A couple of years ago I went on a cruise with 9 scientists, including the former head of the Great Barrier Reef marine park authority and members of the Australian institute of marine science, doing citizen science on the Great Barrier Reef. During the trip, we visited places where corals were being grafted and grown to be attached to areas where corals had reduced. We had lectures about coral bleaching amongst others, and we saw places where coral bleaching had occurred. We did surveys of each place we visited, collecting seawater samples that were genetically tested for the complete biome of several locations. The attached slides are from the trip.
We visited the Great Barrier Reef at a time when it was looking better than it had for 37 years, and some of the places we went were absolutely astounding. But other places, particularly out past the Great Barrier Reef, in the coral sea, were completely destroyed. Cyclones are becoming much more frequently and three had been through the coral sea in the preceding two years - as well, this area tends to be warmer, so the coral here had had more bleaching events. We saw coral graveyards, where the ocean floor was covered by broken bits of white coral about six inches long. We doubled the number of things identified as living in the coral sea, and found crown of thorns starfish there for the first time. We also discovered things at each of the sites we visited in the Great Barrier Reef that hadn’t been found there before. Admittedly, some places we went very rarely have visitors, especially the coral sea. It was a pretty amazing trip.
Yes, coral can survive being bleached. Only if the zooxanthellae rebound to the coral structure before they die - so, within about two weeks. The coral is weakened, and more susceptible to other problems. After all, it’s been starved for the time that the zooxanthellae haven’t been bonded. Bleaching occurs when water temperatures are too hot, so for corals to survive after having been bleached, the temperature needs to reduce very quickly. Some reefs are in warmer areas than others, so there’s a lot of work being done to identify corals that can survive in warmer reefs and work out how to use them to help other reefs.
However, some corals grow much faster than others, and some types of corals are more susceptible than others and bleaching events are increasing, so the composition of reefs is changing - they have less of what might be called boulder corals and more of the Staghorn corals. Bleaching has always occurred naturally, about ten to twenty years apart (if I’m remembering correctly), but it’s now heading to about two or three years apart, and this is putting a lot of stress on reefs worldwide.
