While the greatest threat to coral reefs worldwide is from warming waters and ocean acidification, the marine communities are also vulnerable to other disasters that are generally thought to wreak havoc on land only, such as hurricanes. In September, back to back hurricanes devastated the coral reefs in the Caribbean and scientists are starting to study it as part of a long-term project to understand how corals recover (or don’t) from disasters.

Researchers from University of Buffalo, State University of New York (UB) and California State University, Northridge (CSUN), traveled to St. John, part of the United States Virgin Islands, in November to study the reefs there. Assessing the damage suffered by the corals under the impact of hurricanes Maria and Irma was the first step toward understanding their recovery.

“Hurricanes generate huge waves. The effect is like sandblasting — the waves carry sand and debris, such as bits of broken coral, onto the reefs, striking them over and over again,” UB’s Howard Lasker said in a statement Tuesday.

There were also colonies that had lost branches or were covered in harmful algal growth or bacteria.

“A once-vibrant reef was now littered with broken coral, the remains of soft sponges and soft coral ripped from their homes on the ocean floor, and 5- to 10-ton boulders that had been tossed about as if they were pebbles. Going deeper underwater, the signs of destruction were less obvious but could still impact life on the reefs for generations to come,” according to another statement Tuesday released by CSUN.

Seeing the lower scale of coral injuries deeper they went was taken by the scientists as an encouraging sign, but it also meant the ecosystem was more vulnerable.

“For one, there are a very large number of underwater projectiles now lying on the ocean floor that could cause damage. It certainly wouldn’t take much for a future storm to pick up those pieces and pound them against the rest of the remaining reef. In that sense, the reef is more prone to physical damage… There are a lot of hammers sitting on the sea floor, ready to be picked up and thrown against the anvil,” CSUN’s Peter Edmunds said in the statement.

Corals are tiny marine animals that form colonies on the seafloor, typically in shallow waters not over 200 feet deep. They are home to many algae, which give them their brilliant colors, and the colonies also provide habitat to a large variety of fish and other marine creatures. Therefore, damage to reefs upsets an important marine ecosystem.

Corals Before and After Two views of the coral reef of St. John, U.S. Virgin Islands. The photo on the right shows the reef vibrant and full of life in 2013. The photo on the left was taken in November 2017, after hurricanes Irma and Maria tore through the region. The reef is now more sparsely populated, with many coral colonies either severely damaged or sweated away. Photo: Peter Edmunds

Referring to the damage inflicted by the hurricanes, Lasker said: “It’s an interesting natural experiment. You could not, in good conscience, conduct such an experiment on your own as a scientist, and it is sad to see these beautiful places in the ocean damaged so severely. But we can learn from this — it gives us the chance to better understand the process of recovery.”

Researchers will return to the site multiple times over the coming months and years, to see how the coral recovery takes place, or doesn’t.