Warmer ocean waters around the Hawaiian Islands are threatening reefs — and the state’s tourism-dependent economy — by increasing the frequency of coral bleaching, a new study has found.

A scientific paper published this week in the international journal PeerJ documented effects of climate change at the famed Hanauma Bay Nature Preserve on the east side of Oahu.

While the corals there did not suffer as much as other places in Hawaii, researchers still found nearly 10 percent of the bay’s corals had died after bleaching events in 2014 and 2015. Nearly half the coral at the bay was bleached.

“Global climate change poses a direct threat to the biological sustainability of the protected reefs of Hanauma Bay Nature Preserve and a clear economic and cultural threat to the state of Hawaii,” said Ku‘ulei Rodgers, a coral reef biologist at the University of Hawaii’s Hawaii Institute of Marine Biology.

University of Hawaii biologist Ku‘ulei Rodgers conducts a coral bleaching survey at Hanauma Bay.

Courtesy: Keisha Bahr

Rodgers authored the paper and conducted the studies at the bay with Keisha Bahr​, Paul Jokiel​ and Angela Richards Donà. Jokiel was one of Hawaii’s best known coral researchers; this was his final field research before he died in April 2016.

“Warmer seawater temperatures are again predicted for the Hawaiian Islands in 2017 with the grave possibility of more coral bleaching and mortality,” Bahr said in a statement.

Rising ocean temperatures cause the corals to expel the algae that is living within them, according to the National Oceanic and Atmospheric Administration. That causes the corals to turn white, or bleach, leaving them still alive but in a fragile state.

The 100-acre Hanauma Bay is the state’s most popular snorkeling destination, with roughly 1 million visitors a year.

Despite all the people in the water, the study found that the area with the most coral death was Witch’s Brew, at the far end of the bay where fewer people snorkel. The scientists attributed this to the warm water that accumulates there before exiting the bay.

The lowest levels of bleaching were in the channel that runs through the middle of the bay, which provides access for snorkelers to go from the inner reefs — where corals have been steadily declining since at least 2002 — to the outer bay. The study says this is due to the higher water circulation and, in turn, lower temperatures found there.

Hanauma Bay Nature Preserve on a Tuesday: the one day each week without visitors, so the bay can rest.

Courtesy: Keisha Bahr

The 1.15-degree temperature increase over the past 58 years in the waters around Hawaii has increased the frequency of coral bleaching events at Hanauma Bay as well as elsewhere around the islands, scientists say.

But unlike the bleaching events in 1996, 2002 and 2004, which were relatively short and therefore coral recovery was high, the bleaching events in 2014 and 2015 were longer, leading to unprecedented bleaching and mortality.

The study notes that 2014 marked the beginning of the longest global bleaching event on record, which currently continues and has affected more reefs than any previous worldwide bleaching event, according to the study.

The most recent bleaching, which scientists said was predicted, poses an “imminent threat” to the biological sustainability of Hanauma Bay’s ecosystem and a significant economic threat to the state, the study’s authors wrote.

Tourism contributed $15 billion to the state’s economy in 2015, according to Department of Business, Economic Development and Tourism. And of the 8 million annual visitors to Hawaii, it’s estimated that 80 percent participate in ocean recreation activities — there are more than 1,000 companies to serve them, the study notes.

Early signs of mortality on a bleached coral colony at Hanauma Bay.

Courtesy: Keisha Bahr

In addition to warmer waters, oxybenzone, the active ingredient in most sunscreens, has also made corals at Hanauma Bay and elsewhere more susceptible to bleaching, according to a study by Craig Downs of Haereticus Environmental Laboratory in Virginia.

Seawater testing discovered concentrations of oxybenzone — which is found in over 3,500 sunscreen products — were 12 times higher in Hawaii and the Caribbean. The oxybenzone causes the coral to bleach at temperatures several degrees cooler and inhibits its ability to reproduce.

While Hanauma Bay’s concession stand has pulled sunscreens with oxybenzone from its shelves — zinc sunscreens are a popular alternative — there is no law banning its use. The Legislature considered bills last session, which wrapped up May 4, but ultimately balked.

The scientists in the Hanauma Bay coral bleaching study cite drastic increases in global carbon emissions over the past 100 years as the cause of the elevated sea surface temperatures that are hurting coral reefs.

“From an economic perspective, Hanauma Bay is a major driver of tourism dollars for the state,” Donà said. “But I think the main point is that no reef — no matter how well protected — is able to survive the higher seawater temperatures associated with climate change. The takeaway from our standpoint is that carbon emissions are raising seawater temperatures and that must stop immediately if we hope to have coral reefs in the near future. We must reduce our carbon footprint.”

She said the next steps are more experiments as researchers try to better understand the factors that affect bleaching. HIMB’s Coral Reef Ecology Lab will be looking at how corals respond to the synergistic effects of temperature, light and ocean acidification, which Donà explains as the absorption of carbon dioxide into the ocean, reducing pH and making the ocean more acidic, which is bad for corals.

The lab will also be examining how the coral reef community as a whole responds to these common bleaching stressors, she said, and investigating whether corals are acclimating to the rising temperatures over time.

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