A study published on Thursday found that an estimated 11.1 billion pieces of ocean plastic trash are lodged in coral reefs across the Asia-Pacific region, increasing corals’ susceptibility to potentially deadly diseases by as much as 89 percent. Scientists examined 124,884 corals at 159 reefs from Thailand to Australia, finding plastic bottles, bags, fishing line and even Nike shoes wedged among colorful corals. The region is home to 55.5 percent of the world’s coral reefs, which harbor a quarter of marine species and provide food and livelihoods for hundreds of millions of people.
“Plastics are a triple whammy for coral infections,” said Drew Harvell, a professor of ecology and evolutionary biology at Cornell University who conducted reef surveys in Indonesia for the study published in Science.
First, plastic debris can cut open corals’ delicate skin, exposing them to infection. Second, ocean plastic trash is often colonized by bacteria that can directly introduce disease to corals. And third, plastic can shade corals, blocking light and creating conditions that allow certain pathogens to thrive.
“These diseases are pretty damaging to corals,” Harvell said. “Once a coral has one of these diseases, it can kill the whole colony, and once an infection starts on one coral colony, it can build up steam and spread to other ones.”
“I think it’s a huge new impact to show that these plastics are so dirty that they can be creating wounds and infectious disease,” she added.
The researchers predict the amount of plastic caught on coral reefs will spike 40 percent by 2025 to 15.7 billion pieces.
The findings come as coral reefs are under unprecedented stress from climate change. Rising ocean temperatures have triggered back-to-back coral bleaching events in which the algae that live in corals and provide them with nutrition and their color, become toxic. The corals expel the algae and turn bone white. Deprived of nutrition, corals can die unless ocean temperatures cool and the algae return. A groundbreaking study published January 4 concluded that coral bleaching – a phenomenon virtually unknown before 1980 – is now accelerating at a rate that will not give reefs enough time to recover before the next heat wave hits.
“We don’t have the data to say whether infected corals would be more susceptible to bleaching, but it seems likely,” said Harvell. Also unknown is the extent that bleaching would make corals more vulnerable to pathogens transmitted by contaminated plastic.
Coral scientists are confident of the link between plastic contamination and coral disease, thanks to the study’s extensive surveys of reefs between 2011 and 2014. Researchers, led by Joleah Lamb, a postdoc in Harvell’s lab, laid down transects at each reef, covering an area that ranged from 645 to 1,290 square feet (60 to 120 square meters). The scientists examined every coral colony more than 2 inches (5 cm) in diameter, noting the presence of plastic and disease.
“The real strength of the dataset is that it shows the correlation of coral health with plastic,” Harvell said. “I think we can be pretty definitive that plastics are contributing to the coral dying. It was not uncommon to find dead coral underneath plastic.”
Compared to plastic-free corals, the scientists found significantly higher rates of three debilitating diseases when corals come into contact with plastic: Skeletal eroding band disease (24 percent greater likelihood), white syndromes (17 percent increased likelihood) and black band disease (5 percent higher likelihood).
Study coauthor Courtney Couch, a coral reef researcher at the National Oceanic and Atmospheric Administration in Honolulu, said the scientists did not determine which types of plastic might be more harmful than others. “Our surveys showed that 71 percent of the plastic debris surveyed was associated with textiles, household goods, packaging and consumable items, while the remaining 29 percent was discarded fishing gear,” she said in an email.
One of the more worrisome discoveries was that plastic trash was eight times more likely to affect reefs with greater structural complexity, such as corals with branches that easily snag bags and other debris. Those corals also provide habitat for fish, meaning plastic-induced coral disease could affect coastal fisheries.
But the risk of disease varied dramatically across the Asia-Pacific region, which the study noted is home to 73 percent of the human population that lives within 30 miles (50km) of a coast.
In Indonesia, the researchers observed 25.6 pieces of plastic entangled on reefs per 100 square meters compared to 0.4 items on areas of the Great Barrier Reef in Australia. Indonesia is one of the top five ocean plastic polluters – along with China, Philippines, Thailand and Vietnam, according to a 2017 report from the Ocean Conservancy.