Bleaching Elkhorn coral in Buck Island Reef National Monument

Shown here is a photograph taken on October 20, 2023, of a wild Elkhorn coral colony in Buck Island Reef National Monument, St. Croix, VI, showing the effects of heat stress caused by elevated ocean temperatures. The heat stress causes “coral bleaching,” or the loss of nutrient-giving microscopic algae that normally live within corals. When these symbiotic microalgae are expelled, the coral’s tissues become transparent and allow the white skeleton to shine through. During bleaching, corals are likely to starve, and may die if ocean temperatures do not cool off to less than about 87 degrees Fahrenheit. Of note in this photo is the bleached appearance of the upper surfaces of the coral, demonstrating the compounding effect of direct sunlight that exacerbates the bleaching—the undersides of this colony that are naturally shaded can be seen still containing some color, and therefore, their symbionts.