April 13, 2024

Going back to the future to predict the fate of a dead coral reef in Florida

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A close-up of a well-preserved Late Holocene hard pillar coral (Dendrogyra cylindrus), now genetically extinct along the Florida reef tract due to a disease outbreak in 2014. Credit: Florida Atlantic University

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A close-up of a well-preserved Late Holocene hard pillar coral (Dendrogyra cylindrus), now genetically extinct along the Florida reef tract due to a disease outbreak in 2014. Credit: Florida Atlantic University

Rising temperatures and disease outbreaks are decimating coral reefs in the tropics. Evidence suggests that higher latitude marine environments may provide crucial refuges for many at-risk and temperature-sensitive coral species. However, how coral populations expand into new areas and sustain themselves over time is constrained by the limited scope of modern observations.

What can thousands of years of history tell us about what lies ahead for coral reef communities? Quite. In a new study, researchers and collaborators at Florida Atlantic University provide geological insights into coral range expansions by reconstructing the composition of a suite of Late Holocene dead coral subfossils from an unusual site in southeast Florida and comparing it with modern reefs throughout the region.

Located off one of the most densely populated and urbanized coasts in the continental United States, the Late Holocene coral dieback known as the “Pompano Ridge” records a northward expansion of tropical coral communities that occurred over a period of time. of regional climate warming of more than 2,000 years. years ago.

Could this happen again in the face of climate change? Going “back to the future,” this study offers a unique look at what was once a vibrant set of coral reefs and explores whether history could repeat itself.

Study results, published in the journal Earth and Environment Communicationsreveal significant differences in coral composition between Late Holocene and modern assemblages, suggesting that ongoing environmental stressors that were not present during the Late Holocene will likely limit the ability of modern higher-latitude reefs in Florida to function as long-term climate refugees. term.


Video of the Late Holocene coral death assemblage at Pompano Ridge and soft corals that now live there. Credit: Anton Oleinik, Ph.D., and Alex Modys, Ph.D.

“Today’s environmental conditions and ecology have deviated substantially from long-term Holocene baselines that have occurred on millennial timescales,” said Anton E. Oleinik, Ph.D., co-author and associate professor of geology, Department of Geosciences, FAU Charles E. Schmidt College of Science.

“Based on the findings of our study, we are not overly optimistic that Florida’s subtropical reefs will be able to support expansions of reef-building coral species reminiscent of the Late Holocene anytime soon.”

The findings show that Late Holocene coral assemblages were dominated by now critically endangered Acropora (stony coral) species between 1,800 and 3,500 years ago, reflecting classic zonation patterns characteristic of healthy Caribbean reefs prior to the 1970s.

“What’s really remarkable is that we didn’t find these subfossil corals in the middle of the Caribbean, like Belize or Bonaire. We found them right here in South Florida, far beyond their current range,” said Alexander B. Modys, Ph. . .D., first author and recent doctorate in the Department of Geosciences at FAU. “What existed here thousands of years ago is similar to what we would have seen in Jamaica in the 1950s and 1960s.”


Anton Oleinik, Ph.D., (left) holds a well-preserved subfossil of a Late Holocene hard pillar coral (Dendrogyra cylindrus), now genetically extinct along the Florida reef tract due to a 2014 disease outbreak, and Alex Modys, Ph.D., has a Late Holocene subfossil of an Elkhorn coral (Acropora palmata). Credit: Florida Atlantic University

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Anton Oleinik, Ph.D., (left) holds a well-preserved subfossil of a Late Holocene hard pillar coral (Dendrogyra cylindrus), now genetically extinct along the Florida reef tract due to a 2014 disease outbreak, and Alex Modys, Ph.D., has a Late Holocene subfossil of an Elkhorn coral (Acropora palmata). Credit: Florida Atlantic University

In contrast, modern reefs off Southeast Florida are becoming increasingly dominated by stress-tolerant species such as Porites astreoides (mustard coral) and Siderastrea siderea (round star coral), in part due to their resistance to thermal stress, sedimentation and, more recently, stony coral tissue loss disease.

Researchers conducted surveys along 16 transects across the entire length of the coral rubble zone in the study area and identified 21 unique groups of corals. To determine how current assemblages at the study site compare to Late Holocene assemblages, they collected live coral abundance data along the same 16 transects in the summer and fall of 2018.

They also compared the composition of the Late Holocene assemblages with publicly available data for the entire reef tract off southeast Florida from the Southeast Florida Coral Reef Monitoring and Assessment program, allowing for a comprehensive analysis that spans the entire range. of the region’s coral communities.

A total of 1,949 coral skeletons were identified and qualitatively assessed based on their overall preservation in terms of abrasion, presence of original corallite material and encrusting communities. The researchers used Uranium-Thorium dating to determine its age, which ranged from 900 to 4,500 years.


Alex Modys, Ph.D., diving into the dead coral pool at Pompano Ridge unearthing a subfossil coral is Orbicella annularis.. Credit: Anton Oleinik, Ph.D., Florida Atlantic University

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Alex Modys, Ph.D., diving into the dead coral pool at Pompano Ridge unearthing a subfossil coral is Orbicella annularis.. Credit: Anton Oleinik, Ph.D., Florida Atlantic University


The Late Holocene coral death assemblage at Pompano Ridge. Credit: Alex Modys, Ph.D.

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The Late Holocene coral death assemblage at Pompano Ridge. Credit: Alex Modys, Ph.D.







Subfossil samples included well-preserved Late Holocene elkhorn coral (A. palmata), which represented nearly 50 percent of the samples, montane star coral (Orbicella spp.), and pillar coral (Dendrogyra cylindrus), a hard coral that is now genetically extinct along the Florida reef tract due to a disease outbreak in 2014.

The author’s research suggests that southeast Florida could offer a refuge for corals that are being devastated by climate warming in the tropics. However, they warn that the range expansions observed in the past are unlikely to happen today without direct human intervention.

“The rapid decline of southern source populations and additional anthropogenic stressors that were not present during the Holocene are likely inhibiting the natural expansion of tropical corals that we would expect to see with climate warming,” Modys said. “They won’t get there alone, so more aggressive conservation strategies like assisted migration may be necessary.”

The Late Holocene record of Pompano Ridge provides a basis not only for identifying areas that could serve as critical climate refugia for corals in the future, but also for developing comprehensive restoration and management strategies that aim to replicate the communities’ successful ecological attributes. historic coral reefs, ensuring their long-term sustainability. However, climate change remains a major obstacle, even on reefs at higher latitudes.

“It is important to emphasize that the long-term sustainability of these restoration activities will ultimately depend on the rate and magnitude of current climate warming,” Oleinik said. “If climate warming continues at its current rate, it will become very warm even in historically colder, higher latitude locations like southeast Florida, and unfortunately these restoration programs will not be enough.”

More information:
Alexander B. Modys et al, Modern coral range expansion in southeast Florida falls short of Late Holocene baseline, Earth and Environment Communications (2024). DOI: 10.1038/s43247-024-01283-0

Diary information:
Earth and Environment Communications

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