Plastic pollution poses a grave and growing threat to Earth’s oceans – and to coral reefs in particular. Between 9 and 23 million tons of plastic waste enter marine and freshwater environments every year, where they can cause irreparable harm . Plastic can damage or destroy coral by becoming tangled around it, whilst bacteria growing on that plastic can carry diseases which ravage entire reefs . When swallowed by marine animals, plastic fragments and particles can cause internal wounds, as well as leaching out toxic chemicals which are soon passed up the food chain when those animals are eaten, negatively affecting entire ecosystems .
Given its immense and devastating impact, it is therefore essential that we study plastic pollution – including where it occurs, what sort of plastic is most common and how it entered the environment. The more information we have about plastic pollution, the more effective we can make our solutions to it. However, little is known about plastic pollution in some ecosystems, such as mesophotic coral reefs. Mesophotic coral reefs lie deeper in the water – at depths of between 30 and 150 metres – than the shallow reefs we tend to think of, but can be just as important ecologically. Unfortunately, since we know so little about pollution on these deeper reefs, they are often overlooked for conservation and management.
Between 2016 and 2022, a team of 19 marine biologists from 9 different countries set out to change this situation. Their goal was to survey plastic pollution on a variety of shallow and mesophotic coral reefs, as well as to assess which factors might be affecting its distribution. Using divers and both manned and unmanned submersibles, they surveyed 84 coral reefs from across the Atlantic, Indian and Pacific Oceans. These reefs covered a combined area of almost 68,000 square kilometres and were located in the waters of 13 countries, including the British Overseas Territory of Bermuda and the US territory of Guam. The team’s focus was on identifying macroplastic waste – plastic waste larger than 5 centimetres.
What they found was astounding – and horrifying. They identified 258 pieces of man-made waste, spread across 77 of the 84 reefs surveyed. Concerningly, they even found high densities of waste on reefs more than 10 kilometres from human populations – suggesting that even the farthest reaches of our oceans are being affected by human activity. Of the waste they identified, 88% was made of plastic, with the remainder including fabric, glass and metal.
One of the study’s most important findings was that man-made waste – including plastic – was more common on mesophotic reefs than on shallow reefs. The density of waste was greatest at depths of 50 to 100 metres. The team suggested several explanations as to why man-made waste was more common on the mesophotic reefs they surveyed. One potential reason was that shallow reefs are more exposed to waves, which can dislodge plastic and other waste and cause it to fall towards deeper, more sheltered areas. Another was that, since they are more well-known and accessible, shallow reefs have benefitted from clean-up efforts, reducing the amount of waste found there. It was also possible that some waste was hidden out of sight within the coral on shallower reefs, escaping the notice of the survey.
By far the most common source of plastic waste found on the reefs was fishing activity. Some 73% of the plastic waste the team identified came from fisheries, largely comprising discarded fishing nets and lines. The remainder of the plastic they encountered was composed of everyday consumer items, such as plastic bottles. On only 9 of the reefs surveyed – all located in Comoros waters – was everyday consumer plastic more common than fishing-related plastic, likely due to those reefs being close to unofficial landfill sites. The prevalence of fishing-related plastic may explain why waste was so common on largely-uninhabited reefs, since declines in fish populations have forced fishing vessels to travel further in search of their catch. Worryingly, fishing-related plastic was also found in high densities near to marine protected areas, suggesting that fishing vessels are operating on the edge of these vital conservation zones in order to take advantage of fish spilling over from inside them.
This study highlights the far-reaching nature of plastic pollution – touching even deeper-water reefs in largely uninhabited areas. It also demonstrates the importance of considering deeper, mesophotic coral reefs for conservation and management, since these vital habitats are severely affected by pollution. Furthermore, the study shows that it is essential to reduce both plastic production and consumption, whilst also working to find plastic-free alternatives to fishing gear, which the team predicted would be a significant challenge.
If we are to preserve our planet, its ecosystems and the services they offer us, however, it is a challenge we must rise to meet.
Story by Daniel Tucker-Bailey
Focus Paper – Pinheiro HT, MacDonald C, Santos RG, Ali R, Bobat A, Cresswell BJ, Francini-Filho R, Freitas R, Galbraith GF, Musembi P, Phelps TA, Quimbayo JP, Quiros TEAL, Shepherd B, Stefanoudis PV, Talma S, Teixeira JB, Woodall LC, Rocha LA (2023) ‘Plastic pollution on the world’s coral reefs’, Nature, 619:311-316
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Photo (C) q phia Wikimedia Commons