A slippery slope: coral reefs and oil spills

Written By Danielle Moloney 


Just like oil and water, oil and coral reefs don’t mix. Reefs already face a multitude of threats to their prosperity- warming temperatures, pollution, and overfishing to name just a few- but oil spills present another difficult challenge. Annually, 500 million gallons enter the oceans- mainly via ship maintenance and improper disposal methods, allowing plenty of opportunity for it to find its way into waters containing reefs. Estimates from the U.S Coast Guard suggest that more than half of the oil in the ocean is from sewage treatment plants rather than accidental tanker spills. 

How does oil affect corals?

There are many contributing factors that determine how much harm will be done when oil comes into contact with coral, including age, species, and amount of exposure experienced by the coral. According to the National Oceanic and Atmospheric Administration (NOAA), there are three main methods through which oil comes into contact with coral: oil floating on the surface of the water deposited directly onto corals when the tide level drops, rough tides mixing lighter oil directly into the water column, where it can float downward and settle on coral, and heavy oils mixing with sand and sediment, making them dense enough to float downward and make contact with corals. Furthermore, indirect effects from burning oil for fuel cause an increase in atmospheric CO2, which leads to an increase in ocean temperatures and presents yet another means through which petroleum oil use can harm corals. 

Once oil comes into contact with coral, a variety of negative outcomes can occur. It can hinder their growth, development, reproduction, and behavior, and can ultimately lead to death. This has lasting impacts on the ecosystems surrounding reefs, as they provide habitats and feeding grounds for so many marine species. 

Cleaning up oil spills near reefs is no bargain either. Often, chemical dispersants are used to break up oil spills and alleviate the damage occurring in the immediate area. This would mean that a smaller amount of oil may come into contact with a larger amount of coral. However, studies have shown that even small doses of oil exposure can prove harmful to corals. These long term small exposures, called chronic oil pollution, lower the life expectancy of coral larvae, damage reproductive systems, lead to loss of zooxanthellae, and eventually to death. 

A diagram shows some of the threats faced by coral reefs including oil spills. See the right side panel for tips on how individuals can help combat these negative effects. Photo courtesy of NOAA. 

Oil spills in the news 

On July 25th, 2020, the Japanese ship MV Wakashio ran aground on a coral reef located off the coast of Mauritius. Mauritius, located off the east coast Africa, is home to coral reefs that inhabit its surrounding shallow waters. Regarded as some of the most biodiverse reefs on the planet, the nearby oil spill posed a significant threat to the health of the reef system. The MV Wakashio was travelling from China to Brazil carrying nearly 4000 tons of oil when it hit the reef in Mauritius. Days later, the oil onboard the ship began to spill into the ocean. 

While this particular spill only contained a fraction of the amount of oil spilled in other well-known disasters (for example, the infamous 1989 Exxon Valdez oil spill spewed about 34,000 tons of oil into Alaskan waters), the problem in Mauritius was the location of the spill. The MV Wakashio ran aground near Pointe d’Esny, in close proximity to two environmentally protected marine ecosystems as well as the Blue Bay marine park reserve. Mauritius’ Prime Minister declared an environmental state of emergency while volunteers flocked to the country to help clean up what was sadly named “one of [Mauritius’] worst ecological disasters.” 

An image from CNN shows the MV Wakashio split in two after running aground on a reef in Mauritius. Image courtesy of CNN. 

What’s next?

Based on the above, something has to give in order to keep corals healthy from threats caused by oil use. One major step forward in making this change is the new development of plant-based oils. An Australian study from 2004 exposed corals to both plant-derived and mineral-derived oils, then measured any loss of symbiotic algae. They found that the plant-derived oil alternative was significantly less harmful to coral health than the mineral-based oil. This alternative addresses the CO2 emissions from oil burning that causes reef stress as well. The plant based oil had fewer greenhouse gas emissions than the petroleum oil. 

By moving away from petroleum based oils, the harmful effects of oil spills will become less of a prevalent stressor for reefs around the globe. You can help do your part by following the tips for how individuals can help protect reefs in the diagram above!

Please contact the author with any questions: dmoloney@fandm.edu

*Featured Image courtesy of CNN

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