The world’s coral reefs are currently in the grip of a massive global bleaching event — only the third such event in recorded history. Thanks to unusually warm water brought on by the effects of climate change, a particularly strong El Nino event and a persistent warm “blob” in the Pacific Ocean, corals throughout the Indian, Pacific and Atlantic oceans are at risk of bleaching and possible death.

One of the places most recently affected is the Florida coral reef tract, which spans from the Florida Keys up to Martin County and is the only coral reef tract found off the coast of one of the continental U.S. states. Indeed, at close to 150 miles in extent, it’s the “third largest barrier reef ecosystem in the world,” according to NOAA. And while bleaching is already a big enough problem for coral, the Florida reefs are being hit with a double whammy this year in the form of a widespread disease outbreak, which scientists say could be difficult to recover from. 

Bleaching occurs when warm water causes corals to become stressed. Healthy coral contain a type of symbiotic algae known as zooxanthellae, which provide essential nutrients and give the coral its color. But coral often expel their zooxanthellae when they’re stressed, turning white (or “bleaching”) in the process. Bleaching is not an automatic death sentence for coral, but the process does deprive them of important nutrients and leaves them weakened. This weakened state also makes them more susceptible to disease.  

Toward the end of the summer, scientists in Florida started to notice the beginnings of a bleaching event, said Sean Morton, superintendent of the National Oceanic and Atmospheric Administration’s Florida Keys National Marine Sanctuary.  

“We start to really look out when the water temperature gets about 30 degrees Celsius, or 86 degrees Fahrenheit,” Morton said. “When it hits that level, the corals get stressed.”

The bleaching, itself, isn’t exactly unusual, said Meaghan Johnson, marine science coordinator for The Nature Conservancy in Florida. Every year for at least the past 10 years, some bleaching has occurred on the reef tract. However, the bleaching was particularly severe both last year and this year — and this year’s bleaching event has been compounded by a potentially even more serious threat: an unusually widespread outbreak of disease that eats away at live coral tissue and can cause parts of the reef to die.

The disease outbreaks have been especially severe in the area off the coast of Miami, where bleaching was particularly heavy, Morton said. While several different types of coral disease have been observed, a disease known as “white plague” has been the most prevalent.  

“Coral disease can be either viral or bacteria, or have both components, and their origins aren’t very well understood” said Brian Walker, a research scientist at the National Coral Reef Institute at Nova Southeastern University. “We believe the microbes are…naturally occurring in the background levels of all corals, even ones that aren’t showing disease responses.”

But as corals become stressed and weaken, it’s easier for bacteria or viruses to get out of control and take over. Many coral diseases actually eat away at live coral tissue, making them even more dangerous than bleaching.

“A lot of times you’ll find disease outbreaks with particularly large bleaching events, and this year we’re having a really bad one,” Walker said. This is bad news because disease outbreaks are much harder for coral to recover from than just a bleaching event.

If warm water conditions subside and coral are given enough time, their zooxanthellae will return, and they’ll be able to recover from a bleaching event. Disease, on the other hand, destroys coral tissue and causes reefs to diminish in size. And while coral can slowly grow back over time, sometimes plants and other organisms will move in first and take over the space the coral used to occupy, meaning the reef is permanently disrupted.

How much damage this year’s outbreak will cause remains to be seen. Water temperatures in the region have finally begun to cool down, according to Morton, and the Florida Department of Environmental Protection recently evaluated the risk for mass coral bleaching as “low.” However, the corals that have already bleached will need time to recover, and disease outbreaks are still being observed, Walker said.

Throughout the past several months, the Florida Reef Resilience Program’s Disturbance Response Monitoring system has led teams from universities, nonprofits and government agencies in monitoring the bleaching event. Results from these surveys will be available in early November, but until then it’s unclear exactly how much of the reef tract has been affected or how difficult its recovery will be.

However, Walker said it’s the worst he’s seen the reef — at least the part of it in the region north of the Keys — in the 18 years he’s been working there. He and his colleagues have been examining a cluster of “large corals” (between six and 25 feet in diameter) in the northern extension of the Florida reef tract, some of which are up to 400 years old. They found that about half the living corals were affected by either bleaching or disease, and in some cases had lost up to 60 percent of their live tissue.

And Johnson, from The Nature Conservancy, added that some of the corals affected by disease are listed as threatened under the Endangered Species Act. These species include the boulder star coral and the pillar coral. “The fact that they were being affected by the disease was alarming because there’s not many of them left,” she said.

And while improvement is expected in the coming months, there’s not much scientists can do for the corals in the meantime other than discourage divers from touching them and trying to prevent as few additional disturbances as possible, Morton said. Scientists will also be closely monitoring the water temperature through the winter, as unusually cold water can also stress the corals and cause additional bleaching. As recently as 2010, there was a cold-water bleaching event, Morton said.

He also added that, even if temperatures return to normal and the corals get a break through the winter, scientists are already predicting that 2016 will be another bad year for the reef tract. The Disturbance Response Monitoring program already has preliminary plans to continue monitoring through the winter.

Meanwhile, scientists believe that the larger global bleaching event could also continue on in 2016, a prediction that’s worrying for marine ecologists. Coral reefs, in Florida and throughout the rest of the world, are vital parts of the marine ecosystem, providing habitat for thousands of of organisms. So continued monitoring will be vital for scientists hoping to understand how the environment could change in response to these bleaching events in the future.

“I think in the long run it’s in everyone’s best interest to want to maintain a healthy reef ecosystem,” Walker said.

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