The Development and Demise of Florida’s Coral Reefs

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A new study published in the journal Global Change Biology by scientists at the USGS St. Petersburg Coastal and Marine Science Center suggests that unlike most other reefs, the geological decline of Florida’s reefs preceded modern coral-reef degradation by at least 3,000 years.

This article is part of the October-November 2018 issue of the Sound Waves newsletter.

Series of four photos that show the same reef in 1971, 1988, 1998, and 2004.

Time series of the coral reef at Grecian Rocks (Key Largo, FL) illustrating an example of the dramatic decline in live coral on Florida Keys reefs in recent decades. From Shinn and Kuffner (2017).

The global decline of coral reefs since the 1970s poses a significant threat to the valuable ecosystem services reefs provide. The complex, three-dimensional structure of reefs serves as the foundation for biodiversity, fisheries, and shoreline protection that are critical to coastal communities and economies in U.S. jurisdictions and beyond. The ecological decline of coral reefs in recent decades has triggered a decline in the geological process of reef growth (accretion) and an increase in reef erosion.

Like many reefs around the world, Florida’s reefs have experienced significant coral loss in recent decades from coral bleaching, disease, and human-related disturbances. A new study published in the journal Global Change Biology by scientists at the USGS St. Petersburg Coastal and Marine Science Center (SPCMSC) suggests, however, that unlike most other reefs, the geological decline of Florida’s reefs preceded modern coral-reef degradation by at least 3,000 years.

Beginning in the 1960s, USGS scientists helped to pioneer geological studies of coral reefs and are continuing to build upon that legacy today. USGS researchers are the custodians of coral reef cores from throughout the Florida Keys reef tract and beyond; these valuable records of reef history are preserved in the USGS Core Archive housed at the SPCMSC. The new Global Change Biology paper, for the first time, combines and synthesizes the records from all the cores collected from the Florida reef tract to develop a comprehensive reconstruction of the development of Florida’s reefs during the Holocene (the past ~11,700 years).

Series of four photos, two of divers underwater, one from the sky of an island chain, one of cores of coral.

The culmination of geological studies of coral reefs conducted by USGS researchers from the 1960s to present. A) USGS researchers coring Grecian Rocks reef (Key Largo, FL) in the late 1970s. B) USGS researcher and student collect a reef core from Western Sambo Reef. C) Map of core locations throughout the six sub-regions of the Florida Keys reef tract. D) Photograph of a reef core from Dry Tortugas National Park collected in 2017. Labels indicate the depth of core penetration into the reef in feet. [Read more]

The reconstruction reveals that although Florida’s reefs flourished during the relatively warmer climate of the Holocene Thermal Maximum approximately 7,000 years ago, as climate cooled during the late Holocene, reef accretion declined dramatically. By 3,000 years ago, reef-building had stalled throughout the reef tract, leaving most of Florida’s reefs geologically senescent. The central role of climate in both the millennial-scale decline in reef accretion and in modern reef degradation highlights the vulnerability of both geological and ecological reef processes to climate in the past, present, and future.

The decline in reef accretion was not simultaneous throughout the reef tract, however. Reef accretion ceased earliest in the Middle Keys, where reef development is strongly controlled by the influence of Florida Bay. The researchers hypothesized that the shallow waters of Florida Bay would have been the first place that experienced extreme cold temperatures as the climate cooled, and the transport of that water onto the reef beginning approximately 6,000 years ago would have caused the early decline of reefs in that region. In contrast, reefs in the Dry Tortugas National Park, which is isolated from the influence of the Florida Platform, continued growing the longest. The difference in the timing of reef shutdown explains the differences observed in the thickness of Holocene reefs throughout the region.

This research, for the first time, makes the important distinction between millennial-scale geological decline of the Florida Keys reef tract and the modern, ecological coral reef crisis. The geological collapse of Florida’s reefs left the ecosystem balanced at a delicate tipping point where a veneer of living coral was the only barrier to reef erosion. Modern climate change and anthropogenic disturbances have now pushed many reefs past that critical threshold and into a novel (modern) ecosystem state in which reef structures built over thousands of years are rapidly eroding. The next step for this study is utilizing the new information on Holocene reef thickness to map the shifting vulnerability of adjacent coastal communities to storms based upon how much reef remains to protect shorelines.

The full citation for the article is:

Toth, L.T., Kuffner, I.B., Stathakopoulos, A., Precht, W.F., Aronson, R.B., and Shinn, E.A., 2018, A 3000-year lag between the geological and ecological collapse of Florida’s coral reefs: Global Change Biology, v.24 i.11, p. 5471-5483, doi: 10.1111/gcb.14389.

Plot shows a timeline of coral reef development in the Florida Keys.

Conceptual diagram of the history of coral-reef development in the Florida Keys. The graph shows average (black line; shaded area indicates ±95% confidence intervals) reef accretion across all sites in the Florida Keys. Reef accretion began to decline as climate cooled following the Holocene Thermal Maximum. By 3000 years ago, reef accretion was negligible throughout the region, making the Florida Keys reef tract geologically senescent. Recently, anthropogenic warming and other disturbances have caused declines in coral cover and have pushed Florida’s reefs into a state of net erosion.

View from the sky of land and ocean with labels and dots to show different reef thicknesses.

Map showing generalized variability in average Holocene reef thickness throughout the Florida reef tract. Reefs grew the least in the Middle Keys where the negative influence of Florida Bay caused reef development to shut down by ~6000 years ago. Reefs grew the longest (and thickest) in the relatively isolated Dry Tortugas National Park.

 

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