Hawaii’s Newly Discovered Deep-Sea Reefs Thrive in a ‘Twilight Zone’

Scientists believe the corals may become a refuge from climate change for marine life as warming oceans kill shallow reefs.
(Photo: Courtesy NOAA)
Oct 10, 2016· 3 MIN READ
John R. Platt covers the environment, wildlife, and technology and for TakePart, Scientific American, Audubon, and other publications.

Imagine living your entire life in the weak illumination provided by a single match.

That’s not much different from the conditions hundreds of feet below the surface of the ocean off Hawaii, where researchers have discovered the most extensive deep-sea coral reefs ever encountered.

The extremely fragile yet abundant coral are what are known as mesophotic reefs, in that they live in extremely low light, between 1 and 10 percent of what can be found in shallow waters.

The lack of light, along with the reefs’ remote locations and crushing water pressure, have hidden them and their surrounding ecosystems from humans for, well, pretty much forever.

(Photo: Courtesy NOAA)

“Literally, every dive we do is an area that no human being has ever visited before,” said Randall Kosaki, deputy superintendent of the Papahānaumokuākea Marine National Monument for the National Oceanic and Atmospheric Administration and one of the researchers who detailed the deep-sea reefs in a paper published last week. “And given how remote some of these reefs are, there’s also a fair chance that no human will ever go back there again.”

According to Kosaki, the conventional wisdom had been that there would be fewer corals the deeper the dive because the “twilight zone” before light completely disappeared would not provide enough sustenance for photosynthetic corals. That didn’t prove to be the case. “The big surprise was at 300 feet to 330 feet off of Maui, there were areas that had nearly 100 percent coral coverage, which was really phenomenal,” he said. The corals found there were adapted for low-light conditions, with flat growths designed to suck up as much energy as possible, just like solar panels.

The corals weren’t alone. The researchers also found the reefs to be teeming with other life, including several species of fish and algae that are new to science. In some cases, 90 to 100 percent of the reef communities can only be found around the Hawaiian Islands. “To have communities that are composed of 100 percent Hawaiian endemics, it’s just off the scale globally,” Kosaki said. “That’s just the highest level of endemism known from any marine ecosystem on Earth.”

So what is the fate of these deep-sea corals when so many of their shallow-water counterparts are dying or suffering because of climate change, pollution, and other threats? Kosaki said that’s uncertain. The corals are fairly protected by their depth and location, where they sit between several islands that provide shelter from ocean currents.

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That’s a bit different from shallow-water corals. “You know, in Hawaii we’re known for our 50-foot waves,” he said. “Surfers love them. Coral, not so much.” The deep-sea corals, however, are more protected from buffering waves and have evolved into extremely thin, fragile, platelike forms.

Kosaki expressed concern about how climate change could affect these communities. He pointed to a major coral bleaching event that took place in Hawaii in 2014, during which 85-degree surface water was pushed down to 250 feet. “That’s not too much deeper than the 280 or 300 feet we were diving where we found numerous new species of algae,” he said. “These are beautiful algae, purple and red, not like green pond scum. Algae are fairly ephemeral and require very specific conditions, so if the hot water had pushed another 50 feet, we wouldn’t have found these new species. That underscores that there is some sensitivity here. We need to continue exploration because we’re at risk of losing species before we even know they exist.”

Meanwhile, the researchers hypothesize that some of the deep-sea corals could provide a sort of refuge for shallow-water species that are threatened by climate change. Another coral bleaching event took place in October 2015, at the same time the researchers were exploring the deep reef. During this bleaching event, “many, many of the corals in shallow water just turned white,” Kosaki said, “but a lot of the colonies of these same species at 200 feet were not bleached.” That means some of the mid-depth corals and related communities could recolonize the shallow waters if what exists there now dies off. That remains theoretical for now, but “it looks like the hypothesis holds water,” Kosaki said.

Although he feels the deep reefs have so far missed the “climate change bullet,” Kosaki said it appears inevitable that things will someday catch up with them. “As the frequency and severity of these thermal stress events, as climate change accelerates, I have no doubt that the deep reef will feel some impact from it,” he said.

Kosaki, who wanted to be an astronaut when he was a kid, said exploring the ocean floor is the next best thing. “This is really pushing the limits of human physiology in terms of how deep you can dive,” he said. The explorations were made possible by advanced rebreather machines—which helped to regulate the oxygen in the researchers’ bodies—as well as submersibles and remotely operated vehicles.