View source for Oil spill pollution impact and recovery


==Sources of oil pollution==
The largest oil spills are due to accidents with the production and transport of oil (collisions and groundings of tankers, hull failure, fires and explosions on platforms). A major source of oil pollution is due to a large number of small unintentional or intentional (e.g. tank washing) spills by ships, and to loading and unloading operations. <ref>[https://www.itopf.org/knowledge-resources/data-statistics/statistics/  International Tanker Owners Pollution Federation]</ref> Besides, releases of industrial and domestic wastewater is also a substantial source of petroleum hydrocarbons in the marine environment.

==Oil spill impacts on the coastal ecosystem==
In general, three categories of effects caused by an oil spill can be distinguished: direct lethal effects, direct sublethal effects and indirect effects (Penela-Arenaz et al., 2009<ref name=P9>Penela-Arenaz, M., Bellas, J. and Vázquez, E. 2009. Chapter Five: Effects of the Prestige Oil Spill on the Biota of NW Spain: 5 Years of Learning. Advances in Marine Biology 56: 365-396</ref>): 
*Direct lethal effects are due to physical and chemical responses to direct oil contact, even without ingestion of pollutants by organisms. Mortality is due to smothering, hypothermia (very common in oiled seabirds), coating (which interferes with an individual's movement, hindering food capture, and escape from predators), or acute toxicity of fuel.
*Sublethal effects, are caused by the permanence of different fuel components in the environment. They do not lead to the death of organisms, but reduce the fitness of the affected species owing to the impact on the physiology, behaviour or reproductive capability of the organisms. These alterations may also modify the distribution, abundance, composition and diversity of impacted communities. When the ocean is polluted by petroleum hydrocarbons, species with smaller individuals and faster reproduction will replace those with larger individuals and lower reproduction rates, thereby changing the local community structure.<ref>Yu, L., Xia, W. and Du, H. 2024. The toxic effects of petroleum pollutants to microalgae in marine environment. Marine Pollution Bulletin 201, 116235</ref>
*Indirect effects include changes in habitat, predator–prey dynamics, interactions among competitors, productivity levels and food webs, due to the loss of key species. Species with small populations are more strongly affected. Important losses of reproductive and breeding habitats may occur in low-energy environments such as rías, bays, estuaries or coastal marshes, which tend to trap oil and to accumulate hydrocarbon pollutants in the sediments.

Sublethal effects of oil and to oil dispersants on fishes range from no observed effect to abnormal skin lesions, decrease in the number of lymphocytes and smooth muscle cells, cardiotoxicity, repressed growth, impaired swimming performance, increased lethargy, changes in behavioral performance, reduced aerobic capacity, impaired cellular stress response, damaged DNA, immune dysfunction, reduced recruitment, and reproductive impairment.<ref>Hajji, A.L. and Lucas, K.N. 2024. Anthropogenic stressors and the marine environment: From sources and impacts to solutions and mitigation. Marine Pollution Bulletin 205, 116557</ref>.

The main threats to marine birds and mammals include behavioral abnormalities, impairment of the immune system, extreme hypothermia, respiratory damage, gastrointestinal damage, reduction to insulation, and membrane damage to the eye, skin, and mucous cells. All animals may experience reduced reproductive success and chemical burns to the ectoderm as a result of direct contact with oil. <ref>Helm, R.C., Costa, D.P., DeBruyn, T.D., O’Shea, T.J., Wells, R.S. and Williams, T.M. 2014. Overview of effects of oil spills on marine mammals. Ch.18, Handbook of Oil Spill Science and Technology, Wiley, pp. 455–475</ref>

Fish eggs, embryos and larvae are particularly susceptible to embryotoxic effects, such as oedema and skeletal abnormalities, due to their proximity to the water surface, small size and underdeveloped membranes and detoxification systems. Many of these effects are caused by the PAH component of oil that accumulates in the lipid bilayer of organisms and affects both the structural and functional properties of the membranes.<ref>Barron, M.G., Vivian, D.N., Heintz, R.A. and Yim, U.H. 2020. Long-term ecological impacts from oil spills: comparison of Exxon Valdez, Hebei Spirit, and Deepwater horizon. Environ. Sci. Technol. 54: 6456–6467</ref>

The effects of hydrocarbon pollution depend on the species impacted. Gastropods and polychaetes are usually the least sensitive species, while corals, bivalves, decapod crustacea and echinoderms are the most sensitive

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'''Options for oil spill cleanup''' from Zhu et al. (2001<ref>Zhu, X., Venosa, A., Suidam, M., Lee, K. 2001. Guidelines for the bioremediation of marine shorelines and freshwater wetlands. U.S. Environmental Protection Agency. Office of Research and Development National Risk Management Research Laboratory. Land Remediation and Pollution Control Division, Cincinnati, USA</ref> and 2004<ref>Zhu, X., Venosa, A.D. and Suidan, M.T. 2004. Literature review on the use of commercial bioremediation agents for cleanup of oil-contaminated estuarine environments. National Risk Management Research Laboratory Office of Research and Development U.S. Environmental Protection Agency Cincinnati, USA</ref>)

''Natural methods''<br> 
Weathering and recovery by natural processes are basically a no-action option, allowing oil to be removed and broken down. Natural dispersion (shearing into small oil droplets  by waves) and emulsification (suspension of oil-seawater droplets by turbulence and microbial surfactants) contribute to the weathering processes that occur after oil release. High water temperatures favor fast dispersion. For some spills, it is likely to be more cost-effective and environmentally responsible to leave an oil-contaminated site to recover naturally than to attempt to intervene. Important natural processes that result in the removal of oils include: 
*Dissolution: Immediately following an oil spill, the light aromatic hydrocarbon compounds (which are highly toxic to aquatic life) dissolve in the water under the oil.
*Evaporation: The primary natural cleansing process during the early stages of an oil spill, resulting in the removal of lighter components in oil. Depending on the composition of the spilled oil, up to 50 percent of an oil's more toxic, lighter components can evaporate within the first 12 hours after a spill.
*Photo-oxidation: Photo-oxidation occurs when oxygen reacts with surface oil under sunlight. Photo-oxidation leads to the breakdown of more complex compounds (especially asphaltene<ref>Li, P., Lu, Z., Zou, S. and Yang, L. 2023. Marine oil spill photodegradation: Laboratory simulation, affecting factors analysis and kinetic model development. Marine Pollution Bulletin 197, 115729</ref>) into simpler compounds that are lighter and more soluble in water, allowing them to be further removed by other processes. The photo-oxidation process slows down after a few days<ref>Ward, C.P. and Overton, E.B. 2020. How the 2010 Deepwater Horizon spill reshaped our understanding of crude oil photochemical weathering at sea: a past, present, and future perspective. Environ. Sci. Process Impacts 22 : 1125–1138</ref>.
*Biological degradation: Several types of microorganisms capable of oxidizing petroleum hydrocarbons are widespread in nature. Biodegradation is an important mechanism to remove the non-volatile components of oil from the environment if nutrients and oxygen are sufficiently available. A lack of nutrients can be compensated for by supplying fertilizers. This so-called bio stimulation is less effective in anoxic environments as anaerobic biodegradation is slow. [[#Annex Crude oil constituents and biodegradation|Biodegradation]] typically takes months to years for microorganisms to decompose a significant portion of an oil stranded in the sediments of marine and/or freshwater environments.
*Sinking: See text box MOSSFA below. 

''Physical methods''<br>
Physical containment and recovery of bulk or free oil is the first response option of choice for the cleanup of oil spills in marine and freshwater shoreline environments. Commonly used physical methods include: 
*Booming and skimming: Use of booms to contain and control the movement of floating oil and use of skimmers to recover it. Minimal environmental impact, efficient for small spills in quiet water, but low oil recovery rate on the high seas. 
*Wiping with absorbent materials: Use of hydrophobic materials to wipe up oil from the contaminated surface. Disposing of contaminated waste requires the necessary attention.
*Mechanical removal: Collection and removal of oiled surface sediments by using mechanical equipment. This method should be used only when limited amounts of oiled materials have to be removed. It should not be considered for cleanup of sensitive habitats or where beach erosion may result. 
*Washing: washing of the oil adhering along the shorelines to the water’s edge for collection. Washing strategies range from low-pressure cold water flushing to high-pressure hot water flushing. This method, especially using high-pressure or hot water, should be avoided for wetlands or other sensitive habitats. 
*Sediment relocation and tilling: Movement of oiled sediment from one section of the beach to another or tilling and mixing the contaminated sediment to enhance natural cleansing processes by facilitating the dispersion of oil into the water column and promoting the interaction between oil and mineral fines. Oil penetration deep into coastal sediments and release of oil and oiled sediment into adjacent water bodies are issues of concern.
*In-situ burning: Oil on the shoreline is burned usually when it is on a combustible substrate such as vegetation, logs, and other debris. This method may cause significant air pollution and destruction of plants and animals. 

''Chemical methods''<br> 
Chemical methods, especially dispersants, are routinely used as a response option in many countries. There are contrasting opinions about the effectiveness of these methods and concerns about their toxicity and long-term environmental effects. Major existing chemical agents include: 
*[[#Annex Use of dispersants|Dispersants]]: dispersing agents, which contain surfactants, are used to remove floating oil from the water surface to disperse it into the water column before the oil reaches and contaminates the shoreline. This is done to reduce toxicity effects by dilution to benign concentrations and accelerate oil biodegradation rates by increasing its effective surface area. 
*Demulsifiers: Used to break oil-in-water emulsions and to enhance natural dispersion. 
*Solidifiers: Chemicals that enhance the polymerization of oil can be used to stabilize the oil, to minimize spreading, and to increase the effectiveness of physical recovery operations. 
*Surface film chemicals: Film-forming agents can be used to prevent oil from adhering to shoreline substrates 
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==Recovery from three major oil spill accidents==
The ecological impacts of three major oil spills have each been monitored over a period of more than ten years. There are similarities, but also some differences between the three cases. For each case, this article summarizes some important conclusions regarding the recovery of the impacted ecosystems.

===Exxon Valdez===

[[File:ExxonValdezShigenaka.jpg|thumb|left|530px|Fig. 1. Exxon Valdez at Outside Bay, May 1989. Photo credit Gary Shigenaka, NOAA.]]

The supertanker Exxon Valdez (Fig. 1) ran aground on a reef in Prince William Sound on the Gulf of Alaska just after midnight March 24, 1989, after being loaded with crude oil the previous day. A leak in the tanker caused 37,000 tons of oil to flow into the sea, much of which ended up on the coast a few days later driven by storm waves and currents. The oiling would eventually extend about over more than thousand miles through Prince William Sound and down the Alaska Peninsula. The spill cleanup operation would peak at an estimated 10,000 workers, 1,000 vessels, 100 aircraft and helicopters, and extend into four years. Exxon estimated its cleanup costs to be $2.1 billion<ref>Shigenaka, G. 2014. Twenty-Five Years After the Exxon Valdez Oil Spill: NOAA’s Scientific Support, Monitoring, and Research. Seattle: NOAA Office of Response and Restoration. 78 pp</ref>. 

The oil, with a lower viscosity than commercial asphalt, caused a mass slaughter of marine animals, including more than 100,000 seabirds, thousands of sea otters and hundreds of harbor seals. In the first few years the amount of oil that had washed ashore sharply decreased as a result of evaporation, cleaning, weathering, dispersal and degradation. Microbial biodegradation stimulated by oleophilic nitrogen-containing liquid fertilizers was most effective<ref>Bragg, J.R., Prince, R.C., Harner, E.J. and Atlas, R.M. 1994. Nature 368: 413-418</ref>. Hopane, a saturated multicyclic hydrocarbon, was selected as an indicator of bioremediation effectiveness, because of its great resistance to biodegradation. In 1992 an estimated 2% of the initial oil spill, from which all volatile and most toxic components had been removed, was still present<ref>Short, J.W., Lindeberg, M.R., Harris, P.M., Maselko, J.M., Pella, J.J., Rice, S.D. 2004. Estimate of oil persisting on the beaches of Prince William Sound 12 years after the Exxon Valdez oil spill. Environ. Sci. Technol. 38: 19–25</ref><ref>Boehm, P.D., Page, D.S., Brown, J.S., Neff, J.M. and Gundlach, E. 2015. Long-Term Fate and Persistence of Oil from the Exxon Valdez Oil Spill: Lessons Learned or History Repeated? International Oil Spill Conference Proceedings 2014(1): 63-79</ref>. By 1997, monitoring provided strong inferential evidence that intertidal populations within Prince William Sound experienced a substantial amount of recovery from the effects of the 1989 oil spill. 

A survey 26 years after the disaster revealed that approximately 0.6% of the oil is remaining sequestered in the subsoil below 10–20 cm of clean sediments. These oil residues are protected from hydrological washing and contain a high fraction of polar compounds recalcitrant to biodegradation. These observations suggest that sequestration limits the bioavailability of the oil despite the fact that it still retains toxic compounds<ref>Lindeberg, M.R., Maselko, J., Heintz, R.A., Fugate, C.J. and Holland, L. 2018. Conditions of persistent oil on beaches in Prince William Sound 26 years after the Exxon Valdez spill. Deep-Sea Research Part II 147: 9–19</ref>.


===Prestige===

[[File:PrestigeSinkingWikimedia.jpg|thumb|530px|left|Fig. 2. Sinking of the Prestige. Photo Wikimedia.]]

On November 13, 2002, the hull of the 26-year-old oil tanker Prestige burst during a storm off the coast of Galicia, Spain. The oil-leaking ship was not allowed to go to a sheltered port for repairs, but had to sail away from the coast by order of the Spanish, French and Portuguese authorities. On November 19, the ship broke in two on the high seas (Fig. 2), about 200 kilometers off the coast. Almost the entire cargo, 60,000 tons of heavy fuel oil, ended up in the sea. Part of the fuel sank to the seafloor and part of it drifted to the Spanish, French and Portuguese coasts. More than 2,000 km of coastline and more than 1,000 beaches were polluted with oil.

Manual cleaning and washing using hot pressurized water had limited effectiveness on sandy beaches and even less along shorelines where the average grain size was pebble or cobble size<ref name=G6>Gallego, J.R., González-Rojas, E., Peláezm A.I., Sánchez, J., García-Martínez, M.J., Ortiz, J.E., Torres, T. and Llamas, J.F. 2006. Natural attenuation and bioremediation of Prestige fuel oil along the Atlantic coast of Galicia (Spain). Organic Geochemistry 37: 1869-1884</ref>. Hydro-cleaning machines were the preferential method to remove oil from exposed rocky shores. Areas inaccessible to mechanical cleaning methods (over 60,000 m<sup>2</sup> of rocky surface area) were treated by bioremediation. The Prestige fuel oil that reached the Spanish coasts was characterized by low solubility and low capacity for dispersion, slow degradation, and high viscosity, adherence and density that hindered rapid weathering, specifically biodegradation, suggesting that the bioavailability of heavy fractions was very low at most of the sites. It consisted of approximately 25% aliphatics, 20% resins, 20% asphalthenes and 35% aromatics - the most toxic oil component for marine biota<ref name=P9/> (see [[#Annex Crude oil constituents and biodegradation]]). Due to the very high viscosity of the oil, application of dispersants was judged to be ineffective (see [[#Annex Use of dispersants]]). 

More than two years after the spill, the sites where no remediation treatment was performed still maintained over 50% of the initial amount of aromatic compounds; however, light and medium n-alkanes were almost totally degraded in the first months following the spill. 
Application of the oleophilic fertiliser S200 (a microemulsion of a saturated solution of urea in oleic acid containing phosphate esters) was compared at various sites with natural attenuation. Depending on the fuel compounds, an additional hydrocarbon depletion ranging from 10% up to 30% was achieved. However, at the sites studied, and despite initial successful results, effect did not persist over the following winter and spring. Microbial fuel degradation was enhanced where humidity, dissolved oxygen and nutrient availability were optimal and fuel adhesion was physically weakened, suggesting the increased effectiveness of bioremediation when irrigated with fresh water<ref name=G6>Gallego, J.R., González-Rojas, E., Peláezm A.I., Sánchez, J., García-Martínez, M.J., Ortiz, J.E., Torres, T. and Llamas, J.F. 2006. Natural attenuation and bioremediation of Prestige fuel oil along the Atlantic coast of Galicia (Spain). Organic Geochemistry 37: 1869-1884</ref>. 

[[File:EuropeanShagChristophMoning.jpg|thumb|right|250px|Fig. 3. European shag (''Gulosus aristotelis''). Photo credit Christoph Monin [https://ebird.org/science/status-and-trends ebird.org] ]]

One of the rare documented long-term effects of oil spill pollution regards the European shag (Fig. 3), of which the reproductive success was reduced by 45% in oiled colonies compared with unoiled ones, while reproductive success did not differ before the Prestige accident. This impairment lasted for at least the first 10 years<ref>Barros, A., Alvarez, D. and Velando, A. 2014. Long-term reproductive impairment in a seabird after the Prestige oil spill. Biol. Lett. 10: 20131041</ref>. It was suggested that seabird populations may have suffered from the sub-lethal effects of oil exposure and reduced food availability after the Prestige oil spill. However, this effect was not triggered at the base of the trophic chain because long-term monitoring surveys showed that the effect of the Prestige oil spill on phytoplankton activity and net primary production was ephemeral, if at all present<ref>Varela, M., Bode, A., Lorenzo, J., Teresa Alvarez-Ossorio, M., Miranda, A., Patrocinio, T.,  Anadon, R., Viesca, L., Rodriguez, N., Valdes, L., Cabal, J., Lopez-Urrutia, A., Garcia-Soto, C., Rodriguez, M., Alvarez-Salgado, X.A. and Groom, S. 2006. The effect of the 'Prestige' oil spill on the plankton in the N-NW Spanish coast. Marine Pollution Bulletin 53: 272-286</ref>. 

Five years after the sinking of the Prestige some oil was still leaking from the wreck. There is also some evidence that part of the oil initially accumulated along the continental shelf (300 kg/m<sup>2</sup> in January 2003 and 0.5 kg/m<sup>2</sup> in October 2004) is gradually transported onshore<ref name=B13>Bernabeu, A.M., Fernandez-Fernandez, S., Bouchette, F., Rey, D., Arcos, A., Bayona, J.M. and Albaiges, J. 2013. Recurrent arrival of oil to Galician coast: the final step of the Prestige deep oil spill. J. Hazard. Mater. 251: 82–90</ref>. Even nine years after the accident, oil was detected in the intertidal area of both beaches in all campaigns. Tar balls were highly biodegraded suggesting that the oil was accumulated on the seafloor for a long time before being transported to the coast by the action of waves<ref name=B13/>.

===Deepwater Horizon===

[[File:DeepwaterHorizonWikimedia.jpg|thumb|530px|left|Fig. 4. The Deepwater Horizon on fire. Photo Wikimedia.]]

The Deepwater Horizon floating oil platform in the Gulf of Mexico exploded on April 20, 2010, due to a blowout while drilling an oil well (the so-called Macondo well) at a depth of 1500 m (Fig. 4). The ultimate cause was a deficient valve in the blowout preventer, which caused the high gas pressure in the well to go out of control. Prior to the blowout, several incidents had occurred that had been ignored to avoid delaying the drilling program. The explosion killed nine crew members on the platform and two engineers. Close to half a million tons of oil (about 4,000,000 m<sup>3</sup>) was spilled into the sea. The total cost of the disaster was close to US$150 billion<ref>Lee, Y.G., Garza-Gomez, X. and Lee, R.M. 2018. Ultimate Costs of the Disaster: Seven Years After the Deepwater Horizon Oil Spill. Journal of Corporate Accounting & Finance 29: 69–79</ref>.

The leaking liquid oil consisted by weight of approximately 38% natural gas and 62% liquid oil. The Macondo oil is a light, sweet oil, with a relatively high content of low molecular weight hydrocarbons and a relatively low sulfur and asphaltene content. Methane (20-30 mass percent) completely dissolved during ascent. Approximately 25% of the spilled oil was recovered or burned, 5–15% evaporated, and the remaining 60–70% spread and weathered within the Gulf of Mexico. It was concentrated in two locations:  on the sea surface, where large droplets of liquid oil formed a slick of mostly insoluble, hydrocarbon-type compounds and in a deep intrusion layer that formed at depths between 900 and 1,300 meters<ref>Ryerson, T.B., Camilli, R., Kessler, J.D., Kujawinski, E.B., Reddy, C.M., Valentine, D.L., Atlas, E., Blake, D.R., de Gouw, J., Meinardi, S., Parrish, D.D., Peischl, J., Seewald, J.S. and Warneke, C. 2012. Chemical data quantify Deepwater Horizon hydrocarbon flow rate and environmental distribution. PNAS 109: 20246–53</ref>. Shortly after the accident the dispersants [https://en.wikipedia.org/wiki/Corexit Corexit] 9500A and 9527 were applied onto the surface slick, and approximately 3000 m<sup>3</sup> of Corexit 9500A were released at depths directly into the plume of the escaping oil<ref>Gros, J., Socolofsky, S.A., Dissanayake, A.L., Jun, I., Zhao, L., Boufadel, M.C., Reddy, C.M. and Arey, J.S. 2017. Petroleum dynamics in the sea and influence of subsea dispersant injection during Deepwater Horizon. PNAS 114:10065–70</ref>.  

A variety of physical, chemical, and biological mechanisms helped to transform, remove, and redisperse the oil and gas that was released. Mechanical skimming and burning removed 3-4% and 6-8% of the total spill, respectively<ref>Etkin, D.S and Nedwed, T.J. 2021. Effectiveness of mechanical recovery for large offshore oil spills. Marine Pollution Bulletin 163: 111848</ref>. Biodegradation removed up to 60% of the oil in the intrusion layer but was less efficient in the surface slick, due to nutrient limitation. Photochemical processes altered up to 50% (by mass) of the floating oil<ref name=PO>Passow, U. and Overton, E.B. 2021. The Complexity of Spills: The Fate of the Deepwater Horizon Oil. Annu. Rev. Mar. Sci. 13: 109–36</ref>.

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MOSSFA stands for Marine Oil Snow Sedimentation and Flocculent Accumulation and describes the gravitational settling of oil in association with ballasting particles and its deposition onto the seafloor. Different types of oil–particle associations can produce MOSSFA events, including (a) the aggregation and sedimentation of large phytoplankton blooms that forms MOS; (b) the formation of bacteria–oil aggregations, which are biofilm-like structures initiated by microbes in response to oil exposure; and (c) the formation of oil-particle aggregates, where fine sediment particles, such as drilling mud, coat and penetrate oil droplets. <ref>Passow, U. 2016. Formation of rapidly-sinking, oil-associated marine snow. Deep Sea Research Part II: Topical Studies in Oceanography 129: 232–240</ref>.
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The oil spill from the well resulted in a deep-sea plume of petroleum hydrocarbons and marine oiled snow sedimentation and flocculent accumulation (MOSSFA). About 20% of the unrecovered oil was deposited in this way on the seabed over an area of more than 100,000 km<sup>2</sup>. The flocculent layer remained in place for years until benthic life had recovered sufficiently for soil bioturbation and subsequent biodegradation. Seabed contaminated with oil from the well was found more than 500 km from the accident site<ref name=PO>Passow, U. and Overton, E.B. 2021. The Complexity of Spills: The Fate of the Deepwater Horizon Oil. Annu. Rev. Mar. Sci. 13: 109–36</ref>.

An estimated 10-30% of the surface oil came ashore a few months after the accident, mainly along the Louisiana shoreline, but also on the shorelines of Mississippi, Alabama, and Florida. In total, over 2,000 km of coast were oiled, half of which were beaches and half were wetlands. When oil reached the salt marshes, it was absorbed into sediments or remained on the sediment and grass surfaces. Some stranded oil supplies showed biodegradation within weeks. Oil filtered into the sand of warm, well-aerated, and physically dynamic beaches led to half-lives of less than a month. Alkanes and PAHs buried in sandy beaches were largely biodegraded within 3 years, while slower biodegradation of sediment-oil agglomerates overlying the sand took place through mechanical and photooxidative processes<ref>Bociu, I., Shin, B., Wells, W.B., Kostka, J.E., Konstantinidis, K.T. and Huettel, M. 2019. Decomposition of sediment-oil agglomerates in a Gulf of Mexico sandy beach. Sci. Rep. 9: 10071</ref>. In contrast, biodegradation of PAHs and alkanes hardly occurred in oil mats buried in anaerobic layers of marsh sediments. Oil concentrations that were initially 100-1000 times above pre-spill values, dropped to 10 times after 8 years, demonstrating long-term contamination by oil or oil residues that persists for decades<ref>Turner, R.E., Rabalais, N.N., Overton, E.B., Meyer, B.M., McClenachan, G., Swenson, E.M., Besonen, M., Parsons, M.L. and Zingre, J. 2019. Oiling of the continental shelf and coastal marshes over eight years after the 2010 Deepwater Horizon oil spill. Environ. Pollut. 252: 1367-1376</ref>. Even 10 years after the spill, oil from the accident continued to occasionally wash up on beaches.

More than 80 deep sea octocoral communities at distances up to 20-30 km from the Macondo well contained traces of oil, as well as surfactant used in the dispersant Corexit. Branch loss was observed on some colonies, and hydroids colonized damaged portions of the colonies, impeding tissue regeneration and weakening the coral’s skeleton due to the added epibiont mass. The initial level of total impact in 2011 had a significant positive effect on the proportion of new growth after 2014. However, growth was not sufficient to compensate for branch loss at one of the impacted sites where corals are expected to take an average of 50 years to grow back to their original size<ref>Girard. F., Cruz. R., Glickman. O., Harpster, T. and Fisher, C.R. 2019. In situ growth of deep-sea octocorals after the Deepwater Horizon oil spill. Elem. Sci Anthr. 7: 12</ref>.

Sediment profile and plan view imaging data collected in 2011 and 2014 showed a rapid benthic functional response to the Deepwater Horizon oil spill. Adverse effects related to organic enrichment decreased along a spatial gradient away from the wellhead. Although the spatial signal of these effects was still significant and detectable in a few variables 4 years after the spill, the data indicated that significant and meaningful functional benthic recovery had occurred<ref>Guarinello, M.L., Sturdivant, S.K., Murphy, A.E., Brown, L., Godbold, J.A., Solan, M., Carey, D.A. and Germano, J.D. 2022. Evidence of Rapid Functional Benthic Recovery Following the Deepwater Horizon Oil Spill. ACS ES&T Water.2c00272</ref>.

According to sensitivity analyses<ref>Ainsworth, C.H., Paris, C.B., Perlin, N., Dornberger, L.N., Patterson, W.F.III, Chancellor, E., Murawski, S., Hollander, D., Daly, K., Romero, I.C., Coleman, F. and Perryman, H.  2018. Impacts of the Deepwater Horizon oil spill evaluated using an end-to-end ecosystem model. PLoS ONE 13(1): e0190840</ref>, the biomass of large reef fish may have decreased by 25% to 50% in the areas most affected by the spill, and the biomass of large demersal fish by as much as 40% to 70%. The oil pollution impacts on reef and demersal forages may have caused starvation deaths of predators and increased reliance on pelagic forages. The consequences for the food web indicate possible consequences of the spill far away from the oil area. Effects on age structure indicate possible delayed effects on fishing yields. Generally, recovery of high-turnover populations is predicted to occur within ten years, but some slower-growing populations may take more than thirty years to fully recover.


==Annex Crude oil types, constituents and biodegradation==
Table: Properties of different oil types<ref>Alves, T.M., Kokinou, E. and  Zodiatis, G. 2014. A three-step model to assess shoreline and offshore susceptibility to oil spills: the South Aegean (Crete) as an analogue for confined marine basins. Mar. Pollut. Bull. 86: 443–457</ref>

{| class="wikitable"
|-
! Group !! Oil category !! Density [kg/l] !! Characteristics 
|-
|I || Gasoline and Kerosene || < 0.8  || Non-persistent and dissipate entirely within a few hours; do not form emulsions.
|-
| II ||Gas Oil and Abu Dhabi Crude || 0.8 - 0.85 || Lose up to 40 % by volume via evaporation after spill. Form viscous emulsions, demonstrate an initial volume increase, and some limited natural dispersion.
|-
| III || Arabian Light Crude, North Sea Crude Oils ||  0.85 - 0.95 || Similar to Group II, but with less natural dispersion.
|-
| IV || Heavy Fuel and Venezuelan Crude Oils || > 0.95 || Lack volatile material and are highly viscous. Preclude evaporation and dispersion.
|}

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Crude oil contains four broad fractions:<ref name=V17>Varjani, S.J. 2017. Microbial degradation of petroleum hydrocarbons. Bioresource Technology 223: 277–286</ref> Aliphatics, Aromatics, Resins and Asphaltenes. <br>
'''Aliphatics''' are saturated or unsaturated hydrocarbons consisting of linear or branched open-chain structures. They include alkanes (e.g. methane, etane, propane), iso-alkanes (e.g. isobutane), naphthenes, terpenes and steranes. 
'''Aromatics''' are ringed hydrocarbon molecules. They include monocyclic aromatic hydrocarbons (e.g. benzene, toluene, ethylbenzene, xylenes) and polycyclic aromatic hydrocarbons (PAHs) such as naphthalene (two-ringed), phenanthrene and anthracene (three-ringed), pyrene and chrysenes (four-ringed), fluoranthene and benzo[a]pyrene (five-ringed). 
'''Resins''' are amorphous solids dissolved in oil. They contain numerous polar functional groups formed with N, S, O and trace metals (Ni, V, Fe) and are structurally similar to surface-active molecules in crude oil and act as peptizing agents. 
'''Asphaltenes''' are viscous and high molecular weight compounds composed of polycyclic clusters, variably substituted with alkyl groups, which contributes to their resistance to biodegradation. They are soluble in light aromatic hydrocarbons such as benzene and toluene. 
<br>
<br>
Biodegradation is a process by which microorganisms (bacteria, fungi, algae) mitigate, degrade or reduce hazardous organic pollutants (alkanes, aromatics) to innocuous compounds such as CO<sub>2</sub>, CH<sub>4</sub>, H<sub>2</sub>O and microbial biomass without adversely affecting environment<ref name=V17/>. Bacteria are the primary degraders and most active agents in petroleum pollutant degradation. Microorganisms in polluted areas adapt to the environment through genetic mutations induced in subsequent generations, priming them to become hydrocarbon decomposers. Hydrocarbon-degrading microorganisms in unpolluted ecosystems constitute less than 0.1% of the microbial community, whilst this fraction may increase to 1-10% of the total population in an oil-polluted environment<ref>Atlas, R.M. 1991. Microbial hydrocarbon degradation-bioremediation of oil spills. J. Chem. Technol. Biotechnol. 52: 149–156</ref>. The biodegradation performance is highest for the natural microbial community already adapted to the site pollution<ref>Bargiela, R., Mapelli, F., Rojo, D., Chouaia, B., Tornes, J., Borin, S., Richter, M., Del Pozo, M.V., Cappello, S., Gertler, C., Genovese, M., Denaro, R., Martínez- Martínez, M., Fodelianakis, S., Amer, R.A., Bigazzi, D., Han, X., Chen, J., Chernikova, T.N., Golyshina, O.V., Mahjoubi, M., Jaouanil, A., Benzha, F., Magagnini, M., Hussein, E., Al-Horani, F., Cherif, A., Blaghen, M., Abdel-Fattah, Y. R., Kalogerakis, N., Barbas, C., Malkawi, H.I., Golyshin, P.N., Yakimov, M.M., Daffonchio, D. and Ferrer, M. 2015. Bacterial population and biodegradation potential in chronically crude oil-contaminated marine sediments are strongly linked to temperature. Sci. Rep. 5, 11651</ref>. Pathways of microbial degradation of hydrocarbon pollutants involve various reactions viz. oxidation, reduction, hydroxylation and dehydrogenation. Saturated hydrocarbons are more easily biodegradable than the aromatic hydrocarbons (e.g. PAHs), which have more deteriorating effects on the environment and life forms. Biodegradability of hydrocarbons can be ranked as: linear alkanes > branched alkanes > low-molecular-weight alkyl aromatics > monoaromatics > cyclic alkanes > polyaromatics > asphaltenes<ref>Atlas, R.M. 1981. Microbial degradation of petroleum hydrocarbons: an environmental perspective. Microbiol. Rev. 45: 180–209</ref>.  Complete degradation of complex hydrocarbon mixture requires synergistic action of different microbial species. Microalgae have a positive influence on sustaining and enhancing bacterial growth by supplying organic exudates and oxygen<ref>Denaro, R., Di Pippo, F., Crisafi, F. and Rossetti, S. 2021. Biodegradation of hydrocarbons in marine environment. In: Inamuddin, Ahamed, M.I., Lichtfouse, E. (Eds.), Water Pollution and Remediation: Organic Pollutants. Springer Int. Publ., pp. 195–228</ref>. Addition of enzymes (e.g. cytochrome P450s, laccases, hydrolases, oxygenases, dehydrogenases, lipases) for biocatalysis allows obtaining faster results in the biodegradation of toxic substances, with low impact on the environment and reduced energy consumption<ref>Bhandari, S., Poudel, D.K., Marahatha, R., Dawadi, S., Khadayat, K., Phuyal, S., Shrestha, S., Gaire, S., Basnet, K., Khadka, U. and Parajul, N. 2021. Microbial enzymes used in bioremediation. J. Chem. 2021: 1–17</ref>. Using genetic engineering techniques, microorganisms can be modified to break down pollutants much more efficiently. However, the introduction of genetically modified organisms (GMOs) into the environment is still a controversial issue and generally forbidden by law. There are currently no reports of their use in the marine environment<ref>Tedesco, P., Balzano, S., Coppola, D., Esposito, F.P., de Pascale, D. and Denaro, R. 2024.  Bioremediation for the recovery of oil polluted marine environment, opportunities and challenges approaching the Blue Growth. Marine Pollution Bulletin 200, 116157</ref>.

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==Annex Use of dispersants==
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Dispersion of oil, i.e. the breaking up of large oil slicks into small droplets, is a natural process that depends on the characteristics of the oil, its weathering stage and environmental parameters such as wave energy, salinity, temperature, etc. This process can be enhanced by application of specific chemicals, so-called dispersants. Dispersants are mixtures of surfactants in one or more solvents designed for application to oil spills with the aim of reducing the interfacial tension between the oil and the water phase. Dispersants promote the natural breakup of floating oil into small droplets in the water column.

For being effective, the dispersant must be able to physically mix with the polluting oil. If the oil is too viscous, chemical dispersion will generally not be possible. Dispersion is most efficient with light, low viscosity oils. A minimum wave height and resulting turbulence are required for effective dispersion. On the other hand, too high waves make dispersant application infeasible and also less necessary because of effective natural dispersion<ref>Zeinstra-Helfrich, M., Koops, W. and Murk, A.J. 2015. The NET effect of dispersants - a critical review of testing and modelling of surface oil dispersion. Mar. Pollut. Bull. 100: 102–111</ref>.

Enlarging the overall contact surface of the oil will in most cases promote bio-degradation by naturally occurring marine microorganisms. Breaking up oil slicks not only reduces the oiling of sea birds and mammals, but also reduces the wind drift of oil slicks towards sensitive coastal areas, such as tidal flats and marshes (see [[Oil sensitivity mapping]]). However, the increased concentration of oil components within the water column resulting from the oil dispersion can potentially increase toxic effects on pelagic, demersal and benthic living organisms. Hence, there is a trade-off among different habitats and species with different ecological, social, and economic values<ref name=G18>Grote, M., van Bernem, C., Böhme, B., Callies, U., Calvez, I., Christie, B., Colcomb, K., Damian, H-P., Farke, H., Gräbsch, C., Hunt, A., Höfer, T., Knaack, J., Kraus, U., Le Floch, S., Le Lann, G., Leuchs, H., Nagel, A., Nies, H., Nordhausen, W., Rauterberg, J., Reichenbach, D., Scheiffarth, G., Schwichtenberg, F., Theobald, N., Voss, J. and Wahrendorf, D-S. 2018. The potential for dispersant use as a maritime oil spill response measure in German waters. Marine Pollution Bulletin 129: 623–632</ref>. The priorities for protection may be different between different stakeholders, such as fishermen, tourism managers or environmentalists. Transparency of the decision-making process is therefore essential<ref name=G18/>. Bio-surfactants (e.g. fatty acids, phospholipids, lipopeptides, glycolipids), which break oil into readily biodegradable small droplets, are much less harmful to the environment than traditional synthetic dispersants<ref>Krishnaswamy, M., Subbuchettiar, G., Ravi, T.K. and Panchaksharam, S. 2008. Biosurfactants properties, commercial production and application. Curr. Sci. 94: 736–747</ref>. After the marine oil spill from the Deepwater Horizon, marine bacteria able to produce bio-surfactants became predominant in the area. 
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==Related articles==
:[[Oil spill monitoring]]
:[[Index of vulnerability of littorals to oil pollution]]
:[[Oil sensitivity mapping]]
:[[Coastal pollution and impacts]]
:[[Bioremediation of marine ecosystems]]


==References==
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{{author
|AuthorID=120
|AuthorFullName=Job Dronkers
|AuthorName=Dronkers J}}


[[Category:Coastal and marine pollution]]
[[Category:Oil spills]]