Reef Decay Leaves Coastal Cities at Risk From Waves

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

(CN) – The ongoing death of coral reefs is eroding the ocean floor and leaves coastal communities more vulnerable to damaging waves, according to federal scientists.

The new study, by researchers at the U.S. Geological Survey, examined five coral-reef tracts in the Florida Keys, Hawaii and the U.S. Virgin Islands, finding seafloor drops in all locations. The team found that the island of Maui – the lone Hawaiian site reviewed, and the location where the largest changes were observed – had lost enough sand, rock and other material to fill about 81 Empire State Buildings.

As climate change raises sea levels worldwide, many experts wonder whether coral colonies will be able to grow fast enough to keep pace.

However, the USGS study found that rising seas and seafloor erosion have already increased water depths more than what scientists expected to occur decades from now. Studies that do not consider seafloor erosion have predicted seas will rise by between 1.5 and 3.3 feet 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 lead author Kimberly Yates, a biochemist at the USGS. “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.”

While the study did not determine specific causes of seafloor erosion in the observed ecosystems, the team points out that reefs around the world face a combination of threats including coastal development, overfishing and pollution, among others.

In order to analyze the coral-reef ecosystems, the team collected detailed seafloor measurements the National Oceanic and Atmospheric Administration gathered between 1934 and 1982, along with surveys from the late 1990s to the 2000s completed by the Geological Survey and the U.S. Army Corps of Engineers.

This Elkhorn coral near Buck Island, U.S. Virgin Islands, has died and collapsed. As coral reef structure degrades, valuable habitat for marine life is lost and nearby coastlines become more susceptible to storms, waves and erosion. (Photo: Curt Storlazzi, USGS)

After reviewing the data, the team found that seafloor has dropped at each site. All five reef tracts had also lost large amounts of sand, coral 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.”

More than 200 million people worldwide live in coastal communities protected by coral reefs. These ecosystems provide about one-quarter of all fish harvests in the tropical oceans.

“Coral reef systems have long been recognized for their important economic and ecological value,” said John Haines of the Geological Survey’s 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 involved coastal engineers and ecosystem scientists who plan to use the findings to determine the risks to coastal communities that rely on coral reefs for protection from various natural hazards.


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