Healthy corals (Photo: Steve Webster)
Corals in the tropical Indo-Pacific are often living near their upper limit of temperature tolerance. When an El Niño, or other warming event, arrives, the corals are stressed and they often respond by expelling their symbiotic dinoflagellates algae (zooxanthellae). Because the zooxanthellae usually impart most of the color to the coral tissue the colonies without their zooxanthellae turn bone white. Depending on the severity of the rise in sea surface temperature, and/or its duration, the corals may recover and be re-inoculated with their symbionts or they may not. In which case they die.
Unhealthy corals (Photo: Steve Webster)
During 1997/98 a powerful El Niño bathed Fiji’s waters in unusually high temperatures and the effects appeared to be devastating. Easily 80 percent of the corals bleached giving the reefs a ghostly and ghastly appearance. It seemed inconceivable that the reefs would ever recover.
In the intervening years the reefs have exhibited an encouraging recovery. Today the reefs of Vatu-I-Ra are looking very healthy. Some older colonies have recovered, and young new recruits are thriving. Although there is ample evidence of the old, dead colonies, covered in filamentous cyanobacteria and algae, the living corals are doing well, in some locations providing virtually 100 percent cover of the substrate. [Note: You can read about similar examples of coral bleaching and recovery from the 2009 Phoenix Islands Expedition from Dr. David Obura here and from Dr. Les Kaufman here.]
Will future bleaching events become more frequent and more severe as global warming continues? In all likelihood, they will. The hope is that if other stressors of the reef community (overfishing, coastal development and pollution, logging, etc.) are eliminated by integrated ridge-to-reef management, the reef communities will be better able to recover from the inevitable bleaching events. That’s what makes the WCS and CI marine programs, and the buy-in of the villages and chiefs in this area so important to the future of Fiji’s reefs.
Heathy corals (Photo: Steve Webster)
One of our New England Aquarium/Monterey Bay Aquarium initiatives in support of this work is the creation of a permanent transect study site on Mt. Mutiny in the Vatu-I-Ra area. Each year we run video and still photo sampling techniques along this line from about 2 meters to 20 meters deep on the steeply sloping reef at Mt. Mutiny. As these data accumulate we will be building a solid baseline with which to compare future changes on the reef. Fish and coral diversity and density will be documented, as well as growth rates and health of individual coral colonies. This will provide us the opportunity to monitor the responses of the reef community to the changes that will likely occur over the next few years and decades.
Steve Webster PhD
Senior Marine Biologist (retired)
Monterey Bay Aquarium
