Scientists watching V.I. coral bleaching

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ST. THOMAS — Local scientists are watching the territory’s colorful corals turn white this summer and hoping it is not a repeat performance of the devastating mass-bleaching event of 2005.

“If this were a person, it would be in critical condition,” Jeff Miller, fisheries biologist for the V.I. National Park, said about the territory’s reefs.

In the summer of 2005, an increase in water temperatures led to a massive coral die-off both in the Virgin Islands and around the world. The territory lost 60 percent of its coral colonies to bleaching and disease.

When colorful, living, coral polyps become stressed by high water temperatures, they expel the symbiotic algae that live within it and provide needed food to the coral. Because the algae is the source of the coral’s color, when it leaves, the coral becomes stark white or “bleached.”

While coral can survive the bleaching, it becomes vulnerable to disease and predators.

NOAA’s bleaching outlook

The National Oceanic and Atmospheric Administration’s Coral Reef Watch released their 2010 Coral Bleaching Thermal Stress Outlook this week, and it predicts a high likelihood for coral bleaching this summer — something local scientists can confirm is already happening.

NOAA uses satellites to take readings of sea surface temperatures from points across the globe, including the Virgin Islands, and computer models are run to project the effects of those temperature readings.

NOAA’s V.I. temperature sensor is located at Salt River on St. Croix.

“The region at greatest risk fills the region east from Nicaragua past the island of Hispaniola to Puerto Rico and the Lesser Antilles, and south along the Caribbean coasts of Panama and South America. The intensity of the potential thermal stress is predicted to increase through October,” the report states. 

According to NOAA, the Caribbean typically experiences higher water temperatures during the second year of an El Niño event. The 2009-2010 El Niño ended in May.

Mark Eakin, NOAA Coral Reef Watch coordinator, said temperatures in the Caribbean have been above normal since January. The models are showing similar trends to those recorded in 2005, he said.

“Things are looking very bad for the Caribbean in 2010,” he said in a written statement. “I am not saying that this year will be worse than 2005, but it looks like it could be a bad year.”

Water temperatures

Miller said that from October 2009 until the end of August, the water temperatures have been at or above normal.

“The water temperatures have been warm for almost a year,” he said. “The corals have been stressed; they haven’t had their usual environment, as far as water temperatures go.”

Tyler Smith, assistant professor at the University of the Virgin Islands Center for Marine and Environmental Studies, said UVI takes readings from 40 locations at reef level, unlike NOAA’s satellite sensor, which takes a surface temperature from only one location. 

Corals have a range of temperatures in which they can survive. Every year at this time, corals reach the upper limit of their thermal tolerance. However, global warming is causing the water temperatures to stay in the high end of the corals’ threshold all year round.

Smith said water temperatures in the territory seem to be hovering around 86 degrees. NOAA defines the bleaching threshold for the Virgin Islands at 85.1 degrees.

Without a strong event to force the cooling of the water, sea surface temperatures will remain as warm or warmer for another four to six weeks, Smith said. 

“If they stay at the stressful levels, we’re going to see mass bleaching; if we get some cooler temperatures, they could recover,” Smith said.

Starting to bleach

Scientists across the territory already are seeing signs of bleaching.

“A third to a half are experiencing some level of paling, very few corals are 100 percent bleached, which means stark white,” Smith said.

While he is seeing more bleaching every day, there does not yet seem to be a mass die-off like there was in 2005.

“I’ve seen a few dying corals and some coral mortality, but very few, like maybe 1 percent,” Smith said.

Marcia Taylor, marine advisor with the UVI Center for Marine and Environmental Studies, said she has seen bleaching on St. Croix.

“There are corals that are starting to bleach,” Taylor said. “I get in the water and swim pretty much every day, and I’m seeing it.”

Miller said he has not seen outright bleached coral on St. John, but he has seen paling of many corals, which is the beginning of a bleaching event.

Hurricane help

While Virgin Islanders may pray for hurricanes to skip the territory, a storm may be the only thing to prevent a mass-bleaching event.

Before Hurricane Earl passed through Aug. 30, some water temperatures had reached 87 degrees, Smith said.

Typically, hurricanes churn up the water column, mixing the cooler water below with the hotter water on the surface.

Immediately after Earl passed, water temperatures at reef level dropped three to four degrees, Miller said. While temperatures bounced back a few days later, the storm may have prevented the water from heating up far beyond the coral’s threshold.

“I’m not saying the hurricane was a good thing, but it did help to lower the water temperature a little bit, and that was good for the reefs,” Miller said.

Further study

While two mass-bleaching events within the same decade likely would be devastating to the territory’s reefs, the truth is that scientists do not really know how the reef will react or recover. They do not know how the corals that survived the 2005 bleaching event might fare in another one or how those that were bleached but recovered might survive during a second event.

“The corals may actually do better, but scientists are still not sure about the level of acclamation or adaptation that we can expect,” Smith said.

Kemit-Amon Lewis, coral conservation manager at The Nature Conservancy, is working on a project to study coral’s adaptive capabilities. He is spearheading the V.I. Reef Resiliency Plan, which was crafted in conjunction with NOAA.

He said the goal is to see whether there are some areas in the territory that may be less susceptible to bleaching. Those areas may be more important ecologically, he said.

“It may be physical factors, may be biological factors, we really won’t know until we start looking,” Lewis said.

Lewis said the other objective is to develop a plan to respond to coral damage from vessel groundings, anchors, storms and chemical and oil spills.

“We are trying to approach coral reef conservation in a more holistic way,” he said.

To help in the effort, Lewis is seeking public input. He said divers and snorkelers can take photos of bleaching or dying coral and send it to him or call to report a sighting. Lewis can be contacted at 718-5575 or e-mailed at

— Contact reporter Aldeth Lewin at 774-8772 ext. 311 or e-mail

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