Scientists have long known that reefs provide South Florida’s coast with one of its best defenses, an underwater garden of colorful fish and corals that also stands as a stubborn barrier to fierce hurricanes and pounding waves.
A thornier question — and one becoming increasingly important as natural reefs crippled by disease and climate change get replaced by engineered reefs — is what difference do the corals make?
Now, for the first time, researchers are beginning to get their answer. In a study published this month in the journal Marine Science and Engineering, a team of University of Miami Rosenstiel School of Marine and Atmospheric Science researchers examined staghorn coral, the once plentiful and fast-growing species that has been used for nearly all restoration efforts so far.
In some instances, they found staghorn more than doubled the reef’s ability to shred waves.
“It's a huge amount of protection they provide,” said Landolph Rhode-Barbarigos, a co-author and UM assistant professor engineering. “The fact that we basically have our coral reefs close to the shore is acting to our advantage. The healthier they are, the more structure they have, the more they're going to protect us. So this is very important to understand.”
For the experiment, researchers created both theoretical waves and used coastal buoy data to replicate real-life waves off Florida’s coast. They then sank an artificial reef in UM’s 38,000-gallon wave tank and measured the difference in wave power when waves passed over a bare reef structure and one covered with the skeletons of staghorn coral.
They found, not surprisingly, that the corals had less impact in deeper reefs, but soared to more than 56 percent of a reef’s protective power in shallow water.
While past field studies have documented the power of reefs, Rhode-Barbarigos said they could not distinguish the contribution of corals.
“When we do measurements in the field, it's both the impact of the structure of the reef, the surface [of the reef] and the corals all at the same time,” he said. “We typically cannot differentiate the influence of those corals.”
This study, he said, not only confirmed what field studies have found, that reefs can weaken waves by 98 percent, “but also provides new information regarding how much the actual coral skeletons, the coral structure, if you want, contributes,” he said. “That's the uniqueness.”
The study was also the first to examine the difference on such a large scale and with the ability to combine both wind and wave power in UM’s Sustain tank, Rhode-Barbarigos said..
Friction from the corals as well as the amount of cover they have on a reef can determine how much they break down waves. The size of the wave and depth also matters.
Aside from the scientific value, knowing the kind and amount of protective power from corals can help steer restoration efforts.
For more than a decade, labs including those from UM, Mote Marine Laboratory, the Coral Restoration Foundation and others have been growing staghorn and replanting it on reefs to replace staghorn that nearly disappeared when a disease outbreak beginning in 1988 killed all but 3 percent. The fast-growing branch coral, along with elkhorn, made Florida and Caribbean reefs among the most diverse and beautiful in the world.
In 2014, Florida reefs took another major hit when a new disease was found off Virginia Key that attacked nearly 40 species of boulder coral that helped build the reef’s foundation. The disease has now spread to the Caribbean and Mexican.
The outbreak, combined with the growing threat from warming oceans driven by climate change that both weaken coral and strengthen hurricanes, upped the urgency to restore reefs.
The information from the study, Rhode-Barbarigos said, could be used to help convince resilience planners to incorporate reef restoration in efforts, including a $4.6 billion plan created by the U.S. Army Corps of Engineers for Biscayne Bay. Miami-Dade County has rejected the Corps plans and wants to revise it to include more natural solutions. But to get federal funding, the Corps will need to justify the work under a cost-benefit analysis required by law, something the agency has struggled to do with reefs and mangroves.
Rhode-Barbarigos said the study sheds light on the importance of combining efforts.
“We have to understand this: Storm surge relates to flooding, and if you want to keep water out, a seawall is what you're going to use,” he said.
But it’s also impractical to wall off the coast, he said.
“If we have a more sustainable investment and investment that goes towards hybrid solutions, green, gray, purely green, so nature-based solutions with some mangroves, some coral restoration, we're going to get a lot of benefits,” he said.
And while staghorn was selected for this first test because of its significance in Florida, Rhode-Barbarigos said the experiment itself is flexible enough to consider other coral species that can grow in other parts of the world on different kinds of reefs.
“So whatever results we find can be applicable to other locations in other skills,” he said.
For the next phase of the work, the team plans to measure other coral species, including boulder coral ravaged by stony coral disease and now being bred in labs to replant, as well as a mix that reflects what actually grows on a reef. Early next year, they plan to construct an artificial reef off Miami Beach to test some of the designs.
“We already acquired the Army Corps permit. We have been green-lighted and addressed all the comments by [state and county enviromental regulators,” he said. “So stay tuned for more.”
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