By Chelsea Harvey  May 4th, 2016

The long-suffering Florida coral reef tract — the largest reef in the continental U.S. and third-largest barrier reef ecosystem in the world — may have bigger problems than anyone thought, according to new research from the University of Miami and Florida International University. Scientists have discovered that part of the reef is actually dissolving into the water, likely thanks to the effects of human-induced ocean acidification.

The research, published earlier this week in the journal Global Biogeochemical Cycles, took surveys of coral throughout the Florida Keys from 2009 to 2010, using chemical analyses of water samples to examine the rates at which corals were either calcifying — that is, building new parts of the reef — or disintegrating into the water.

They found that the reef was dissolving, at least during some parts of the year (generally the fall and winter), in various places throughout the Keys. In the spring and summer, some areas to the south were able to make up for these losses — but, worryingly, the researchers found that the part of the reef in the northern Keys, closer to Miami, was already eroding more quickly than the corals were able to rebuild themselves.

The culprit appears to be ocean acidification, a chemical process that happens when carbon dioxide dissolves in the water and undergoes a reaction that lowers the ocean’s pH. When this happens, several consequences can occur, according to the new study’s lead author, Chris Langdon, chair of the University of Miami’s department of marine biology and ecology.

First, when water becomes more acidic, limestone — which is what makes up the hard, rocky skeletons secreted by corals — can start to dissolve, “just like you dropped a sugar cube in water,” Langdon said. Additionally, a lower pH can make it easier for certain types of organisms to burrow into the coral skeletons and make their homes there, further breaking down the reef.

The results are especially worrisome, given that the northern part of the reef appears to have hit a tipping point in which more limestone is being lost than rebuilt. While it’s well established that acidification is bad for coral, previous research had suggested that reefs around the world likely wouldn’t hit this net erosion threshold until closer to mid-century, when carbon dioxide levels were higher.

Most of these previous experiments were either conducted in laboratories or involved very small-scale, controlled studies of the environment, said Langdon, which did not reflect all the complexities of a natural reef. More recently, some field experiments have suggested that the world’s reefs may hit their tipping points sooner than previously predicted — and Langdon’s study represents the most recent of these, as well as “the largest natural experiment that’s been done to date,” he said.

It’s still unclear why the scientists observed such a gradient in the Keys from north to south, with the northernmost corals getting the worst of the deal. According to Langdon, acidity is known to vary from north to south, since colder water is able to hold more dissolved carbon dioxide. But it’s also possible that the northern part of the reef’s proximity to the heavily populated Miami-Dade County could be having a negative impact because of pollution and other human activities, making the corals more vulnerable to environmental stressors.

Acidification can cause disintegration of reefs even when they’re otherwise healthy — but the new study is all the more concerning because the Florida reef tract has already suffered its share of stress over the past few years. In 2014 and 2015, the reef experienced an unprecedented bleaching event and widespread disease outbreak, likely brought on by both the effects of climate change and 2015’s unusually severe El Nino event.

Local reports have noted that the bleaching and disease remain ongoing in some parts of the reef. Brian Walker, a research scientist at the National Coral Reef Institute at Nova Southeastern University, noted that there was some sign of bleaching recovery back in November on the northern part of the reef tract, where his research is focused — but the disease outbreak did not appear to slow down over the winter.

“This is concerning because the 2016 summer is predicted to be as hot if not hotter on the reefs,” Walker said in an email to The Washington Post.

And, although bleaching and dissolving can happen independently and don’t always occur side by side, both processes weaken coral reefs and can exacerbate one another — meaning the Florida reef tract is in an especially vulnerable state. That’s a problem not only because of its importance to fish and other marine life as a natural ecosystem, but also because of its significance to the local economy. Reef-related tourism brings nearly $3 billion per year to the region, according to Langdon.

He pointed out, though, that the Florida reef tract is unlikely to be the only location in the world suffering from dissolving limestone.

“The right studies haven’t been performed yet to see if this is happening in other places,” Langdon said. But ocean acidification is happening throughout the world, meaning other reefs are likely vulnerable to the same kinds of effects. And investigating the processes affecting corals around the world is increasingly important as reefs become more and more vulnerable to environmental stressors.

At the moment, a global bleaching event — triggered by the 2015 El Nino event, but likely exacerbated by the ongoing warming effects of climate change — has ravaged reefs all over the world. The Great Barrier Reef in Australia, for instance, has experienced bleaching on more than 90 percent of its extent, according to Australia’s National Coral Bleaching Task Force.

As the climate continues to warm, these types of bleaching events are only expected to become more common — and the acidification of the seas will also increase the more greenhouse gases we pour into the air. On that note, the most meaningful action that can be taken to protect the world’s corals is strong mitigation efforts when it comes to our carbon output, according to Langdon.

“The really important point is the cause of both [bleaching and acidification] is carbon dioxide emissions,” he said. “And reducing emissions will reduce the severity of both of them.”

https://www.washingtonpost.com/news/energy-environment/wp/2016/05/04/the-largest-coral-reef-in-the-continental-u-s-is-dissolving-into-the-ocean/