31 Jan 2018 - In the relatively pristine waters of the Great Barrier Reef marine disease ecologist Joleah Lamb spent years looking for the ways human activities—from pollution that warms the ocean to commercial fishing to scuba diving and other tourist activities—could affect how often the legendary corals off the Australian coast get sick.
One thing she and her team did not see much of was plastic trash. “So it wasn’t something I thought about a lot,” Lamb says. That changed when she and colleagues began studying the reefs off Indonesia, Myanmar and other parts of Southeast Asia. They were floored by the ubiquity of diapers, water bottles and plastic bags littering the fragile ecosystems. The researchers kept a record of the detritus they came across in their work in the region—and the data shows that after plastic comes into contact with a reef, the coral is 20 times more likely to be afflicted by disease.
“This is just astounding to see that much of an increase,” says Mark Eakin, who heads up the National Oceanic and Atmospheric Administration’s Coral Reef Watch program. He was not involved in the latest study but has collaborated with several of the authors. The problem “is far more important than we realized,” he adds.
Plastic is only the latest in a litany of human-linked stressors that are imperiling coral reefs—the incubators of ocean biodiversity. And the synthetic substance is apparently driving up disease rates even in the types of reefs that have proved hardiest in the face of other forces. The new study shows that plastic must be taken into account when planning how to manage and protect reefs, and that many countries need to overhaul their waste management systems to keep plastic from getting into the ocean, several experts say.
Reducing plastic pollution will not be enough on its own to save reefs, but it could help ease the pressure they face from the threats that still worry coral experts the most: overfishing and warming-driven bleaching. Because of these pressures reefs face an impending “climate bottleneck,” with overall reef numbers expected to drop and changes in the types of reefs that survive. But removing as much plastic as possible from the equation could make that bottleneck just a little bit wider.
Images of sea turtles trying to eat plastic bags they have mistaken for jellyfish, and the vast mass of debris swirling in the Great Pacific Garbage Patch, have been making news for years. But the sheer amount and pervasiveness of plastic in the ocean is enough to astound even seasoned marine researchers. Terry Hughes, director of the Australia-based ARC Center of Excellence for Coral Reef Studies, says even in the remote reefs of Indonesia he has “just been shocked by the density of plastic bags washing by.”
The new study, detailed in the January 26 Science, provides hard data and a new understanding about the mechanics of how all that plastic affects reefs. Lamb, a postdoctoral researcher at Cornell University, and her colleagues found that where plastic was present on reefs around Southeast Asia—home to more than half the world’s coral reefs—the likelihood of seeing one of the key coral-killing diseases rose from 4 to 89 percent. Considering the 11 billion pieces of plastic the team estimates are entangled on reefs from Myanmar to Australia, plastics could significantly affect the health of reefs that are not only linchpins of ocean biodiversity but key to many economies in the region.
Just how corals become diseased from their encounters with bags, water bottles and fishing lines is still something researchers are exploring, but they have some suspicions. For one thing, the pathogens that cause some of the most common and deadly diseases may be hitching rides on the plastic pieces. Plastic can also damage the delicate surfaces of corals as it rubs against them, introducing bacteria and forcing the coral to expend energy on its immune response as it tries to repair itself. Plastic also smothers the coral, covering the surface and blocking sunlight and oxygen to create conditions that help many kinds of bacteria thrive. They include those that cause black band disease, in which a dark strip of infection eats away at coral tissue.