National Park Service

South Florida/Caribbean I&M Network (SFCN)

Coral Reef Bleaching & Disease in the U.S. Virgin Islands

National Park Service and US Geological Survey scientists have been monitoring coral reefs in the US Virgin Islands for over 20 years and successfully documented the devastating 2005 coral bleaching-disease event in the U.S. Virgin Islands and the subsequent 2010 coral bleaching event.

About Coral Bleaching & Diseases

Coral bleaching example

Coral Bleaching is the term used to describe the discoloration of coral tissue resulting from the loss of the symbiotic algae, zooxanthellae. These algae are present in the coral in concentrations of approximately a million plant cells per square cm of coral tissue. The zooxanthellae are very important to the survival of coral and the growth of the coral reef. Corals can tolerate the loss of zooxanthellae, but only for short periods of time (weeks). When conditions occur that cause the coral to be without these symbionts for more than that, coral mortality can result.

There are many factors that can cause coral bleaching such as changes in salinity and sedimentation, however the only factor that can cause corals to bleach over a wide spatial scale (e.g., on many reefs around an island, or regionally) is elevated water temperatures. This is the condition that has led to the current coral bleaching event in the US Virgin Islands.

Bleaching – When the water gets warm, like it was in the Caribbean in 2005, the coral expels its symbiotic algae, zooxanthellae. Since the algae give the coral its color, the coral appears to be pale or white. This is because the tissue has lost the components (the zooxanthellae) that provide the color. But more importantly, the coral has lost a significant portion of its energy source. They can tolerate this loss for an unknown but short period of time before they begin to die.

This phenomenon is known as "coral bleaching." It has nothing to do with the chemical bleach or Clorox®. It is due to the coral being stressed, and in summer/fall of 2005, the excessively warm water is stressing the corals

Coral Disease

Coral disease comes in many forms and at least four types have been seen in US Virgin Islands corals. Investigations are ongoing concerning the causes and mortality rates of infected corals.

Black band disease example

Black Band Disease – has been documented in some hard corals around Virgin Islands National Park (VIIS) and Buck Island Reef National Monument (BUIS). Although this disease has not been positively linked to human activities there is speculation that the disease is correlated with higher seawater temperatures.

White band disease example

White Band Disease – was first observed in the U.S. Virgin Islands in the early 1970's (Robinson 1973; Gladfelter 1982). White band disease often affects primary USVI reef building corals such as elkhorn and staghorn corals. This disease is not positively correlated with human activity. Most infected colonies are killed by the disease, although sometimes small patches survive.

White plague disease example

White Plague Disease – is similar in appearance to White-band disease although it affects different coral species. There is more information on the exploration of White Plague disease in the USVI below.


2010 Coral Bleaching Events in the U.S. Virgin Islands

Beginning in May of 2010, monitoring revealed that several corals were beginning to become pale in color, a condition known as coral bleaching. These four pictures below were taken in Tektite Reef on May 14, 2010 at 55' (16.8m) depth. This is one of the earliest recorded observations of bleaching during the summer-warm water season. Elevated water temperatures triggered an increase in monitoring frequency at sites around VIIS in order to document the event.

Coral Bleaching Photos and video in Haulover Reef

These pictures were taken on September 29, 2010, and show a sample of the variety of bleaching conditions that existed on the reef at that time.

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Coral Bleaching Video at Haulover Reef- September 29, 2010

 

Coral Bleaching Photos at Mennebeck Reef

These pictures were taken on October 13, 2010, and show a sample of the variety of bleaching conditions that existed on the reef at that time.

 

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Coral Bleaching Video at Mennebeck Reef- October 13, 2010

 

Seawater temperatures on reefs within National Parks in the US Virgin Islands were exceptionally warm in 2010. From October 2009 through September 2010, the monthly average temperature equaled or exceeded the maximum monthly averages from 1988-2004, with temperatures for August 2010 exceeding the threshold at which most corals begin to lose their symbiotic zooxanthellae and bleach. See Figure 1 below.

Figure 1. St. John ocean temperatures at reef depth in 2005-2010 compared with 1988-2004 data. Solid line represents average, minimum, maximum of monthly average temperatures recorded at 4-5 reefs at St. John from 2005 - Oct. 2010 (HA, NF, MB, TK, YZ). Grey shows range (minimum-maximum) of monthly average temperatures recorded at 3 reefs (HA, NF, YZ). Number of sites with data varied by month and year. Vertical lline shows mean values for October 2005-2010. October mean temperature of 29.02, or 84.2°F, is lowest October mean temperature of the previous six years. Horizontal line is bleaching threshold (29.5°C, or 85.1°F)
Figure 1. St. John ocean temperatures at reef depth in 2005-2010 compared with 1988-2004 data. Solid line represents average, minimum, maximum of monthly average temperatures recorded at 4-5 reefs at St. John from 2005 - Oct. 2010 (HA, NF, MB, TK, YZ). Grey shows range (minimum-maximum) of monthly average temperatures recorded at 3 reefs (HA, NF, YZ). Number of sites with data varied by month and year. Vertical lline shows mean values for October 2005-2010. October mean temperature of 29.02, or 84.2°F, is lowest October mean temperature of the previous six years. Horizontal line is bleaching threshold (29.5C, or 85.1°F).

Hurricane Earl passed the VI on August 29-30, 2010 (See Figure 3). NOAA sea surface temperature (SST) anomaly data (See Figure 4) shows the cold water wake left by Hurricane Earl. This SST anomaly data for the Virgin Island shows no storm effect, however satellite data records only the sea surface temperature (top millimeters) and cloud cover can obscure satellite data interpretation. SFCN in-situ data loggers showed Hurricane Earl decreased temperatures >1°C (~3.8°F) in 24 hours but caused physical damage to corals, primarily on the south side of St. John, especially in shallow water (< 20 feet, or 6 m). Water temperatures were still above the theoretical bleaching threshold (29.5°C, or 85.1°F) until September 21, 2010. Historically, water temperatures are warmest in August and September, cooling in October, typically, one of the rainiest months. In early October 2010, a weather system developed near St. John that would eventually become Hurricane Otto. This system produced torrential rains during October 5-7 2010, dumping over 16" (40.6 cm) of rain in those three days; lowering the water temperature 0.5°C (1.8°F). See Figure 2 below.

This rapid drop in temperature may have helped avoid a repeat of the devastating 2005 Bleaching-Disease event. Although graphs show a small drop in coral cover at some sites in 2010, this drop was not statistically significant (see Status and Trends by SFCN Park).

Figure 2. Water temperatures before and after Hurricane Earl
Figure 2. Water temperatures before and after Hurricane Earl.

Figure 3. Track of Hurricane Earl, courtesy of Weather Underground
Figure 3. Track of Hurricane Earl, courtesy of Weather Underground.

Figure 4. NOAA/NESDIS Sea Surface Temperature Anomaly display for September 6, 2010, overlaid upon Google Earth with Hurricane Earl Storm track; Image mosaic from NOAA Coral Reef Conservation Program.
Figure 4. NOAA/NESDIS Sea Surface Temperature Anomaly display for September 6, 2010, overlaid upon Google Earth with Hurricane Earl Storm track; Image mosaic from NOAA Coral Reef Conservation Program.

Yawzi Reef

Monitoring was conducted September (7-10). This was just 7 days after Hurricane Earl passed 40 miles North of St. John (see Hurricane Earl's tracks above). Damage to the corals could be observed with numerous corals tumbled and broken, and many seafans, Gorgonians and sponges, dislodged.
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2005 US Virgin Islands Bleaching and Disease Mortality Event

In 2005, SFCN responded to one of the most devastating coral bleaching events on record in the Caribbean. SFCN scientists working in the US Virgin Islands noticed record-breaking water temperatures at reef depth in April 2005 and began observing some early coral species bleaching in July 2005.

Water temperatures at sites at St. John, U.S. Virgin Islands during the 2005 bleaching and disease mortality event
Water temperatures at sites at St. John, U.S. Virgin Islands during the 2005 bleaching and disease mortality event

As a consequence, SFCN scientists increased their coral monitoring frequency during and after the bleaching event to capture the impacts on coral cover and disease with major findings including:

  • 90% of coral cover bleached
  • Combined effect of bleaching followed by coral disease caused most mortality
  • Although bleaching was associated with record high seawater temperatures in 2005, mortality continued through 2006.
  • By 2007, two-year losses of live stony coral cover at VIIS ranged from 48-64% and BUIS monitored reefs ranged from a 41-79% of pre-2005 event levels.

SFCN scientists, Jeff Miller and Rob Waara, were assisted in this episodic monitoring by USGS scientists, Dr. Caroline Rogers, Erinn Muller, Tony Spitzack, and NPS GIS specialist Christy Loomis.

The SFCN coral monitoring program captured the coral disease event that occurred between the 2005 coral bleaching event and subsequent mass die-off. The relationship between coral bleaching, coral disease, and coral mortality is poorly understood. However, some sort of correlation was observed in 2005 when corals began to recover from bleaching but then suffered mortality from disease (primarily white plague disease).

Images- Bleaching and Disease Progression at Tektite Reef

Tektite Reef, bleaching September 2005
Tektite Reef, bleaching September 2005
Tektite Reef, disease outbreak, November 2005
Tektite Reef, disease outbreak, November 2005
Tektite Reef, catastrophic mortality, January 2006
Tektite Reef, catastrophic mortality, January 2006

 

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Images- Bleaching and Disease Progression at South Fore Reef

South Fore Reef with high live coral cover, 2003
South Fore Reef with high live coral cover, 2003
South Fore Reef, with bleached coral, 2005
South Fore Reef, with bleached coral,
2005
South Fore Reef, mostly dead, 2007
South Fore Reef, mostly dead, 2007

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Bleaching and Disease Progression at Sites with added Episodic Monitoring

Haulover Reef sample data 2005-2006
Haulover Reef sample data 2005-2006
Mennebeck Reef sample data 2005-2006
Mennebeck Reef sample data 2005-2006
Newfound Reef sample data 2005-2006
Newfound Reef sample data 2005-2006
South Fore Reef sample data 2006-2006
South Fore Reef sample data 2006-2006
Tektite Reef sample data 2005-2006
Tektite Reef sample data 2005-2006

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Last Updated: January 26, 2017 Contact Webmaster