Natural Degradation Of Oil Spills Biology Essay


An oil spill can be defined as an accidental or deliberate dumping of oil or petroleum products into the ocean and its coastal waters, bays, and harbors, or onto land, or into rivers or lakes (Holum 1977). Between one and ten million metric tons (one metric ton is 1000 kilograms) of oil are put into the oceans every year. The oil is released, most often, in small yet consistent doses from tankers, industry, or on shore waste disposal (Boesh, Hersher, et al. 1974). Tanker spills cost the United States more than one hundred million dollars every year. Spill frequency increases proportionally with tonnage carried, in a linear manner. Non-tanker spills also increase linearly and account for thirty percent of all spills. The Atlantic area near Europe averages eight spills a year, the American area seven, and the Pacific two. Spills of more than ten thousand metric tons account for about two and a half percent of total spills, and spills above fifty thousand metric tons occur on average once a year. The average spill size is around seven thousand metric tons (Smets 1982).

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If left alone, oil spills will eventually break up naturally. The natural degradation is influenced by temperature, wind, wave action, the thickness of the oil, the degree of dispersion, and the oil's tendency to form emulsions with water (Smith 1968). There are eight primary processes in the natural degradation of oil. The first step is spreading and motion. This step can be broken down into three smaller steps: gravity, viscous forces, and surface tension. Gravity initially spreads the oil into an even layer across the surface. Viscous forces then take over and account for even more spreading. The oil is finally spread into a monomolecular slick by the surface tension of the water.

The second step of natural degradation is evaporation. The amount of evaporation that occurs is dependent on how far the oil slick has spread. There is a higher amount of evaporation when the slick is spread out to a greater degree. Fifty percent of hydrocarbons found in oil are removed from a light spill within ten days, by evaporation (Beer 1983). With crude oil, twenty-five percent of its volume can be evaporated during the first few days. Evaporation continues at a diminishing rate for several weeks (Smith 1968).

The third step in natural degradation occurs when the wind and the waves cause the hydrocarbons to enter into a solution with the ocean water. Typically, the most soluble hydrocarbons are the most toxic.

Emulsification is the fourth step in the natural degradation process. Insoluble components of the oil emulsify with the ocean water and are described as oil in water or water in oil emulsions, depending on the ratio of one to another (Beer 1983). Emulsions can contain up to eighty percent water. As the mass of the oil in the emulsions decreases through evaporation, the specific gravity or density increases, which makes the emulsions more likely to sink (Smith 1968).

The next step is direct sea to air exchange. Wave-produced spray transfers hydrocarbons to the air in the same way salt is transferred (Beer 1983). There is a constant cycle of hydrocarbons between the sea and the air, but these are more localized along tanker routes, and coastal areas (Garrett 1975).

The sixth part of this process is photochemical oxidation, which occurs when sunlight induces chemical changes in the oil and it begins to harden.

The seventh step in the natural degradation of oil is sedimentation. As the density continues to increase, the oil and its other components become heavier than water, so they eventually sink to the bottom of the ocean (Beer 1983).

The final step in the process is bacterial degradation. The remaining sediments on the ocean bottom are broken up by bacteria. If an oil spill stays at sea for longer than three months, only fifteen percent of the original amount should theoretically remain.

The impact of oil on a marine ecosystem is not the same on different organisms. Organisms living at or near the surface are impacted more than others in the area. These organisms include those in intertidal areas, heuston, seabirds, and bottom dwellers. The type of oil spilled influences the extent of damage that is done to the organism.

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Crude oil is the most common petroleum product that is found in oil spills. Crude oil is a complex mix of hydrocarbons with small amounts of sulfur, oxygen, and nitrogen and traces of inorganic and organometalic compounds. There are three classes of hydrocarbons: alkanes, alkenes, and aromatics. Alkanes, in low concentrations, cause anesthesia and narcosis, and in high concentrations, cell damage and death, in marine invertebrates. Alkenes, which are not typically found in crude oil but in refined products such as gasoline, are more toxic than alkanes, but less toxic than aromatics. Aromatics are not common in nature but they can kill ocean organisms, not only at full strength, but in diluted concentrations as well. Aromatics are the most immediately toxic component found in oil (Boesch, Hershmer, et. al. 1974).

Some dangerous hydrocarbons include benzene, toluene, naphthalene, and phenanthrene. Benzene inhibits blood cell formation, and they all cause local irritation of the respiratory system, excitation or depression of the central nervous system, and are mutagenic, carcinogenic, and teratogenic. The effects of oil on organisms can be broken down into two subdivisions, chemical and physical. Chemical effects are caused directly by the involved components and usually take place immediately. Physical effects such as oil coating and reduction of the photosynthetic rate from oil films, are indirect and can take longer to occur (Duffus 1980).

In the intertidal area, physical affects of oil include smothering, abrasion, and removal and alteration of the substrate. Organisms with mucus, such as macroalgae and anemones are often unaffected by the physical effects of oil. Direct toxicity is rare in the intertidal area. Most organisms are harmed by the non-toxic results of the oil being in the environment. Snails often retract into their shells to avoid the oil, lose their grip on a solid surface, and are washed away. Mollusks can become so encrusted with oil that they are swept away. Mobile animals sometimes are cemented to rocks with the thick oil. Sessile animals such as barnacles can become covered in oil and suffocate (Boesch, Hershner, et al 1974).

Bird death tolls from oil spills are recorded for the animals that reach the shore. It is believed that this accounts for roughly five to fifteen percent of the actual bird mortalities from oil spills. Diving sea birds have inordinate mortalities from oil spills. Other birds, such as seagulls and shearwaters, appear to avoid oil slicks on the water. One reason for the difference in mortality may be that when the diving birds resurface, they come up under an oil slick. The primary effect of oil on birds is the fouling of feathers, which disrupts insulation and buoyancy. This causes them to sink, drown, or lose body heat. Birds can also ingest the oil, which inflames the digestive track. After the digestive track becomes inflamed, the bird is not hungry so it does not eat and eventually dies from starvation. Less than twenty percent of birds that are affected by oil and are treated survive. Birds are the only victims that are known for certain to be affected by routine intentional oil discharge by tankers. Due to this, several populations in the northeast Atlantic face local extinction and one species of penguin is endangered with global extinction (Boesch, Hershner, et al 1974).

Mammals and fish are not as affected as birds by oil spills. Mammals, such as pinnipeds and cetaceans are suffocated by the oil and lose their insulation when it attaches to their fur. They can also be poisoned when the oil is ingested (Boesch, Hershner, et al 1974). When the oil becomes matted in the fur of sea lions, many mothers die trying to lick their pups clean (Alvaro and Fernandez 2001). Soap and water are used by rescuers to try and clean their fur, but is not always successful. Milk is used to clean the heads of marine mammals because it does not sting their eyes (Dorfman 2001). Fish may be more resistant to the oil because their surfaces and gills have mucus that repels oil. It is also believed that fish swim away from oil slicks. It is unknown how fish are affected by feeding on contaminated benthic invertebrates (Boesch, Hershner, et al 1974).

There are various clean up techniques for oil spills. These include containment by barriers and the physical removal of floating oil, applying an absorbent that is meant to soak up the concentrated oil, sinking, burning, chemical dispersal, and steam cleaning. Containment and removal is the most desirable method to avoid biological damage, because no foreign substances are added to the water. A downside is that booms and skimmers to remove the oil can only be used in calm water. This method must be applied immediately after the accident to prevent sedimentation.

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Using absorbents is the favored method in North America. Straw is applied to soak up the oil. There is a slight ecological impact because the dried straw and oil deposits are more persistent than oil alone in intertidal areas. Peat moss and synthetic absorbents such as oleophilic plastic polymers are also used, but they increase the problem of the persistence of non-biodegradable absorbent material.

Sinking, which is prominent in Europe, is the least acceptable method from a biological standpoint. Chalk or sand is thrown onto the oil to make it heavier, so it will sink. While this may save it from polluting other areas, large deposits of oil end up on a large area of the ocean bottom. This affects benthic life that supports finfisheries and shellfisheries.

Burning is biologically innocuous. Wicking agents or oxidants often need to be used so that the oil will burn better. This is only feasible on small, contained oil slicks. Chemical dispersal is more of a cosmetic treatment than an overall clean up, because more often than not, it injects oil more solidly into the ecosystem rather than removing it. Chemical dispersal should only be used in extreme cases such as fire hazards and the restoration of important amenities. Steam cleaning exterminates intertidal life and does more harm than good. The general consensus is that physical removal and absorbents are the most reliable non-destructive methods (Boesch, Hershner, et al 1974).

Disposal after clean up also presents another problem. Oil is not the only thing that needs to be disposed of at the end of a spill clean up. Things like seaweeds, dead animals, sand, and supplies (such as straw, plastics, chalk, or chemicals) must also be disposed of properly. There are three main disposal methods used after oil spill clean ups. They are refining, incineration, and burial. The most ecologically and economically sound option is refining, depending on the location of the nearest refinery. If the oil is contaminated to a great degree, it cannot be refined. The probability of using incineration for disposal depends on the location of the incinerator and also the presence of sufficient organic matter in the wastes to make for efficient burning. The final option is to truck wastes to a disposal site and bury them underground. However, ground water contamination is a strong possibility when burying wastes (Boikema and Cairns 1984).

Oil spills have significant long-term effects. These effects can be broken down into the long-term recovery of an ecosystem, chronic pollution, and persistent contamination of ecosystems. In the recovery of an ecosystem, organisms that have been depopulated, have to reproduce. This is a slow process. At times, when a dominant species has been removed, others can gain an advantage through atypical survival of their young, and rearrange the order of the ecosystem. Marine communities have a more rapid rate of succession than land communities such as forests, but it still takes several years. The amount of time needed for total recovery depends on the structural complexity of the system and the degree of damage that was done. Persistent contamination or chronic pollution severely hinders and slows an ecosystem's ability to recover. Chronic pollution occurs when an environment is exposed to the persistent, continuous release of petroleum from refineries, petrochemical plants, oil ports, and other waste discharges.

The production of petroleum accounts for a large portion of the oil put into marine environments. Oil refineries release substantial amounts of petroleum during normal production. Almost any marine community is affected by ecosystem contamination. Chronic pollution can keep a community in an immature stage with a low diversity rate. Much of the ocean is contaminated to at least some degree with petroleum. High concentrations can be found even outside of oil spill areas. Although some petroleum is released naturally through seeps, the rate of petroleum released by man is higher than the natural release rate. Once hydrocarbons have contaminated an ecosystem they become further concentrated in the food web by biomagnification. This disrupts photosynthesis, respiration, and other metabolic functions; has carcinogenic and mutagenic potentials; and can destroy normal behavior patterns associated with survival, feeding, schooling, reproduction and development in the entire ecosystem food web (Boesch, Hershner, et al 1974).

Until oil spills can be prevented, these problems will continue. The best prevention mechanism to help stop large spills usually associated with tankers, is by careful boat navigation (Beer 1983). As to how to stop the general spillage and leakage that occurs continuously, there is no answer yet. Human error accounts for a lot of the destruction of marine ecosystems, and until people become more concerned and aware of the problem, outside of the large tanker spills, the problem will continue and probably get worse.