2015 Expedition: Coral bleaching on Kanton Island

Aquarium researchers and staff are on expedition to the remote Phoenix Islands Protected Area (PIPA) to study the natural history of the islands, surrounding reefs and connecting open water ecosystems. Research on the 2015 expedition will directly inform the management and maintenance of this world-renowned MPA. Today's post comes from the expedition's chief scientists, Drs. Sangeeta Mangubhai and Randi Rotjan.

For last two months, we have been looking at maps on NOAA’s Coral Bleaching Watch website following the formation of a warm pool of seawater over the central Pacific associated with El Nino conditions. We are watching it because we know that if the seawater temperatures increases even by a degree, there is potential for coral bleaching to occur.

Coral bleaching occurs when corals expel the symbiotic microscopic algae (called zooxanthellae) living within their tissue. It is called bleaching because when the coral animals lose their symbionts, they appear white, or "bleached". These symbionts (in the genus Symbiodinium) help produce much needed food for their coral host. Corals expel the algae and turn white when they are under stress. If the algae does not repopulate coral tissues quickly enough, the coral host will eventually starve to death. What is of concern to scientists is that with increasing climate change, we will get more frequent coral bleaching events. Under this scenario, we are not sure how quickly corals will be able to adapt to more frequent stress and how well they will persist.

Bleached corals off Kanton / Photo Credit: Dr. C. Cook

The last time we had a large bleaching event in the Phoenix Islands was from 2002 to 2003. This event resulted in almost 100% mortality of corals in Kanton lagoon and large decreases in coral cover on leeward facing reefs. The reefs in the Phoenix Islands are still in a recovery phase, and many sites such as Kanton lagoon have shown remarkable recovery, especially in the last three years.

Hydnophora corals starting to bleach on Kanton / Photo Credit: S. Mangubhai

For the last five days, along with Yashika Nand, temperature loggers have been retrieved and deployed to document how hot the waters are at depth (with corresponding predictions of bleaching). Seawater temperatures are currently reaching as high as 30.5°C in Kanton lagoon as well as on leeward reefs down to 37m depth. With these temperatures we are seeing coral bleaching all around the atoll, from the shallows down to 45m. Some of the genera most affected include solitary corals (called fungids), branching corals (especially Hydnophora, Pocillopora), plating corals (such as Montipora), encrusting corals (such as Pavona, Coscinarea) and some massive and submassive colonies (such as Porites, Hydnophora, Pavona). Interestingly, branching and plating Acropora which were highly susceptible to coral bleaching in 2002/2003 appear to be more resistant to the current bleaching stress.

Peter Gawne and Sangeeta Mangubhai collecting bleached coral samples on Kanton / Photo Credit: Dr. C. Cook

However, if the temperatures do not decrease soon, we can expect the coral bleaching to become more extensive, and coral mortality will start occurring in the Phoenix Islands. If the mortality is on a large enough scale, this is going to set back the recovery of these reefs another decade.

Let’s hope that weather conditions will change and help dissipate the warm pool of water that is sitting directly over Kanton. Luckily, at this stage this water is not extending much further south, so Randi and I are expecting less bleaching in the more southern islands in the Phoenix group. Stay tuned to see if our prediction is correct!

— Sangeeta & Randi

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