Crude oil is a fossil fuel, a type of ancient carbon stored within sedimentary rocks. It is derived from organisms living in past geological epochs and is found as droplets within porous rocks deep below the continental shelf or in the ocean floor away from coastlines, as well as in land based sedimentary basins. Modern society uses oil products in just about everything-- from something as negligible as petroleum in beauty products, to global energy sources such as transportation fuel or liquid petroleum gases. This is done by refining mined crude oil. Production starts at land or offshore based rigs drilling down into the earth and pumping oil, which is then transported by pipelines, ships, or other transportation methods. Accidents during transport are usually responsible for spills, thus spills can occur anytime and anywhere, although they are more likely to happen in locations with a high concentration of oil platforms, simply because there is ample room for error. Such locations include the Persian Gulf or the Gulf of Mexico.
Factors such as storms, seismic activity, miscommunication, or human error are responsible for the catastrophic accidents that occur at offshore oil platforms and ships, resulting in large quantities of oil being released into the ocean. Perhaps the most infamous spill in recent history, the beaching of Exxon Valdez in Alaska in 1989, as it released about 37000 metric tonnes of oil into the sound and devastated the environment for years to come. However, the spill had one positive impact: it sparked conversation about the effects of crude oil on the ecology. The effects of accidental spills are also largely unpredictable as physical factors based on location of the spill and the geological or climatic conditions negate the immediate and long term outcomes of the event. A good example of this is shown in a study looking at the oil patch spread following the 2010 Deepwater Horizon incident in the Gulf of Mexico. In his study, Poje (2014) found that ocean flows below 10 km contain significant energy and fluctuations in such flows determined the initial spread of pollutant clouds after the Deepwater Horizon incident. The canyon in which this spill occurred lies between areas with deep-water ocean flows below 10 km, wind driven surface currents, and the Mississippi River delta, the plume of which also affects circulation in the spring and summer. In addition to oil release from spills, up to 47% of all oil input into the ocean is assumed to be added naturally through seeping, at about 600000 metric tonnes per year, although there is a large degree of uncertainty.
The reason why oil spills are devastating to the oceans, is because oil impairs marine life both physically and chemically. In previous years, it was assumed that organismal communities suffered due to immediate mortality after the spill, however the persistence of toxic hydrocarbons at sublethal levels continues to affect wildlife, with negative impacts on the ecology driving population cascades and hindering recovery. Immediate impacts of oil spills include oil lathering fur/feathers/scales and hindering mobility, suffocation, reduced visibility, and poisoning. Such effects are best represented in birds, as oil disrupts feather structure, thus altering flight, leading birds to drown or fall victim to predation. The bird will then try to preen and accidentally ingest or inhale oil, leading to poisoning. Another impact of oil on birds is when oil coats their eggs, and effectively kills the embryo within it by limiting access to gas exchange, which can lead to population declines and affect the ecological balance of the environment. Toxic compounds from crude oil are very persistent and can be found up to 1.5 years after the spill in salt marshes, with effects still being recorded up to 6.5 years since then. Similarly, although the local economy improved in Prince William Sound, effects of the Exxon Valdez spill are still being felt today (32 years later) in the ecology of organisms like invertebrates, seabirds, and herring.
Oil spills affect a variety of environments based on where the spills occur, as well as oceanic weather conditions and currents. Typically, if a tanker spills tonnes of oil, the habitats in the pelagic zone and the water column are the first to be affected, yet the winds blow and carry the pollutants closer to shore, where there is more primary productivity and biodiversity, thus having a much larger effect on the organisms that live there. As time goes by, benthic ecosystems become affected as the oil degrades and its products seep down towards the substrate, impacting the small meiofauna, epifauna, and other burrowing or filter feeding animals. The benthos could also just get slicked up and covered by oil, depleted of oxygen if the spill occurs close to shore.
The eutrophic zone of shoreline and pelagic ecosystems can be susceptible to oils and their degraded products due to natural wave action and climatic events like storms, which disperse the pollutants on the surface of the water, creating a high surface area, toxic sheen.
Oil influences fur-bearing mammals and their ability to insulate their bodies in cold water. When animals such as sea otters and birds are exposed to oil, its water repellency nature exposes these animals to harsh elements. Without the ability to repel water and insulate their bodies, these mammals can die due to hypothermia. Though marine organisms such as corals, fish, and shellfish may not be exposed immediately, they can come into contact with oil as weathering occurs. This causes the spilled oil to break down and become heavier than water. Exposure to oil in adult fish can result in impairment of reproduction, reduced growth, changes in respiration and heart rates, fin erosion, and enlarged livers. Species such as shellfish are susceptible to oil as they are consistently exposed in the intertidal zone. This makes them unsafe for other organisms to consume. Juvenile sea turtles can be trapped in the oil or mistakenly consume it. In addition to immediate effects of oils, the chemical components found in crude oil can impair cell membrane function and behaviours of a wide array of organisms, as well as inhibit phytoplankton production, by having the surface film block out sunlight penetrating into the mixed layer. Poisoning from oil toxins can also occur years after the spill-- as biomagnification, when the levels of toxins increase up the food chain and become concentrated in top predator species.
Small, larval stages of fish and invertebrates are particularly susceptible because they are weak, delicate, and developing organisms, and toxins from oil directly inhibit their growth. Corals are especially sensitive to oil detergents and dispersed oil, and they can rapidly deteriorate due to a combination of the spilled oil and anthropogenic attempts for cleanup.
Many marine ecosystems have been drastically affected by oil spillage. Around 126 000 hectares of mangrove vegetation have been destroyed since 1958 due to oil spills that covered 1.94 million hectares of mangrove habitat worldwide. 238 spills of note contributed to this destruction and a lack of data suggests that the extent of the damage is an under-estimate. Figure 3.1 illustrates the damaging effects of an oil spill that pollutes an Indonesian mangrove. Oil Spills also have a significant effect on benthic communities and can have latent effects in coastal regions through the transportation of pollutants by waves and currents, prolonging losses in biodiversity by months. The recovery from oil spills is affected by several factors, with it taking longer in cold climates with thick, heavy oils and in sheltered settings. Warm climates, like many spills in Louisiana and Texas, have comparatively faster recovery rates. Heavy treatment can also negatively affect recovery rates, making it more important to adjust treatment measures to the conditions of the spill. This is particularly significant in marshy areas where erosion can prolong vegetation recovery and habitat loss. The broader reach of oil spills also targets coral habitats. The high susceptibility of small animals that build coral reefs to oil components causes damage to the structural integrity of reefs. Without these animals, the reefs consequently erode and break off, causing more damage to the ecosystem as a whole.
According to the ITOPF and the analyses of occurrence rates of offshore oil spills by Anderson and LaBelle, the rate of occurrence of tanker spills has greatly decreased worldwide since the late twentieth century. Figure 3.2 depicts this drop-off in large oil spill occurrences from tankers from 1970 to 2020. Decreases in spills have also been observed in barges and pipelines in the US during this time as well with the Oil Pollution act of 1990 necessitating greater prevention and estimation measures in the United States . On a global scale, with the exception of the Sanchi spill, 19 of the 20 largest oil spills occurred before the year 2000. Global Oil Spill Trends are slowing into the 21st century as well. Most recently, three medium spills were recorded in 2020 (7-700 tonnes) with around 1000 tonnes2 lost during the year total. Not only has the frequency of large and medium spills decreased, but so has the total spills and the total oil spilled. Occurrences of spills have held fairly steady since 2010, but a drop-off in the last two years gives hope for the future impact of oil spills on ecosystems. Still, cleaner and more efficient recovery strategies are still necessary as residues can linger. 5 years after the Ixtoc oil spill, residues were still found on the Mexican beaches. Even with the recovery of these habitats, the tendency of history to repeat itself requires continued measures to prevent the effects of oil spills in the future.
To limit the occurrence of oil spills, many prevention methods are used by offshore operators. Some of these include:
Despite these efforts of prevention, these mechanisms fail on occasion, resulting in unintended spills. Oil spills in the ocean are often difficult to clean up. Each oil spill has its unique set of aspects including oil type, location and weather conditions that need to be taken into careful consideration. The spread and behavior of each oil spill is dependent on various physical, chemical, and biological processes. Thus, there is no one way to approach oil spills. Some of the most important and commonly used methods are outlined below. These methods can be used locally, and/or globally depending on the nature of the oil spill, and the economic capacities of a Nation.
Oil booms act as a fence that prevent oils from spreading or floating away. Once the oil is confined, skimmers are then used to separate the oil from the water on the surface level so that it can be collected, processed, and reused. This allows the collected oil to still be economically viable. Unfortunately, this method can only be used effectively within a few hours of spillage. Once enough time has surpassed, the area of the spill will become too large to manage.
In-situ burning is the ignition of surface floating oil in an attempt to burn it off. When used correctly, in-situ burning is able to reduce the amount of oil on the water surface significantly while minimizing the negative effects of oil on the environment. However, this method could produce toxic fumes that can potentially result in significant damage to the environment and marine life. There is also a minimum concentration (thickness) of 3mm of oil required for ignition.
Dispersants, such as Corexit 9500, are chemical emulsifiers that are sprayed onto the spills with the help of aircrafts and/or boats. These dispersants assist the formation of small oil droplets, promoting the rapid dilution of oil through water movements. Small oil droplets increase the surface area of the oil, which increases exposure to bacteria action and natural evaporation. Dispersants are more effective when used within the first couple of hours of the initial spill. Successful dispersion of oil can help reduce the immediate risk to seabirds and shorelines, but can also have negative impacts on deep water corals and sea grasses.
Biodegradation is the use of bacteria and other microorganisms to breakdown the spilled oil into harmless substances such as carbon dioxide and fatty acids. The natural process of biodegradation can be sped up through fertilizing the surface of the ocean with nitrogen and phosphorus to stimulate the growth of these microorganisms. Biodegradation is one of the best methods when it comes to combating oil spills in sensitive areas that are not under threat. However, it is a lengthy process and fertilizers could provide optimal conditions for the growth of unwanted algae that consume a considerable amount of oxygen, cut off sunlight, and negatively impact marine life.
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