UVI scientists explore coral reef 'twilight zone'

ST. THOMAS - Marine scientists at the University of the Virgin Islands have been exploring what they call the "twilight zone" - a huge, deep-water, coral reef system that stretches along the southeastern Puerto Rican Shelf.

Diving 130 feet down and deeper, researchers at UVI's Center for Marine and Environmental Studies are the first to study the mesophotic coral ecosystem to the south of St. Thomas and St. John.

UVI coral reef researchers Tyler Smith and Richard Nemeth led the study.

"We definitely didn't discover the reefs, but the studies that we've been doing have helped us to understand the systems," Smith said.

The 100-square-mile area is home to million of corals found between 100 feet to 350 feet below the water's surface.

Mesophotic coral reefs are some of the least studied and most poorly understood light-dependant coral habitats in the world, Smith said. The reefs form in dimly lit waters around the globe in what has been called the "twilight zone."

Smith said diving the deep reef is a very different experience from the territory's typical shallow reefs.

"It's a very blue-washed twilight zone, full of what looks like thousands and thousands of these mushrooms, which are corals," he said. "Some are the size of a dinner plate; some are larger, like a trash can lid."

The ocean floor along the Puerto Rican Shelf had been mapped by the National Oceanic and Atmospheric Administration, but scientists had not actually gone down to the reef.

"We had batholithic maps of the sea floor, which revealed the structure, but no one knew what was controlling that structure," Smith said. "So our task was to go down and describe the animals that were living in the area."

The UVI researchers focused their study on the Marine Conservation District, an area just slightly smaller than St. John on the southeast section of the shelf. Smith said they estimate 50 million to 100 million corals are living within the 19-square-mile area of the district.

The researchers found that the corals had not been as affected by the massive bleaching event in 2005, but they did suffer during subsequent disease outbreaks in 2006. Smith said scientists also discovered that the reef was not the same - there were different types of coral formations with distinct habitats.

They found "bank top" formations along the top of the shelf and "knolls" and "hillocks" a little deeper, he said. They also found pedestals of coral - live coral stacked on top of dead coral like pancakes. While the scientists do not yet understand why the coral formations differ, the first step is simply identifying the habitats and formations.

"This is the first time this has been described to science," Smith said. "Every time we look at them, we find something that is brand new."

Smith and Nemeth's research also found the deep-water reef to be a critical site for commercially important grouper and snapper spawning.

Divers used special equipment such as closed circuit re-breathers and air tanks of mixed gases to gain more time on the bottom. Smith said that they could usually get about 30 minutes on the bottom to explore and document their findings - if they had used regular scuba equipment, they would have only gotten about 10 minutes on the ocean floor. Most of the diving was done on the upper ridge of the shelf, in 100- to 150-foot water, even though the reef extends much deeper, Smith said.

The researchers had some heart-racing encounters with marine life not typically seen in shallow reefs. Smith said he was charged by a 500-pound, 8-foot-long blue marlin while he was at a decompression stop on his way back to the surface.

"It was exhilarating," he said. "He came right at me, with the sword pointing right at me, I just went into the fetal position and he went past me."

He said they also saw some large sharks, and they could often hear whales singing.

"It's a pretty amazing and magical world down there," Smith said.

The UVI researchers wrote a paper on their findings and presented it in February at an Aquatic Scientists meeting in San Juan.

Smith said the next step is to find funding to continue the research. He said they need to complete their assessment of the mesophotic systems and connect the maps provided by NOAA to their own habitat descriptions. Future studies will look at what processes control the health and formation of the corals and predict what the reefs will do in the future given local human impacts, climate change and ocean acidification, Smith said.

"These systems are a real treasure in the U.S. Virgin Islands and it should be very exciting for anyone in the Virgin Islands to know that these systems are here in abundance," Smith said. "It's a great aspect of our territory."

The study was funded by the Lana Vento Charitable Trust, NOAA's Caribbean Fishery Management Council and National Ocean Sciences.

- Contact reporter Aldeth Lewin at 774-7882 ext. 311 or e-mail alewin@dailynews.vi.

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