By: Kayla Ripple

Coral bleaching is characterized by the loss of zooxanthellae that inhabits coral tissue. In addition to the color it provides, zooxanthellae are vital to the overall health and life support of coral colonies, supplying the coral with nutrients and proteins to sustain growth (see Coral Chronicles Feb 2016 edition). When zooxanthellae are lost, the coral is still alive, but turns stark white appearing bleached, hence the name “coral bleaching”. If conditions persist, bleached corals are likely to experience high mortality throughout the colony. Extreme temperature and weather patterns are usually the culprit of coral bleaching events. In the face of a changing climate, coral reefs have experienced more intense and sustained bleaching events in past decades. Scientists in Australia became alarmed this year with possibly one of the most severe bleaching events ever recorded on the Great Barrier Reef (GBR). Signs of a widespread bleaching event became apparent beginning in March 2016 along the entirety of the GBR. The ARC Center for Excellence in Coral Reef studies began an extensive project to survey 911 reefs for levels of bleaching severity (see photo below). Of these reefs, nearly 60-100% of corals were affected by the bleaching event. Andrew Baird, one of the ARC scientists documenting the bleaching event, claims that “…we’re already measuring an average of close to 50% mortality of bleached corals. At some reefs, the final death toll is likely to exceed 90%.” “Tragically, this is the most remote part of the Reef, and its remoteness has protected it from most human pressures: but not climate change.”

Untitled911 reefs were surveyed for level of bleaching severity. Southernmost reefs experienced the lowest levels of bleaching severity, likely due to more hospitable water temperatures. Results are show in this figure from the ARC Centre of Excellence. – ACR Centre of Excellence for Coral Reef Studies

Bleaching events in the Caribbean typically occur in the summer months of July to September, but can occur in cold winter months as well. With the summer upon us, the coral bleaching event in Australia brings some concern for potential bleaching threats in the Florida Keys and Caribbean. CRF “spreads the risk” by housing large numbers of distinct genetic strains of threatened coral in offshore nurseries across the Florida Keys Reef Tract. Because of their location, coral nurseries can serve as repositories for protection of genetic material that might otherwise be lost on reef sites (Schopmeyer et al. 2012). The protection of these various genetic strains is vital to the continued restoration and persistence of coral populations.

Click here to read the full article on the GBR bleaching event from the ARC Center for Excellence for Coral Reef Studies: https://www.coralcoe.org.au/media-releases/only-7-of-the-great-barrier-reef-has-avoided-coral-bleaching

Scientific paper cited: Schopmeyer, S. A., Lirman, D., Bartels, E., Byrne, J., Gilliam, D. S., Hunt, J., Johnson, M. E., Larson, E. A., Maxwell, K., Nedimyer, K. and Walter, C. (2012), In Situ Coral Nurseries Serve as Genetic Repositories for Coral Reef Restoration after an Extreme Cold-Water Event. Restoration Ecology, 20: 696–703. doi: 10.1111/j.