Bleaching Progression around Virgin Islands National Park and St. John

Warmer than normal seawater temperatures in the US Virgin Islands being detected in mid-late July and into August scientists with the SFCN became alert to potential coral bleaching around VIIS and BUIS.

Corals are animals that contain microscopic algae (= plants) called z ooxanthellae in their skin. These plants give the corals their greenish-brown color. More importantly, the plants use sunlight (through photosynthesis) to promote coral growth. If corals didn't have these plants helping them grow, coral reefs wouldn't exist the way we see them today. Corals can't get enough energy from the food they eat; they have to have an extra energy source, and it comes from the plants within their tissue.

When the water gets warm, like it is now, something causes the coral to expel the 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 right now, the excessively warm water is stressing the corals.

Bleaching was observed in August on some deeper reefs however, with even warmer temperatures in September, bleaching spread rapidly. The September mean monthly seawater (at depth) were some of the warmest observed in over 15 years of continuous (every two hour) data collection. This triggered the SFCN to perform episodic monitoring of the permanent coral sites around St. John and Buck Islands .

SFCN conducts annual monitoring using permanent, randomly selected video transects at selected reef sites (Areas of Particular Concern = APCs); five sites around St. John , and 2 sites around Buck Island (in Buck Island Reef National Monument ). In response to the bleaching event, each transect (n=20) at each monitoring site was filmed (sampled) and will be analyzed for benthic cover and percent cover by bleached coral.

Data for Tektite Reef in Virgin Islands National Park have been analyzed revealing 24.7% coral cover at that site with 90.2% of the coral being bleached.

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

Reef Sites (images and videos)

Haulover

Mennebeck

Newfound

Tektite

 

 

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