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Hurricanes ‘sandblasted’ these Caribbean coral reefs

Damaged gorgonian corals on a reef on St. John. The wispy, white strands clinging to the edges of the coral are harmful cyanobacteria, which have grown on injured areas. (Credit: image by Howard Lasker; video by Georgios Tsounis, Cal State Northridge/Jacqueline Krawiecki, U. Buffalo)

Coral reefs off St. John, part of the US Virgin Islands, suffered severe injury during hurricanes Maria and Irma, say scientists who traveled there in late November to assess the damage—the first step in understanding the reefs’ recovery.

Some coral colonies lost branches. Harmful algal growth cloaked others. Many—weakened by the hurricanes—were left with ghostly, feather-like strands of bacteria hanging off open wounds where bits of coral had been scraped off.

The researchers also observed sites where whole coral colonies, akin to individual trees in a forest, had been swept away by the fury of the storms.

damaged pillar coral
A damaged pillar coral on a reef off the coast of St. John. The tops of some of the pillars have broken off as a result of the recent hurricanes. (Credit: Howard Lasker)

“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,” says Howard Lasker, professor of geology at the University at Buffalo.

“In shallow waters, what we found certainly lived up to our expectations—holy, moly this was bad news.”

Lasker led the research trip with Peter Edmunds, professor of biology at California State University, Northridge.

The team, with funding from the National Science Foundation’s rapid response research program, spent two weeks aboard the F.G. Walton Smith, the University of Miami Rosenstiel School of Marine and Atmospheric Science’s research vessel.

Scientists say damage varied by location.

“In shallow waters, what we found certainly lived up to our expectations—holy, moly this was bad news,” says Edmunds, who has spent 31 years studying St. John’s reef.

“But when we went deeper, it became more nuanced,” Edmunds continues. “It was still beautiful. There were corals, sea fans, and some fish swimming around. Then you would look more closely, and you would see tumbled corals and missing corals in spots where you had seen corals just three months before. There were changes, but there certainly was a tremendous amount still there. I think it’s very encouraging.”

damaged gorgonian coral
Damaged gorgonian corals on a reef off the coast of St. John. The living tissue on many of the branches have been chipped or torn off, leaving only the dark non-living core of the branch. (Credit: Howard Lasker)

A natural experiment

Coral reefs act as habitats for fish and other wildlife, providing food for communities worldwide and generating tourism dollars for seaside economies.

The recent hurricanes presented a rare opportunity for Lasker and Edmunds to study how corals recover from disasters—an important line of research in a warming world where rising ocean temperatures are stressing reefs.

“It’s an interesting natural experiment,” Lasker says. “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.”

Corals, often thought of as plant-like, are actually tiny marine animals. They build colonies that rise from the ocean floor to form the colorful, whimsical structures that people know as reefs. As time goes on, Lasker and Edmunds will study how quickly coral recruits repopulate damaged sites, and whether injured colonies bounce back or die.

Lasker and Edmunds have been researching St. John’s reefs for years. Their team has been documenting which coral species live there, and examining photographs dating back to 1987 to determine how the reefs’ composition has changed.

The focus is on understanding the balance between hard, stony corals, which form the backbone of ocean reefs, and softer, more flexible gorgonian corals—tree-like species that form an underwater forest of sorts, providing habitat for small fish and other aquatic life.

The hurricanes add an unexpected variable to this work.

cyanobacteria on damaged coral
Cyanobacteria on damaged coral. (Credit: Howard Lasker)

It’s like a forest fire

During the recent research trip, the team assessed damage at sites off of St. John’s southern coast, in an area that is part of the Virgin Islands National Park. In the coming months and years, the scientists will revisit many of these spots to see how coral communities are—or aren’t—recovering.

“I don’t see signs that they are going to disappear. They will persist, but in what form?”

Edmunds says what remains of the St. John’s reef is still quite beautiful, populated with an abundance of coral with the ability to reproduce, if given time. Despite these encouraging signs, however, he notes that today’s reefs are much more vulnerable than in past decades, given climate change and other stressors.

Lasker compares the effect of storms on reefs to the effect of wildfires on forests.

“Hurricanes have always occurred,” Lasker says. “They can cause extensive damage, but then the populations start to recover. It’s analogous to forest fires: After a number of years, the forest starts returning. There’s a period of disturbance, and then the system recovers.”

But scientists still have a lot of questions about this rebound.

For example, few investigators have looked in detail at the plight of soft corals—Lasker’s area of expertise. There are clues that these species may fare better than their stony counterparts after a disaster, but more research needs to be done to understand how storms, warming waters, and ocean acidification can alter the composition of reefs and whether these changes are permanent or short-lived, Lasker says.

El Niño has made these coral reefs into ‘ghost towns’

“These are magnificent ecosystems,” he says, “and we really know very little about how they change and recover after disasters.”

When it comes to the future of reefs, Edmunds says, the question is not “are reefs going to disappear?” The bigger question is: in what form will they exist?” he says. “I don’t see signs that they are going to disappear. They will persist, but in what form?”

The team also included scientists from the Woods Hole Oceanographic Institution, the National Park Service, and the Georgia Aquarium.

Source: University at Buffalo