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Florida's coral reefs suffer severe damage after a summer heat wave

NOAA's Mission: Iconic Reefs field team member Cate Gelston, co-lead scientist on the assessment cruise, retrieves a transect tape after completing an outplanted coral health assessment survey.
Ben Edmonds
NOAA's Mission: Iconic Reefs field team member Cate Gelston, co-lead scientist on the assessment cruise, retrieves a transect tape after completing an outplanted coral health assessment survey.

A preliminary study of the damage done by last summer's record heat wave to Florida's coral reefs is in — and the results are not good.
Barely one-fifth of the staghorn corals survived. And elkhorn corals weren't even found at two of the five reefs surveyed. These are the biggest, most visible corals found in the world's third-largest reef off the state's southeast coastline.

Researchers surveyed 64 locations at five of the seven Mission: Iconic Reef sites — Carysfort Reef, Horseshoe Reef, Sombrero Reef, Looe Key Reef, and Eastern Dry Rocks — to examine the reef-building corals planted by the Coral Restoration Foundation, Mote Marine Laboratory & Aquarium, and Reef Renewal.

Map of the reefs investigated in the survey
Map of the reefs investigated in the survey

This research follows a mission in August that assessed coral health during the height of the marine heat wave. It uses new data about how eight additional weeks of high temperatures affected corals.

Preliminary findings from the latest assessment include:

  • Preliminary data indicate that less than 22% of approximately 1,500 staghorn coral (Acropora cervicornis) surveyed remain alive.
  • Only the two most northern reefs surveyed, Carysfort Reef and Horseshoe Reef, had any living staghorn coral.
  • Of the five reefs surveyed, live elkhorn coral (Acropora palmata) was found at only three sites: Carysfort Reef, Sombrero Reef in the middle Keys, and Eastern Dry Rocks off Key West.
  • No live staghorn or elkhorn corals were observed at sample areas surveyed at Looe Key Reef in the lower Florida Keys.

Scientists are trying to resuscitate the reefs. They first took some of the coral that survived the heat wave to tanks in the Florida Keys. They are now transplanting them back on the reefs.

We talked with Erinn Muller, a senior scientist at Mote Marine Laboratory in Sarasota, who studies coral resilience, about what she found.


It looks like the staghorn corals and the elkhorn corals aren't doing well. Tell our listeners about what you have all found down there.

MULLER: So the staghorn corals and the elkhorn corals, which are critically endangered species and have been a really big focus of our restoration efforts for the last decade or so, are some of the most sensitive coral species. So they're really susceptible to lots of different stressors, including really warm water temperatures, like what we saw this past summer. So as those temperatures increased during those summer months, we definitely saw those two particular species bleach (or) lose their algal-symbiotic partners. And when that happens to those species, they don't last very long.

So a lot of them definitely died due to that bleaching event, whereas other coral species seem to show signs of stress, but then are now showing signs of recovery.


Staghorn corals are probably the most charismatic of all the all the corals. Does that make them any more susceptible their size than the other corals? Do we have any reason why they seem to be particularly affected by this?

Yeah, those particular species, they have these beautiful branches that come out. So they create a lot of space for other organisms to kind of hide within. But another characteristic of those species too is their tissue is really thin. And because of that, they don't have a lot of nutrient reserves to kind of get them through stressful events.

So when things like increasing water temperatures start affecting them, they're much more sensitive to some of the boulder and the brain corals, which put a lot of resources into shunting energy into reserves like lipids and proteins. So that’s one of their characteristics, they put a lot of energy into things like growth, and growing really quickly, and not a lot into energy reserves.

I imagined going down there and not seeing these beautiful creatures must be kind of heartbreaking.

Yeah, those particular species are so beautiful to see for many different reasons. I mean, they've been very rare within Florida for the last decade or so. So even just seeing them out on the reef is a beautiful moment. And a lot of those corals that were out on the reef were ones that restoration practitioners like Mote and our partners put out there.

So going down there in the summer and seeing the corals that we put out there, and those remnant individuals that were remaining from the wild population undergoing so much stress was really hard to see.

But we also saw signs of really interesting hope, in the sense that some of the corals that we put out there that we created within our lab, through assisted sexual reproduction, actually were doing well. And not all of them. By and large, we lost a lot, but many of them did survive. And some of them didn't even show signs of stress.

So for me, as a research scientist, my job is to now dig into that and understand why those corals did so well compared to the ones that we've lost, and figure out how to integrate more of that within a restoration pipeline.

So are we seeing any staghorn and elkhorns that are less susceptible to heat? Is that something that might be worthy to replace the population with in the future?

Yeah, that's exactly right. And we saw kind of a sprinkling of individuals throughout the reef tract that seem to be more resistant to heat. And so we want to figure out why are those corals so resistant? And then how do we utilize them moving forward without bottle-necking the population, we still want to make sure there's lots of diversity within the corals that we put out there. But maybe we want to do more of those individuals that seem to be more resistant, because those are the ones that are able to survive these changing environmental conditions.

Do you see that you all are going to have to be doing this every summer? Or is this maybe a temporary thing that maybe eventually will build up a population of more heat resistant corals that you won't have to go in there and do triage, so to speak, every summer?

Yeah, I mean, ideally, we see this combination of both where we have things like climate change mitigation, reducing the frequency and severity of these really warm summers. And then at the same time, we are doing the best that we possibly can to make sure that the population is able to cope with the things that we can't control right now. So both of those things have to kind of happen in concert for us to be really successful moving forward.

Any surprises that you saw from your preliminary reports, maybe some corals that did better than you were expecting?

It was a surprise to see that a lot of the corals that we created through sexual reproduction, basically collecting eggs and sperm during their spawning season each year and settling them in our lab. They actually did better than a lot of the corals that had been put out there through asexual production.

So the process of fragmentation, we're trying to still figure out why those corals are doing better, but it seems like this new shuffling of alleles as that sexual recombination occurs, maybe giving a lot of these corals a boost. And the fact that we have been really hand-selecting parents for the last couple of years to try to increase the resilience of those corals seems to be giving them an advantage as well.

Do you have any hope for the future that maybe our efforts will eventually be able to save the Florida reef?

I don't think we would come to work every day if we didn't have hope for the future, right? And we wouldn't need restoration if we hadn't been in this situation for the last 50 years or so. So yes, this last summer was hard to see. But it's par for the course sometimes when you're dealing with disturbed ecosystem like Florida's coral reef.

But every time things like this happen, we learn a ton. A lot of the corals that are the boulder and the brain corals seem to be doing much better within this high water temperature of last summer compared with the branching corals. So we're going to continue to grow and build our boulder and brain coral infrastructure to get more of those out there, and then capitalize off of those corals that survived within the elkhorn and staghorn corals and crack the code to figure out how do we make that population even more resistant, moving forward, recognizing that the environment still has to be come more hospitable for us in the future.

 In the meantime, we continue to do what we need to do and make sure that when the environment gets better — and I do hope that that comes to fruition — that we have corals remaining to come back from because without our efforts really all would be lost.

Copyright 2024 WUSF 89.7. To see more, visit WUSF 89.7.

Steve Newborn is WUSF's assistant news director as well as a reporter and producer at WUSF covering environmental issues and politics in the Tampa Bay area.
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