I have drilled skeletal cores from massive coral colonies all around the world, from the Caribbean, to remote Pacific Islands, Taiwan, Australia, and Palau. Finding live corals was never very hard, there were always plenty to choose from and I would search for the largest (oldest) corals. Then, in 2019, I went to the Farasan Banks, in the southern Red Sea. I could certainly see the amazing diversity and coral cover that used to exist here. But, it's almost all dead, following a terrible bleaching event in 2015. The Red Sea is not hit by large storms or waves, so the corals are all standing dead, creating an eerie reef graveyard. I've never spent so much time just snorkeling over reefs to find live corals to drill. At some reefs, there were none. For the first time, I resorted to drilling cores from dead corals. And the live corals that I did find often had partial mortality scars.
So what happened? Why was this reef so devastated? Well, our new paper in Science Advances shows that an unusual conflux of heat and nutrients led to an exacerbated bleaching response. Initial strong monsoons in early 2015 caused strong upwelling, supplying the reef with excess nutrients. Then, in late summer, the monsoon winds suddenly died off earlier than usual, leading to anomalous heating. While a few extra nutrients might normally be good for corals, our paper shows that it is bad when combined with heat stress. The reason? I suspect that the corals' symbionts quickly soak up those nutrients and grow in size / reproduce. Then, when the heat stress comes, causing the symbiont photosystem to produce toxic oxygen species, the large symbiont populations are harmful to the coral, leading to exacerbated bleaching.
Imagine it's Sunday. You're planning a BBQ for a week from today, but the weather person on TV just forecast rain for next Sunday. How much confidence do you really have in that prediction? Turns out, that's about the confidence we can place in predicting coral bleaching from SST alone. Our new paper in PeerJ dives into these statistics and proposes some initial steps forward. The main conclusion is that we should critically evaluate our understanding of the causes of bleaching events, think beyond SST, and work together to improve our predictive ability.