Seafloor Erosion in Coral Reef Ecosystems Leaves Coastal Communities at Risk

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A new USGS study of changing sea depths at five large coral reef tracts in Florida, the Caribbean, and Hawai'i, found the seafloor is eroding in all five places, placing coastal communities protected by the reefs at increased risk from storms, waves, and erosion. 

This article is part of the April 2017 issue of the Sound Waves newsletter.

In the first ecosystem-wide study of changing sea depths at five large coral reef tracts in Florida, the Caribbean, and Hawaiʻi, USGS researchers found the seafloor is eroding in all five places, and the reefs cannot keep pace with sea-level rise. As a result, coastal communities protected by the reefs are facing increased risks from storms, waves, and erosion.

In the Florida Keys, the U.S. Virgin Islands, and Maui, coral reef degradation has caused seafloor depths to increase as sand and other seafloor materials have eroded over the past few decades, the USGS study found. In the waters around Maui, the seafloor losses amounted to 81 million cubic meters of sand, rock, and other material—about what it would take to fill up the Empire State Building 81 times, the researchers calculated. 

As sea levels rise worldwide due to climate change, each of these ecologically and economically important reef ecosystems is projected to be affected by increasing water depths. The question of whether coral colonies can grow fast enough to keep up with rising seas is the subject of intense scientific research.

But the USGS study, “Divergence of seafloor elevation and sea level rise in coral reef ecosystems,” published April 20, 2017, in the journal Biogeosciences, found the combined effect of rising seas and seafloor erosion has already increased water depths more than what most scientists expected to occur many decades from now. Other studies that do not factor in seafloor erosion have predicted seas will rise by between 0.5 and 1 meter, or between 19 inches and 3 feet 3 inches, by 2100.

“Our measurements show that seafloor erosion has already caused water depths to increase to levels not predicted to occur until near the year 2100,” said biogeochemist Kimberly Yates of the USGS St. Petersburg Coastal and Marine Science Center, the study’s lead author. “At current rates, by 2100 seafloor erosion could increase water depths by two to eight times more than what has been predicted from sea-level rise alone.”

The study included areas of the reef tract in Florida’s Upper Keys and Lower Keys; looked at two reef ecosystems, St. Thomas and Buck Island, in the U.S. Virgin Islands; and also included the waters surrounding Maui. The researchers did not determine specific causes for the seafloor erosion in these coral reef ecosystems. But the authors pointed out that coral reefs worldwide are declining due to a combination of forces, including natural processes, coastal development, overfishing, pollution, coral bleaching, diseases, and ocean acidification (a change in seawater chemistry linked to the oceans’ absorption of more carbon dioxide from the atmosphere).

Underwater photo of a vast area of dead corals on the seafloor at Buck Island, U.S. Virgin Islands

Elkhorn corals (Acropora palmata) on the seafloor along the northeastern coast of Buck Island, U.S. Virgin Islands, have died and collapsed into rubble. As coral reef structure degrades, habitat for marine life is lost and nearby coastlines become more susceptible to storms, waves, and erosion. Photo credit: Curt Storlazzi, USGS.

A view of healthy elkhorn corals on the seafloor in the U.S. Virgin Islands

Healthy elkhorn coral (Acropora palmata) on the seafloor along the southeastern coast of Buck Island, U.S. Virgin Islands. Elkhorn coral is one of many important reef-building species that create 3D structure on the seafloor. Coral reef structure provides habitat for marine life and helps break up waves as they approach the coastline. Photo credit: Curt Storlazzi, USGS.

For each of the five coral reef ecosystems, the team gathered detailed seafloor measurements from the National Oceanic and Atmospheric Administration (NOAA) taken between 1934 and 1982, and also used surveys done from the late 1990s to the 2000s by the USGS Lidar Program and the U.S. Army Corps of Engineers. Until about the 1960s, seafloor measurements were done by hand, using lead-weighted lines or sounding poles with depth markings. From approximately the 1960s on, most measurements were based on the time it takes an acoustic pulse to reach the seafloor and return. The USGS researchers converted the old measurements to a format comparable to recent lidar data.

They compared the old and new sets of measurements to find the mean elevation changes at each site. The method has been used by the U.S. Army Corps of Engineers to track other kinds of seafloor changes, such as shifts in shipping channels. This is the first time it has been applied to whole coral reef ecosystems. Next the researchers developed a computer model that used the elevation changes to calculate the volume of seafloor material lost.

They found that overall, seafloor elevation has decreased at all five sites, in amounts ranging from 0.09 meters (about 3.5 inches) to 0.8 meters (more than 2.5 feet). All five reef tracts also lost large amounts of coral, sand, and other seafloor materials to erosion.

“We saw lower rates of erosion—and even some localized increases in seafloor elevation—in areas that were protected, near refuges, or distant from human population centers,” Yates said. “But these were not significant enough to offset the ecosystem-wide pattern of erosion at each of our study sites.”

Worldwide, more than 200 million people live in coastal communities protected by coral reefs, which serve as natural barriers against storms, waves, and erosion. These ecosystems also support jobs, provide about one-quarter of all fish harvests in the tropical oceans, and are important recreation and tourism sites.

“Coral reef systems have long been recognized for their important economic and ecological value,” said John Haines, Program Coordinator of the USGS Coastal and Marine Geology Program. “This study tells us that they have a critical role in building and sustaining the physical structure of the coastal seafloor, which supports healthy ecosystems and protects coastal communities. These important ecosystem services may be lost by the end of this century, and nearby communities may need to find ways to compensate for these losses.”

The study brought together ecosystem scientists and coastal engineers, who plan to use the results to assess the risks to coastal communities that rely on coral reefs for protection from storms and other hazards.

You can read the study at Biogeosciences: “Divergence of seafloor elevation and sea level rise in coral reef ecosystems.”

The full citation for the article is:
Yates, K.K., Zawada, D.G., Smiley, N.A., and Tiling-Range, G., 2017, Divergence of seafloor elevation and sea level rise in coral reef ecosystems: Biogeosciences, 14, 1739-1772, [https://doi.org/10.5194/bg-14-1739-2017].

USGS Science Makes an Impact

This research has been featured in 25 major stories published in various newspapers, newsletters, and magazines since the paper was published on April 20, 2017. The research was featured in the Washington Post and Tampa Bay Times; made front-page news in the Miami Herald and Honolulu Star Advertiser; and prompted two stories in Energy and Environment News, among others.

The article was viewed and/or downloaded over 1,934 times in the first three weeks it was available online. The authors have received an outpouring of thanks from both the scientific community and concerned citizens for the work and for getting the information out to the public. 

Resource managers in both Hawaiʻi and the U.S. Virgin Islands have requested the paper and results to include in their regional assessments of coral reefs; the U.S. Army Corps of Engineers has also requested data.

 

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