Coral Bleaching Part II...Past and Future Coral Reefs

So it's been a while since my last post on the problem of coral bleaching (which you can find here if you need to refresh your memory!) but having got distracted by a number of other interesting stories, I want to return to this issue.

In order to understand what may happen to our coral reefs in the future, it is important to look at what happened in the past and how ancient reef systems responded to climatic changes.

Today, our coral reefs are composed of scleractinian corals however in the past, other types of reef-building organisms dominated. In fact, 100 million years ago when atmospheric carbon dioxide levels were double the current levels, corals were only a minor component of tropical reefs with bivalves and other non-coral organisms dominating instead. This was largely as a result of the elevated carbon dioxide levels altering the pH of the water which favoured those organisms that secreted their shells in the calcite phase of calcium carbonate (corals secrete their shells in aragonite).

The earliest reefs were formed by algae (which often replaces corals once bleached) but by 500 million years ago, reefs were composed of calcifying organisms such as rugose and tabulate corals in addition to mollusks, crustaceans and echinoderms. The development of coral reefs was more or less continuous over a long time span (hundreds of millions of years) with only 'brief' interruptions where no reefs occur in the fossil record coinciding with the major mass extinctions.

Around 250 million years ago, both rugose and tabulate corals became extinct and corals more similar to the modern-day scleractinian corals, appeared. It is thought that the disappearance of the ancient types of coral is linked to changes in ocean chemistry, especially alterations to carbon dioxide concentration.

This paloecological evidence suggests that tropical reef systems are variable, with different states existing depending on the climatic conditions. Based on the fact that ancient reefs were composed of non-coral organisms, it seems likely that at some point in the future our current tropical reef systems could be pushed into a new state as a result of human influences. Given the fact that bleached reefs are frequently replaced by algae, it seems possible that the world's reefs could be replaced by algal reefs like the ones which dominated in the past. Although there have been few studies to investigate this idea, those that exist imply that the alteration of coral reefs to algal reefs would significantly impact upon the species composition of reef systems and given our reliance on them for fisheries and tourism, it is in our favour to address the issue of coral bleaching before it is too late...

Next time...can we save our reefs by breeding new 'super' corals?