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BVI Coral Bleaching Bulletin

Week of 23 August 2010 BLEACHING PREDICTION: Based on current BVI weather, field observations and the National Oceanographic and Atmospheric Administration’s (NOAA) Coral Reef Watch predictions, the threat for mass coral bleaching in the BVI this week is MEDIUM.

BACKGROUND: In 2005, the Caribbean experienced its worst bleaching event ever recorded due to extended warm water temperatures. As a result, nearly 60% of our reefs died. Since then, summer water temperatures have remained below the threshold in which corals “bleach”, a condition where symbiotic algae called zooxanthellae (zooxs) are expelled from coral polyps and expose the coral’s white skeleton. If corals are without their zooxs for too long, they will die. The threshold in which corals bleach is 1º-2º C above what they are used to (the summertime maximum) within the environment in which they live. However, water temperatures throughout the region are beginning to warm and conditions are becoming favorable for another mass bleaching event throughout the Caribbean. We will be producing this newsletter weekly throughout the summer months to keep you posted on the potential (low, medium or high – see below) of another mass bleaching event. All our predictions are based on local weather conditions, field observation and products1 from the National Oceanographic and Atmospheric Administration’s (NOAA) Coral Reef Watch website (http://coralreefwatch.noaa.gov).

*Just because the water is warm, doesn’t mean the corals will bleach!

Understanding the predictions:

LOW – Satellite imagery may show warm water temperatures over an extended period of time, but because of local weather conditions (high cloud cover, wind, waves) there is lower chance
of corals to bleach.

MEDIUM – Warm water temperatures have remained above the summertime maximum for extended periods of time or temperatures are 4°+ above the maximum for over a week. Local
weather conditions are also sunny, clear and calm but may have a storm system nearing. Corals may begin appearing “pale” or lighter in color.

HIGH – Water temperatures are extremely high and weather conditions remain sunny clear and calm. BVI corals are appearing pale to white in color.
1 Maps can be found on the Coral Reef Watch website http://coralreefwatch.noaa.gov

“Degree Heating Weeks” (DHW) maps indicate the cumulative stress over the past 12 weeks. 1 DHW is equivalent to one week of the surface water temperature at 1° C greater than the expected summertime maximum. 2 DHW’s is equivalent to two weeks at 1° C above the expected maximum OR one week at 2° C above the expected maximum. When the DHW reaches 4, bleaching is likely to occur because the temperature is either 4° C above the summertime maximum for one week or 1° C for four weeks. Once the DHW reaches 8, mass bleaching is occurring and coral death is likely.


The NOAA Coral Reef Watch now has the USVI & Puerto Rico under a bleaching warning. Sea surface temperatures are currently 29.8°C, 1.3 degrees warmer than the maximum high for this area. Weather conditions have been optimal for thermal stress (clear & calm) and will most likely continue throughout this week. Maps show the DHW for 22 Aug 2005 and 24 Aug 2009 to show what conditions looked like the same time during those years.

22 Aug 2005 – DHW was at .5 this time in 2005, it is currently at 1.9 DHW. Sea surface temperature was 28.6°C this time in 2005. Later this same year, the BVI experienced the worst mass bleaching event ever recorded.

24 Aug 2009 – Last year this time, temperatures were below the summer maximum and no bleaching occurred.

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