There's a Whole New Threat to Florida Reefs Damaged by Pollution and Overfishing

Florida's coral reefs are already in big trouble. Scientists around the globe have noted serious problems for the delicate but vital ecosystems, especially from "bleaching," a process that occurs when high heat and sunshine cause devastating effects. 

But that's not the only threat reefs in the Sunshine State face. A Florida Keys study has found a new and alarming problem: Bites from natural reef inhabitants such as parrotfish are also killing corals weakened by overfishing and pollution. 

Even worse, the study published earlier this month in the journal Nature found that the weakened coral die at a rate of up to 80 percent during the warmest months of the year. In other words, this could be a very bad summer for the state's marine ecosystem. 

The four-year study, which was conducted on a coral reef off the coast of Key Largo, was authored by a team of researchers from Oregon State University, University of Florida, North Carolina-based SymbioSeas, and University of California–Santa Barbara, including Andrew Shantz, who recently earned a PhD from Florida International University. 

Globally, coral reefs are dying off due to several factors, mainly agricultural runoff and population explosions along coastal areas, along with broader threats such as coral mining, rising ocean temperatures, and acidification. One report published by the D.C.-based World Resources Institute in 2011 estimated that up to 60 percent of coral reefs are at risk from human activity. 

The idea behind the experiment was to study the effects of overfishing and nutrient pollution, according to Shantz.

But the researchers stumbled onto an unexpected result: A completely normal environment can turn deadly for coral when they're already stressed by climate change and pollution. 

From the summer of 2009 to 2013, Shantz and the rest of the team covered portions of coral reefs off Key Largo with cages made with welded frames covered in mesh to simulate the lack of fish. Then researchers dropped polymer-covered Osmocote slow-release fertilizer inside the cages every six weeks to mimic nutrient pollution. 

After the coral was uncovered and exposed to parrotfish, the researchers saw something they didn't expect: Parrotfish, which usually share a symbiotic relationship with the coral, were essentially killing the coral with their bites. 

Normally, the coral can withstand the bite from a parrotfish as it feeds on the algae and other microorganisms. While feeding, the fish grind the coral with their beak-like teeth and excrete the coral material as sand, which creates beaches and other parts of the reef. 

But because of the weakened state, Shantz says, the coral isn't able to recover and the bites became infected. While previous experiments have studied the impact of pollution and overfishing to reefs, Shantz says there's been nothing as long or in-depth into the study of the coral reef microbiome, or the community of microorganisms that live within the coral. 

Shantz makes the comparison of a coral microbiome to a healthy gut of a human being.  

"Almost every animal exists with a consortia of bacteria and microbes in/on them, and that is very healthy and normal," Shantz says. "These groups of microbes are often regulated and controlled by the host to keep healthy beneficial ones and eliminate harmful ones. Just like when we get sick we lose the ability to regulate those communities and sometimes harmful bacteria take over and run amok, enrichment and overfishing seem to overwhelm coral's ability to regulate those communities."

Adds Shantz: "It's like when you get a small cut. In nice clean water, it's no big deal, but if it's dirty, then chances of it becoming infected are much greater."

Losses of coral habitat throughout the world could spell certain doom for people in the Caribbean, Shantz says, because the ecosystems provide protein for up to a million people and could also result in a loss of billions of dollars to Florida's economy alone. 

According to an estimation posted to the website of the National Oceanographic and Atmospheric Administration, coral reefs in South Florida are worth up to $8.5 billion, generating $4.4 billion in local sales, $2 billion in local income, and 70,400 jobs.

Also, according to Shantz, the world is a much uglier place without the intrinsic beauty of coral reefs. 

Though the new study offers a grim prospect for coral reefs, Shantz says, the good news is that it identified at least two main drivers of decline. More important, he says, they can be managed. 

Shantz says what's needed is (1) a fishing management policy, particularly in the Caribbean, that helps sustain some of the bigger fish and (2) good water regulation and management plans. 

"It's certainly not good news to see, but great to understand the mechanisms that are often driving local decline," Shantz says. "Major threats [such as warming and ocean acidification] are still looming, but it's great to see empirical evidence that managing local threats like pollution and fishing can help mediate and protect from those big global threats."