Climate change will continue preventing coral reef growth in subtropics

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Based on new research, USGS scientists predict future climate change will continue to suppress coral reef growth in the subtropics.

If you could travel back 10,000 years ago to a time period known as the early Holocene Epoch and swim through the shallow seas across what is now known as the Southeast Florida Continental Reef Tract (SFCRT), you would be surrounded by a seemingly endless garden of bright, beautiful corals. During this time period, the branching, elkhorn coral (Acropora palmata) and many other stony corals flourished, leading to the construction of more than 100 km—about eleven times the height of Mount Everest—of reef habitat along the coast of South Florida.

Fish swim among pointed coral stems

The reef-building staghorn coral, Acropora cervicornis, has recently expanded its range to the nearshore environments of southeast Florida, offshore of Broward County, as the climate has warmed. Its prospects for building reefs that far north will be severely limited by cold fronts, which climate change is making more common.

Credit: William F. Precht. June 2018.

During the early Holocene, the Earth’s climate was warm and stable, creating favorable conditions for reef development in South Florida. At this time, coral reefs built by elkhorn coral, the dominant reef-building coral in most shallow-water habitats throughout the western Atlantic, began to grow throughout South Florida. In fact, early Holocene rates of reef growth or “accretion” were so rapid that reef height was increasing by more than 8 meters (~26 feet) every 1,000 years on average. This rapid growth allowed the reefs to keep pace with the rapid pace of sea-level rise taking place at the time. Between 7,800 and 5,800 years ago, Earth’s climate began to cool, and changes in wind patterns caused winter cold fronts to become more common in South Florida. These natural, gradual changes in environmental conditions caused cold-sensitive, reef-building corals like elkhorn to retreat south, contracting the extent of the SFCRT. By around 3,000 years ago, reef accretion was negligible throughout South Florida.

Corals are colonial animals that produce a calcium carbonate framework that can grow for thousands of years to create the structures known as coral reefs. These animals are very sensitive to large changes in temperature, such as extreme winter cold snaps. Therefore, coral reefs tend to thrive in tropical latitudes where water temperatures are warm and stable year-round. At the latitudinal boundaries of these systems—the subtropics (like South Florida)—some species of corals still survive, but the most critical reef-building corals are less common because they are very sensitive to large environmental fluctuations.

Since 1959, USGS scientists have studied the geology, growth, and decline of coral reefs around the world. The major declines in coral populations around the world in recent decades has driven USGS scientists to work to disentangle the primary reasons why corals flourish in certain environmental conditions and perish in others. Some of the causes of recent coral and reef declines include bleaching due to warming sea surface temperatures, coral disease, and local human impacts. Recently, Dr. Lauren Toth of the USGS St. Petersburg Coastal and Marine Science Center led a team to investigate the geographic extent and the primary drivers of reef growth in South Florida throughout the Holocene.

To understand the primary drivers of reef growth, the team collected data from samples of fossil reefs throughout South Florida. Some data came from the newly dredged Government Cut Channel as part of the Port of Miami expansion. This dredging activity exposed ancient logs of elkhorn coral (Acropora palmata) that were buried in the reef, and therefore provided the scientists access to additional geologic records.

 

Left: logs of large, branching fossil corals. Right: flat coral reef dominated by weedy, soft corals

(a) Holocene Acropora palmata reef framework on the Outer Reef in Miami, FL exposed by dredging in Government Cut that suggests reef‐building once occurred throughout the region (b) Modern habitats in the same location dominated by octocorals, sponges, and macroalgae, which do not build reefs.

(Credit: William Precht, Dial Cordy & Associates, Inc.. Courtesy of William F. Precht)

Dr. Toth worked with William Precht, a private consultant for Dial Cordy & Associates, Inc., to collect new data from the exposed areas of these old reefs. Precht also called upon Dr. Harold Hudson, retired scientist who spent time at both USGS and the National Oceanic and Atmospheric Administration (NOAA) and is now a private consultant (Reef Tech), to collect new core samples from the surfaces of reefs in the area. The involvement of Dr. Hudson lent decades of valuable knowledge and expertise to the project team. The team also involved several other respected partners from the Florida Atlantic University, Florida Institute of Technology, Nova Southeastern University, and the University of South Florida.

The team analyzed the ages of the coral samples to determine where and when reefs were growing in the region. They then compared the timing of changes in the latitudinal extent of reef building to existing records of sea level, temperature and other climatic changes that would indicate changes in cold-front frequency in South Florida. Although the researchers showed that sea level controlled when and how far reef development extended from the shore, they found that climate was indeed the most important driver of variations in reef growth over the last 10,000 years. Reefs expanded northward during the relatively warm, stable climate at the beginning of the Holocene ~10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts resulted in contraction of reef building until 3,000 years ago when those conditions caused reef development to terminate throughout South Florida.

Map of South Florida displaying outer reef extent from West Palm Beach to Miami, inner reef about half that length

Map of the extent of the Southeast Florida Continental Reef Tract (SFCRT) Inner Reef (red line) and Outer Reef (yellow line) and timing of Acropora palmata reef growth at sampling locations (boxplots). Boxplots represent the medians (solid verticals) and interquartile ranges (boxes) of radiometric ages of A. palmata from each location. Error bars (whiskers) are 1.5 × the interquartile range. Points indicate data outside this range. The two stars indicate ages from A. palmata sampled at the northern limit and southern limit of the Southeast Florida Continental Reef Tract.

Map image is the intellectual property of Esri and issued herein under license.

Copyright 2020 Esri and its licensors. All rights reserved.

Although ancient reefs grew during warm periods and contracted during cold periods, Toth and her team concluded that future conditions will not simply reverse this trend. Unlike the warm periods of the early Holocene, modern climate change is occurring very rapidly, increasing global average temperatures and rates of sea-level rise at an unprecedented rate. Modern climate change is expected to cause more extreme warm and cold fronts, and more frequent and intense storms. These conditions, as well as other disturbances like high-temperature events, coral disease, and human impacts, will decrease the growth and survival of corals in Florida’s high-latitude environments—impeding their ability to build reef structures. In other words, the forecasted conditions will continue to suppress—not accelerate—future reef growth in the subtropics.

Read the results of the study in Nature Scientific Reports.

“We’ll still see some corals living throughout South Florida, but extreme weather events driven by climate change will continue to prevent those corals from actually being able to build up the reef,” said Dr. Lauren Toth, lead author on the study. “Without new growth, the reefs will start to erode. If that happens, the reef structure that took thousands of years to build, and the benefits it provides to society and the reef ecosystem will be lost.”

Coral reefs, especially reefs topped by large, branching corals such as Acropora palmata, are important for protecting coastlines by causing waves to break away from shore. This defense amounts to over $1.8 billion in annual protection from the impacts of storms and coastal flooding. Reefs also provide habitat for diverse marine life and economically important fisheries species, adding an additional $2 billion annually to the Nation’s economy.

Some scientists have predicted that as climate change continues to warm our oceans, reefs may be able to find refuge in cooler, high-latitude environments like South Florida. These new findings from USGS and their team demonstrate that modern climate change will instead prevent the reef growth that halted 3,000 years ago from beginning again. Although future conditions threaten to eliminate the benefits of reefs for good, management interventions such as reef restoration provide some hope for buying reefs some time until the most dominant drivers of climate change can be addressed.

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