Bruno and his colleagues surveyed 48 colonies spread over 1,500 kilometers (932 miles) of the Great Barrier Reef every year for six years. The team measured general health and looked specifically for outbreaks of white syndrome. The cause of this affliction is unknown, but by comparing the health data they gathered with satellite measurements of ocean temperatures, the scientists were able to confirm that the disease tends to follow summers with higher than usual sea surface temperatures. They also discovered that only thriving reefs—where tiny coral polyps cover 50 percent or more of the ocean's bottom—suffered from the syndrome, according to their paper in PLoS Biology. "The irony is that the healthiest reefs are the ones where the disease outbreaks are occurring," Bruno says.
Because scientists do not know what causes white syndrome, it is impossible to say why denser reefs suffer more. But it is possible that, just like in human cities, proximity gives a pathogen a better chance of spreading. Corals compete with their neighbors, Bruno says, attacking them at night with special tendrils. "It could plausibly open up lesions that allow a bacteria to colonize the coral tissue," he says.
Determining the exact cause of white syndrome, which was first observed only about a decade ago, will require parsing the hundreds of bacteria that thrive in corals' mucus secretions. One thing is clear, however: rapidly increasing temperatures are subjecting the tiny animals to undue stress. "It is faster than they can acclimate," Bruno says. "Coral reefs are an ephemeral feature of the ocean. The concern is that we are going to lose the habitat that they create when they are super-healthy."