups of organisms have been identified.

Various reviews have been conducted on the toxicity of petroleum spills on marine ecosystems and populations (Clark 1982; Geraci and St Aubin 1990), to which the reader is also referred.

Reproductive, developmental and behavioural processes are very sensitive to exposure to hydrocarbons. Generally, early life stages are more sensitive than adults and many juvenile and adult crustaceans and echinoderms are more sensitive than juvenile and adult fish. It is well established that different oils vary in their toxicities and that acute toxicity is largely due to components of the water soluble fractions and dependent upon exact conditions and duration of exposure to them (GESAMP 1993).

Chronic sublethal effects caused by petroleum hydrocarbons spilled or discharges into low energy, shallow coastal waters remain a valid concern.

The major habitats affected are intertidal and subtidal sediments and to, a lesser extent, the water column. GESAMP (1993) highlighted low-energy, marshes and seagrasses as being among the most sensitive and vulnerable shorelines to oil pollution. Threshold effects concentration for various species and different hydrocarbons and their mixtures in water were reported as low as a few mg l^-1.

Elliott and Griffiths (1987) provided a summary of the biological effects of petrochemical pollution in the Forth (see table below).

The physical effects arise from the tendency of oils to coat surfaces, including sediment surfaces, rocky shores and vegetation. Large deposits are associated with discharges or spillages of crude oil where large expanses of intertidal mud and sand flats, rocky shore and saltmarshes can be covered. When such deposits are not removed as part of a spill clean-up, they smother benthic organisms and prevent feeding by birds and fish. The response of benthic invertebrate communities is very similar to the response to organic enrichment and pollution because the primary effect is to create anoxia in the sediment by preventing exchange of water between the sediment surface and the water column.

The toxic effects of crude oil and a range of petrochemicals have been demonstrated on a range of estuarine invertebrates, including polychaetes (Reish 1979), decapod crustaceans, including crabs (Williams and Duke 1979), shrimps (Couch 1979) and larval decapods (Epifanio 1979), amphipod crustaceans (Reish and Barnard 1979) and molluscs (Menzel 1979). While comparisons are difficult because of the use of different oils and various ways of preparing oil for testing, a number of laboratory tests were performed using water soluble fractions of No. 2 fuel oil and Venezuelan crude. The fuel oil was generally more toxic.

Studies on the changes in fish and macrocrustacean community structure in relation to a discharge of oil and petrochemicals in estuaries are few and have been hampered by the lack of appropriate fish community descriptors and the high levels of natural variability (GESAMP 1993).

Impacts at the fish population level are also difficult to detect but are unlikely to be great because fish will avoid localised areas polluted by oil. In enclosed systems, such as estuaries, extensive pollution by oil may reduce the holding capacity of the system for fish such that its value as a nursery or feeding ground may be reduced.

Lancaster et al (1998) studied the recruitment of sea bass in relation to the 'Sea Empress' oil spill in 1996 and concluded that although some differences were detected between estuaries, they were unlikely to be more significant than natural year-to-year variation.

Laboratory and field studies in the 1970s and 1980s demonstrated acute and chronic effects in adult fish exposed to waters and sediments contaminated with high levels of hydrocarbons (GESAMP 1993). Lethal effects on estuarine crabs, shrimp and lobsters have been demonstrated (Williams and Duke 1979, Couch 1979 and Epifanio 1979). Petroleum spills generally have a low acute toxicity potential for adult fish but fish kills may occur due to high exposure to emulsified oil in shallow waters (i.e. the Braer spill, January 1993, Shetland Isles) (GESAMP 1993). However, the creation of oil water emulsions requires a high energy system which is unlikely to occur in many European estuarine systems.

In laboratory studies with a range of fish (rainbow trout, perch, sea trout, fathead minnow and pike), the effects of exhaust from a two-stroke outboard engine were studied. Estimated environmental concentrations of 0.27 to 1.6 ml l^-1 of exhaust condensate in the wake of the engine were used in the laboratory experiments and effects were observed at the subcellular level (including enzyme activity) and on physiological functions (carbohydrate metabolism and on the immune system) (Balk et al 1994).

Marine wildlife (seabirds and mammals) are often the most conspicuous victims of oil spills. Diving and surface-dwelling populations of seabirds and sea otters are known to be vulnerable and sensitive to oiling.

Following a spill, seabirds may be affected in a number of ways. Although oil ingested during attempts to clean plumage may be lethal, the most common causes of death are from loss of body heat, starvation and drowning following damage to the plumage by oil. Plumage is essential to flight, heat insulation and waterproofing and even small effects on any of these functions can result in mortality. As well as external effects, birds can ingest oil when eating contaminated food. This can cause direct toxicity and lead to decreased survival, density and fecundity of bird populations (GESAMP 1993).

Documentation for the field on the effects of oiling on mammals, especially cetaceans, is scarce (GESAMP 1993).

Seals and dolphins are highly mobile animals which are generally able to avoid any prolonged encounter with an oil slick. The main threats to these animals are not so much the reduction in insulation but internal damage resulting from ingesting contaminated food. Seals are vulnerable to hydrocarbons and other chemicals evaporating from the surface of oil. Exposure to these pollutants causes symptoms which include irritation to the eyes and lungs and breathing difficulties.

Elliott and Griffiths (1987) demonstrated bioaccumulation of hydrocarbons by flounder and plaice in the Forth estuary system and suggested that the primary route of uptake was via the food.

Biomarker studies have revealed that fish detoxify bioaccumulated hydrocarbons and the degree to which this process has been initiated. The induction, through the cytochrome P-450 pathways, of mixed function oxidases (MFO) activity has become a biomarker for hydrocarbon exposure and contamination (Payne and Fancey 1982, Elliott and Griffiths 1987). Initial studies in the Forth in 1987 indicated that MFO activity was greatest closer to the main source of hydrocarbons to the estuary (Elliott and Griffiths 1987). This response has also been detected in the Elbe estuary .

Tainting (an odour or flavour foreign to the product) has occurred in commercial species contaminated with crude and refined oils. GESAMP (1993) report studies detecting taints in fish and macro-crustaceans resulting from exposure from acute incidents, chronic discharges and in experimental studies. Tainting from acute incidents involving crude oil has been reported for mackerel, sea trout, plaice, carp, mullet, salmon, crab and lobster and involving refined oils for mackerel, herring, flounder, sea trout, salmon, haddock, saithe and lobster. Chronic discharges of refinery wastes have resulted in reported taints in grey mullet, eel, 'flatfish', and rainbow trout. Taints have been induced in plaice, eels, salmon, saithe, cod, trout, shrimp and crab in experimental studies using a variety of crude and refined oils.

Experimental studies indicate that taints can be detected when fish are exposure to concentrations of oil in water in the range 0.01 to 1 mg l^-1. Alkylbenzenes have been indicated as capable of causing a taint in fish but this is not the only class of tainters in crude and refined oils. Fish can be tainted very rapidly on exposure - within a few hours at concentrations of oil above 1 mg l^-1 - and have been shown to lose their taint within 1 to 4 days (experimental study on cod). However, field studies have indicated fish were still tainted days or weeks after a spill of fuel oil (GESAMP 1993).

Potential effects include:

• intertidal habitats are under greatest threat from the physical effects of oil pollution. The most vulnerable habitats are sheltered rocky coasts, intertidal sand and mudflats and saltmarshes;
• subtidal habitats and their associated flora and fauna may be threatened in high energy coastal situations where the likelihood of oil/water emulsions forming is greater;
• seals and dolphins are threatened by the consumption of contaminated food. An additional hazard for seals is the inhalation of volatile components of oil causing eye and lung irritation and breathing difficulties;
• damage to intertidal habitats used as seal haul-outs could be significant if the incident occurred during the breeding season;
• birds are affected by oil through the physical damage to plumage and by the consumption of contaminated food.

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