In August 2018 National Geographic published a story titled "Half of the Great Barrier Reef is Dead" and compared the destruction to a forest after a forest fire. 50% of the reef has become barren and has little scope for recovery. This is an extreme situation and implies that, if the present rate of destruction continues there will be virtually nothing left in 10 years. Is this true? How has this happened?
Pressures on the environment
The blame has been put squarely on rising sea temperatures. The average global sea surface temperatures have risen by about 0.13ºC per decade, by 2100 there is predicted to be a rise of global ocean temperature of between 1-4ºC. The IPCC have stated that if global temperatures rise by 2ºC then 99% of coral reefs will die, it is thought that whatever we do now this will probably happen. The present target is 1.5ºC, "only" 70-90% will die! However the local changes in temperature are what is killing coral. Corals like it warm, about 23-29ºC and they cannot live below about 18ºC. The Great Barrier Reef corals have a range of 27ºC to 32ºC, unfortunately if that rises to 32.5ºC, which it has been in some areas, particularly in the north, then the corals die, it is just too much for them. Some will bleach at anything over 30ºC for more than a few days.
In fact there are many more pressures on the reef. I studied a reef near the docks in Aqaba, Jordan where phosphate ore was loaded into cargo ships, there was always a cloud of grey dust blowing along the coast. The corals there were very degraded and there was poor visibility on the reef heavy growth of algal turf up to 1km from the dock. There were also a greater proportion of branched corals in the polluted area compared to an unpolluted site further along the coast. This and other studies show that pollution from sources such as sewage and fertilisers in rivers strongly affect corals, as well as sporadic events such as oil spills and cyclones. All of these are increasing worldwide, even the cyclones and hurricanes as global warming sets in with a vengeance.
What happens to the corals?
All corals contain algae within their cells, zooxanthellae, most of the 798 reef‐forming species have developed a symbiosis with unicellular dinoflagellate algae of the genus Symbiodinium. They carry out photosynthesis from inside their safe haven and pass food onto the coral host. Corals also eat small planktonic organisms which are captured by the stinging tentacles and transferred to the mouth and gut. If things get too hot the corals throw the zooxanthellae out, expelling them from their cells into the sea. This is coral bleaching; the corals do not die straight away and can sometimes recover. It seems an odd thing to do! Like a hotel throwing out all the paying guests because the air conditioning doesn't work. So why do they do it? One possibility is that the algae become toxic, when they heat up, or are given high doses of ultraviolet from the Sun, their photosynthetic pigments, like chlorophyll, become damaged and as well as producing oxygen they also produce active substances like hydrogen peroxide and free oxygen radicals, oxygen atoms which charge around sticking onto proteins and DNA and damaging them. This becomes dangerous for the coral cells so they quickly kick them out. The paying guests have are no longer paying and have turned into assassins. Another possibility is that the coral stops being able to provide enough carbon dioxide to the algae, the guests leave the hotel because they are not being fed. If the corals do not take in more zooxanthellae within weeks they will probably die.
Acidification of the Oceans
When carbon dioxide dissolves in water it makes carbonic acid, which is what we doing in fizzy drinks. Clearly as world carbon dioxide levels increase so the oceans become more acid. This will affect any organism which has a shell or skeleton made from calcium carbonate, if you put a seashell or bone or piece of coral into ann acid it will fizz and give off bubbles of carbon dioxide. A recent study in "Science" points out that by about the year 2050 carbonate sands (the white sand of coral reef areas, not the silicate sand we use for building) will be dissolving faster than they are forming (from breakdown of coral reefs, coralline algae and shells). This could mean that coral reefs ould be destroyed. Not only that but more carbon dioxide will be released into the atmosphere! Carbonate rocks are a good way to remove the gas, normally.
So, Are Coral reefs doomed?
No one knows for sure, but it is not looking too good. It seems certain that global temperatures will continue to rise by at least 1.5ºC, I would guess to at least 2ºC before beginning to decline. If this is true then the present evidence suggests that most corals will be destroyed. On the positive side it has been shown that corals and zooxanthellae can adapt to higher temperatures and a lot of research is going on with replanting resistant corals and there is another area of research which is looking at the coral microbiomes, the microbes growing on and in the coral polyps. They could have an effect in protecting them against disease (like our gut microbiome and skim bacterial flora), maybe it can be manipulated. The big problem is the timescale, climate change is taking place too fast for "normal" evolutionary adaptation to take place. Coral reefs in cooler waters at the lower end of the range, will not be affected to the same extent, many reefs are still healthy. A major, 2016, study concluded that there are many Bright Spots among the World's Coral Reefs.
Corals resistant to heat stress: Cyphastrea, Goniopora, Galaxea, and Pavona.
Vulnerable corals: Stylophora, Pocillopora, and Acropora
Coral Reef Bleaching: An Ecological and Biological Overview 85 . 2018 http://dx.doi.org/10.5772/intechopen.69685