On this remote island in the middle of the Pacific Ocean, corals are being put to the test of their lives as water temperatures soar to a staggering 31C (88F). The current El Niño event is responsible for the large-scale warming across the equatorial Pacific, but water temperatures have been above the threshold for coral bleaching at this site for months now. Worst of all, waters will remain far above average for the next 1-2 months, before returning to near-normal values by spring 2016.
The last time water temperatures reached such levels was during the 1997/98 El Niño event – the largest El Niño event on record. Presumably the reef at Christmas Island was devastated by widespread bleaching and mortality, but nobody was around to document the effects. As it happens, I snorkeled the reefs at Christmas during November 1997 as a baby graduate student, but didn’t think to take photos or make any systematic observations at the time.
Over the last week, our field team has been systematically documenting the status of the many different types of coral reefs on Christmas Island. In close collaboration with Dr. Julia Baum (U. Victoria) – a marine ecologist by training – we have taken hundreds of photos along preset transects that she and her students have been surveying for the last 7 years. Such baseline data are critical to understanding how this off-scale thermal stress event has impacted the reef. And up until July, 2015, her field teams had documented thriving reef communities all around Christmas Island. This July, they observed the beginnings of the current bleaching event, with some species showing widespread bleaching but little mortality.
This time, the underwater damage is jaw-dropping. It is hard to describe the gut punch that I took when I first witnessed the devastation on the remote windward reefs. Here, entire species of coral, such as Acropora, were already coated in brown-green algae, pushed far beyond the point of bleaching many weeks ago. Others, such as Favia, were 100% bleached, with some algae-coated colonies having lost their months-long battle against the warm waters. Consistently, we observed Porites and Pocillopora in various states of bleaching. For Pocillopora, every colony showed bleaching at the tips of its stocky branches, but very few were completely bleached. While some Porites colonies were completely bleached, a Porites colony only a few meters away may have only exhibited mild paling. We did not see any dead Porites colonies – at least not yet. This genus is particularly important to my lab’s work, as we use Porites colonies to reconstruct El Niño events for the last 7,000 years, calibrating our reconstructions with colonies growing on the reef today. Ecologically, it is one of the more resistant species to bleaching, so its resilience to the current thermal stress event is a key benchmark of reef health. I would estimate total coral bleaching and/or death at 50-90% (very site-dependent).
We will be returning to Christmas Island in March 2016 with the Baum team, at the tail end of the current El Niño event, to document the maximum extent of bleaching and mortality. Judging by how much the reefs have deteriorated from July to November, the next four months may well witness near-100% mortality for most of the Christmas Island coral reefs.
Our latest field observations confirm our fears – that the current bleaching event will reshape the reefs at Christmas Island for years to come. Monster El Niño events are nothing new to Christmas, as recorded in the geochemical variations of centuries-old coral skeletons from my lab’s work, but the rapid succession of very strong El Niño events in the last decades stands out against the backdrop of natural variability in our records. For the reefs at Christmas Island, the 2015/2016 El Niño event may be a tiding of things to come under continued anthropogenic climate change. If so, then the current event represents a golden opportunity to study if and how this reef is fundamentally (and possibly permanently) altered under acute thermal stress. After all, while we cannot stop the ravaging effects of the current El Niño on coral reef ecosystems across the Pacific, we can certainly learn some key lessons from it. It isn’t far-fetched to think that one day, such lessons may provide the blueprint for engineering the resilience of corals reefs to climate change.
Below are some of the photos from the last week's expedition. UPDATE: A special thanks to Danielle Claar (U. Victoria) for helping to identify the species in these photos.