Critical Florida coral ravaged by summer’s marine heat wave

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This summer’s record-breaking marine heat wave may have been the “nail in the coffin” for an iconic species of coral that serves as a building block of marine life around Florida. Still, scientists see other “signs of hope” in the state’s reefs.

Elkhorn coral populations – which had already been teetering on the brink of local extinction in Florida – have been “decimated” by the extreme ocean heat, according to Liv Williamson, a coral expert and assistant scientist from the University of Miami.

“This heat wave was the nail in the coffin for these populations,” Williamson said. “There were already so few elkhorn coral individuals on Florida’s reefs that various genetic rescue plans were underway, but now almost all the corals we would have used for such efforts have died.”

Elkhorn and staghorn coral are some of the only so-called branching corals found in the Carribean. They were also the first coral species to gain protected status under the Endangered Species Act, Jennifer Moore, a threatened coral expert for NOAA told CNN.

The branching part of these corals is key; their tree-like appendages grow faster than other coral and spread out like a rainforest canopy, providing protection for fish and other vertebrates, which helps the overall ecosystem thrive.

Both coral species are slightly more heat-tolerant than other corals to begin with, Moore told CNN, but more likely to die once they bleach – a process in which they turn white as they expel their algal food source in response to heat stress.

This summer’s die-off happened to both wild elkhorn and to corals bred to be more heat-tolerant. Coral conservationists have been trying for years to use those varieties to restore the disease-ravaged population.

Some of the planted corals were bred to withstand ocean temperature up to 2 degrees Celsius above normal. But the water around Florida and the Caribbean this summer was up to 3 degrees Celsius above normal, causing mass bleaching and the die-off, Williamson said.

As the world continues to warm because of human-caused climate change, marine heat waves are becoming more common and extreme, scientists say.

“This summer has just illuminated how extreme things can get so quickly and I just don’t think we are prepared for that,” Williamson told CNN.

Back in the 1960’s and 70’s elkhorn and staghorn corals “were so common it was like blades of grass,” Moore told CNN, but have become so rare “you cry in your mask when you see a live one on the reef.”

A 2020 study of the elkhorn coral population in the upper Florida Keys found it was “functionally extinct,” or unable to reproduce effectively on its own and contribute to the ecosystem, and may face local extinction over the next 6 to 12 years. The researchers said the trends likely applied to all of Florida’s elkhorn.

“There are simply too few, too far away from each other,” Williamson said.

Staghorn coral are bleached near Key Largo. When coral are stressed, they expel their algal food source and slowly starve to death.

“Although there are a small number of individuals still alive, the species has dwindled so much that they no longer play an effective role in the ecosystem in the way that they once did, and they no longer have a viable population,” Williamson said.

Any deaths would have a “dramatic impact” at restoration sites just starting to see enough coral density to make an ecological impact, Moore said.

Staghorn coral may have faired slightly better than elkhorn this summer, Williamson said, but still faces similar long term challenges.

The grim news comes despite other signs of hope at the region’s reefs. Florida reefs are only just able to start recovering now that ocean temperatures have dropped from bathtub-like 90s to levels the heat-sensitive corals can better tolerate.

Scientists fear this summer's ocean heat was the
Elkhorn coral used to be widespread around Florida.

Scientists have known since the summer that a mass bleaching event and die-off was happening, but they still don’t know the full extent of it or how bad it will be in the long run. Bleached coral may still be alive and recover now that water temperatures are cooler. Conversely, more coral could die because of vulnerability to disease in the months that follow bleaching, coral experts said.

“We are definitely looking at a major mortality event, we just won’t know the extent of it for a couple more months,” Moore told CNN.

For now, some coral scientists like Moore are hanging their hats on “shockingly fast” signs of recovery at reefs recently surveyed and on the prospects of using science learned from this event to give the species a better chance to survive the next heat wave.

“To see corals that were 100% bleached two or three weeks ago regaining their algae and regaining their color also shows there’s resilience in the system,” Moore said. “That gives me a lot of hope. I don’t really know where it’s all going to land, so I can’t really tell you if it’s worse or better than I feared in July, but I am cautiously optimistic because of these little glimmers of hope. We just need to figure out how to maximize it so that we can help this system recover.”

Others are still struggling to cope with the loss and the prospect of what feels like a Sisyphean effort to save such a vital species, especially in the face of climate change. Scientists like Williamson are left feeling “heartbroken” after witnessing their life’s work obliterated in a matter of weeks.

“It’s hard to express the loss that my fellow coral conservationists and I feel, watching the pillars of this vital reef ecosystem collapse and the fruits of our labors destroyed,” Williamson wrote on Instagram.

“Even if we do plant these nursery fragments back onto the reefs, what’s to say they will survive next summer, or the one after that?” Williamson told CNN.

The prospects for coral recovery lie in a herculean rescue effort this summer. Coral conservationists moved corals to deeper water, cooler nurseries and harvested diverse genetic specimens and then put them in a “living gene bank” on land. Scientists like Moore plan to use the specimens to plant corals yet again.

“Emotional fatigue was across everyone, because in some cases these were corals that they grew from babies and put out on the reef,” Moore said. “To see them bleach and potentially die is really, really emotionally draining. Yet, because we didn’t just sit there and watch them die – that’s what give me hope.”

“I think we have lots of tools to prevent extinction and I’m not going to quit,” Moore told CNN.

Scientists are cautiously optimistic that some of the coral can recover.


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