La Parguera, Puerto Rico

Dark skies hint at the hurricane that is moving quickly across the Caribbean on this early July morning, but the sea off Puerto Rico’s southwest coast is glassy. Marine scientists Ernesto Weil and Juan Cruz Motta of the University of Puerto Rico-Mayagüez (UPRM) swim along the La Parguera reef, stopping to examine a fan coral marred by black patches of disease and a staghorn coral bleached white.

For decades, ecologists had thought that La Parguera and other reefs in the dimly lit ‘mesophotic zone’, 30-150 metres below the ocean surface, were sheltered from storms and temperature fluctuations — unlike corals in shallow water. But several recent studies suggest that deep-water reefs are susceptible to the increasingly powerful hurricanes and ocean warming caused by climate change . And that casts doubt on the long-standing notion that deep-water corals could serve as refuges for marine life displaced from increasingly vulnerable shallow reefs.

In Puerto Rico, such questions are more than academic: in September 2017, Hurricane Maria — a deadly category-4 storm — devastated the island and many of its surrounding reefs. Weil, Cruz Motta and their colleagues are tracking the health of deep-water corals at the La Parguera site, which escaped significant damage, and that of deep reefs that were directly in the storm’s path. This natural experiment could help to reveal the extent to which turbulence from the increasingly frequent and extreme hurricanes predicted by climate models could endanger deep reefs in the coming decades.

“If we’re going to have one of these or two of these [storms] per year, it’s going to be very hard for reefs and other coastal communities to recover,” Weil says.

Twilight zone

The UPRM marine-science research facility is on a tiny island just off Puerto Rico’s main island. Downed trees, humming power generators and houses covered with blue tarpaulin reveal that the US territory is far from recovered, nearly a year after Maria barrelled through. During a rainstorm, water pours through the ceiling in one disused laboratory, drenching abandoned equipment.

The UPRM researchers expected to see similar devastation at offshore reefs. “We pretty much thought it was the end of the world,” says Nikolaos Schizas, a marine biologist at the university.

But when the team ventured out to La Parguera reef a few months after the storm, its deep-water corals seemed to have weathered the event surprisingly well. Schizas suspects that this is because Maria travelled east to west across Puerto Rico, and lost strength on its journey over land.

The scientists are now analysing data collected by sensors that they studded on the reef’s 80-metre-tall face before the hurricane. These continuously record information on water temperature, quality and other factors at various depths. The team’s preliminary analysis suggests that Hurricane Maria sent waves crashing into the reef with unusual frequency, lifted sediment off the reef shelf and pushed a swell of cold water from the ocean bottom up to the reef.

It could take years to understand whether and how the hydrological changes observed during Hurricane Maria affected the La Parguera reef ecosystem. But the data collected during the storm are intriguing because most previous measurements of hurricanes’ hydrological impacts have been collected off the coasts of continents, says Curt Storlazzi, an oceanographer at the US Geological Survey in Santa Cruz, California, who analysed the UPRM data. “No one has imaged what has happened on an island,” he adds.

East meets west

Now the UPRM team is eager to visit a reef near Vieques, an island 13 kilometres off Puerto Rico’s eastern coast that suffered severe storm damage. The researchers plan to survey the health of corals at depths of 30 metres or more, and to compare that information with photos and samples that the US National Oceanic and Atmospheric Administration collected before the storm. But the UPRM expedition has been delayed because of continuing infrastructure problems on Vieques.

Comparing data from the relatively unscathed La Parguera reef and the Vieques site may reveal the hurricane’s impact on shallow and deep-water corals alike. Reefs evolved to adapt to storms — even the occasional monster hurricane. But today, many in Puerto Rico face a variety of other threats, including ocean acidification and invasive sargassum algae. And warming waters have created a more hospitable environment for pathogens that attack corals.

The results of the UPRM team’s surveys might help researchers to understand whether deep-water reefs could serve as a refuge for species fleeing damaged or destroyed corals in shallow waters. A study published last month in Science ¹ has cast doubt on that idea: survey data from four reefs in the Atlantic and Pacific oceans revealed little overlap between the species found on shallow and deep reefs. That makes it unlikely that residents of one type of reef could ‘reseed’ the other. The study also reported evidence of storm damage to some of the deep-water reefs: portions seem to have been buried by sediment stirred up by hurricanes.

And research by Tyler Smith, a coral-reef biologist at the University of the Virgin Islands in St Thomas, has found that hurricanes can scramble ocean temperature gradients, which could make deep-water reefs inhospitable to their native species ² .

Still, he says, scientists will need more data to verify, or discard, the deep-refuge-reef idea. He and his colleagues are surveying reefs around the Virgin Islands that were hit by Hurricane Maria. Their preliminary data suggest that visible hurricane damage stopped about 10 metres below the ocean surface.

But reefs may not be so lucky in the future. More-frequent major storms, warming waters and other threats could wear these ecosystems down over time. To lose structural complexity here would be unbelievable,” Schizas says. “But it’s a little by little process.”