How Five Trillion Pieces of Plastic Impact Coral Reefs

Written by Danielle Moloney 

Marine debris in the world’s oceans has been a major topic of conversation in recent years as the global population continues to grow, and the rate of debris entering the ocean grows along with it. Ocean debris can refer to plastics, fishing nets, or any other man-made products that make their way into the marine environment (accidentally or otherwise) – though plastics constitute between 60 and 80% of all marine debris. According to The Ocean Cleanup project, there were 5 trillion pieces of plastic in the oceans as of 2019 – and there are still all the other types of materials to consider! The mismanagement of waste, along with a high demand for plastics and other disposable products, has facilitated the inordinate amount of debris that has entered the global oceans. News outlets have painted a picture of huge floating patches of litter and marine life interacting in an alarming way with marine debris – but how does all of this impact coral reefs? 

Given the severity and persistence of the global marine debris problem, researchers across the globe have worked to assess what ocean debris means for coral reefs in particular. Corals tend to respond poorly to dramatic changes in their environment, which can be brought about by high levels of debris in their waters, and even via direct contact with the corals themselves. For example, fishing nets can become entangled in the branches of corals, resulting in both physical damage as well as an introduction site for pathogens (Sweet et. al 2019). Disruptions in healthy coral tissue due to damage (such as scraping from contact with marine debris) allow disease to infiltrate the system, which facilitates the spread of pathogens into nearby tissue, which ultimately spreads and leads to degradation. In 2019, a group of scientists from the United Nations Environment Programme (UNEP) released a comprehensive compilation of current scientific knowledge about the effect of plastics on shallow water corals. Their goal was to distill scientific knowledge in such a way that policymakers can make the most informed decision possible to ameliorate the marine debris problem. 

Fig. 1 Plastic water bottle on top of a stony coral. Photo courtesy of Encyclopedia of the Environment.

Where is all of this debris coming from?

The UNEP report breaks down marine debris sources into ten major categories, the top three most significant sources being rivers/land based runoff, direct dumping, and lost fishing gear (also known as ghost gear), as shown in the table below. Tourism and coastal communities are cited as the two main contributors to marine debris, especially plastics. The report also points out that while ghost gear is not as widely covered in the media, it contributes a significant amount of debris to the oceans. Other sources such as straws and microbeads from cosmetics are more discussed, and while damaging, do not contribute to the same degree as the aforementioned major sources. This being said, all types of marine debris affect different aspects of reefs in different ways, all of which are negative. 

Table 1. A compilation of the ten major sources of marine debris by estimated metric tons of plastic entering marine ecosystems. Table courtesy of UNEP Report on Plastics and Shallow Water Coral Reefs. 

Why does this matter?

There is a reason why coral reefs are so well studied, and it is not solely for their natural beauty. Reefs are estimated to support over 500 million people, whether via their economic, social, or ecological relevance. This amounts to billions of dollars in the global economy which depend on the continuity of healthy coral reef ecosystems. An estimated 4.5 billion people rely on fish from reef habitats as a significant portion of their daily protein. Other examples of how humanity relies on reefs include the income, coastal protection, and fisheries that they provide to their dependents. The marine environment is a boundless and ever moving ecosystem, marine debris is an international issue that spans country lines and reaches all corners of the oceans, even those that are far from human habitat. Furthermore, estimates suggest that a significant portion of marine debris entering the oceans is never accounted for – meaning that the potential implications may be more severe than expected. 

Fig. 2 A map of estimated plastic on coral reefs from 2010 to 2025 projections. Data for this figure include marine debris levels assessed at the time as well as an estimate of mismanaged waste that would have the potential to enter the ecosystem as marine debris. Figure courtesy of UNEP Report on Plastics and Shallow Water Coral Reefs, with adaptations from Lamb et al. 2018 and Jambeck et al. 2015.

Implications and Future Steps 

While the scope of the marine debris issue can seem insurmountable, there are still important steps that can be taken to address and decrease the scale of the issue. This is especially important moving forward and change must take place now. In an effort to bridge the gap between science and policy, the UNEP report concludes with a list of recommendations that, based on their scientific assessments, will aid in protecting reefs and other marine life from the harmful effects of debris. In general, these recommendations can be categorized into two major areas of improvement: creating strong partnerships between various stakeholders (i.e. international governments, scientists, policymakers, coastal communities, etc.) and investing in concrete actions (e.g. educating fishermen, funding research/monitoring projects, enforcing both incentives and punishments for compliant vs. noncompliant stakeholders) in order to reduce the amount of marine debris produced and to decrease the amount already present in the oceans. 

Awareness is the first step in tackling the marine debris crisis. Although the persistence and severity of this issue are great, the steps outlined above can make a significant difference in terms of whether or not this issue declines or continues to worsen in the coming years and decades. 

This article only addresses a small subset of the information provided by the UNEP Report on Plastics and Shallow Water Coral Reefs. If you would like more information, or to read the report in its entirety, please visit this link:

Please contact the author with any questions:

1 thought on “How Five Trillion Pieces of Plastic Impact Coral Reefs

  1. Douglas Fenner, Ph.D. April 3, 2020 — 8:56 pm

    I am a scientist who studies corals and coral reefs. I have been diving on coral reefs for about 40 years. I have been diving in many places in the Caribbean, Pacific, and Indian Oceans. I have about 2800 dives under my belt. I live in American Samoa. There are streams with loads of plastic in them, and beaches crammed with plastic on them. On a few rare occasions I’ve seen plastic bags stuck on corals, and indeed the coral under the bag is usually dead. I spent 3 months on an island off of Belize once, and the shoreline was plastered with trash, mostly plastic, tons of it. We dove twice a day, 6 days a week, and I snorkeled on Sundays. I never saw any plastic on any corals. I lived 2 years in the Philippines, and dove on most weekdays. Villages had plastic trash like snow in some places. At one place I swam near the surface with plastic all around and on me. But I do not remember seeing any plastic on corals.
    Plastics in the ocean are a huge problem, we all agree. But plastic floats and much of it ends up on beaches. Some of it sinks, but almost all the world’s oceans are deep water without reefs, reefs are a tiny proportion of the ocean. And in my experience plastics are one of the most minor threats of all to coral reefs. I have seen reefs turned white as snow from high temperature bleaching, and I’ve seen fields of coral killed by high temperature bleaching. There is a long laundry list of things that damage coral reefs, and plastics are near the bottom. In 1998, high temperatures killed about 16% of all the world’s coral. I know of no empirical studies or statistics that show that coral reefs are heavily damaged by corals. I think this story is built on pieces that are real, the huge amounts of plastics in the environment, and the fact that coral reefs are indeed being destroyed, and assumed that plastics were a major part of that damage. That is NOT the case. Assumptions must be tested. If this one is tested, it will fail, I have 40 years of experience that show that.


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