Hakai Magazine

caribbean coral reef
In the Caribbean, long-term environmental changes mean that hard corals are being replaced by soft, non-reef-building species. Photo by Fotograferen.net/Alamy Stock Photo

How a Coral Reef Holds Up in a Hurricane

Two Category 5 hurricanes that slammed into the US Virgin Islands gave researchers a glimpse into how corals weather storms.

Authored by

by Jason G. Goldman

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In the autumn of 2017, the island of Saint John, in the US Virgin Islands, suffered under the howling winds and pummeling rains of two Category 5 hurricanes in a row. Hurricanes Irma and Maria were, at the time, the two most powerful hurricanes ever recorded in the open Atlantic Ocean—and they slammed into Saint John just two weeks apart.

Two months later, University at Buffalo ecologist Howard Lasker and his team were diving the coral reefs fringing Saint John. They had been monitoring these reefs continuously since 2014, chronicling a suite of long-term changes caused by ocean warming and acidification, overfishing, and the extirpation of charismatic animals like sea turtles, seals, and manatees. Superimposed on this background decay came the sudden wallop of the dual storms. “Huge boulders just got flipped over,” Lasker recalls. Even in areas where the hurricanes’ impacts were less obvious, giant waves whipped sand off the seafloor and sent it raining upon the reefs, scuffing them like a massive sandblaster.

By at least some metrics, the reefs managed to survive. While the storms caused coral densities to decline by as much as 47 percent, the reef retained the same relative composition, Lasker explains. The diversity of species around the reefs was similar before and after the storms, even if there were fewer individuals overall.

Counterintuitively, some of the resiliency may be attributable to the same long-term trends that have been changing Saint John’s reefs for centuries. Hard corals, also known as scleractinians, are becoming less common, while octocorals—sea fans and other non-reef-building soft corals—are increasing in prevalence.

“Octocorals are pretty darn tough,” says Lasker, “they can put up with a lot of wave action.”

In the summer following the hurricanes, the reefs saw fewer new baby coral recruits. But by 2019, recruit densities had returned to pre-hurricane levels. “Our prediction is that the system will be resilient. Barring further disturbances, those abundances will return back to where they were,” says Lasker.

While the resiliency of octocorals is good news—especially since climate change is expected to bring more storms slamming into the Caribbean—the shift on the reef from hard corals to soft corals isn’t necessarily beneficial. Octocorals don’t build calcium carbonate skeletons, so they are unlikely to replace the functions that hard-coral reefs currently provide, such as protection against coastal erosion.

Given ongoing ecological shifts, “the reefs of the future will look very different from the reefs of the past,” says marine scientist Dominic Andradi-Brown of the World Wildlife Fund, who was not involved with the study. “It’s not clear that an octocoral-dominated reef would have the same fishery species; it’s a very different habitat.”

“Even the most optimistic interpretation of our results should not be, oh, this is fine. I would characterize [octocorals] as a moderately stable placeholder,” says Lasker.

“The reefs that we see today are a pale shadow of the reefs that were there 50 years ago,” Lasker says. “The reefs that we will see 20 years from now will be even worse.”