In March of this year, Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies at James Cook University, spent nine days flying over the Great Barrier Reef, conducting an aerial analysis of its entire length. What he saw was unprecedented. A stretch of about 1,500 miles from the Torres Strait in the north, right down to the reef’s southern boundary had been severely bleached, leaving white structures where once there would have been colour.
Of the 1,036 reefs surveyed by Hughes and others from the Great Barrier Reef Marine Park Authority, about a quarter were severely affected, while a further 35 per cent had modest levels of bleaching. It marks the largest bleaching event on record and the second most severe. What’s more, it is the third mass bleaching event to take place in the region in the last five years.
Bleached coral is not necessarily dead, but it is severely weakened and much more vulnerable to disease. In the Great Barrier Reef, scientists believe the effects of the latest event will be lasting, with some damage irreparable.
Bleaching occurs when coral expel the algae which usually live in their tissues. The algae, which in good times gain shelter, carbon dioxide and nutrients from coral, in turn provide their hosts with photosynthetic products to fulfil their energy needs. When the algae are lost, the coral’s white limestone skeleton shines through its transparent tissue, leaving it exposed to erosion and damage. One of the main culprits of these events is warming waters (although poor nutrient levels are also thought to be relevant). Scientists believe that warming of just 1C above normal levels can cause algae to be expelled.
Some coral however, react to bleaching events in an entirely different way. Rather than fading to a milky white as the algae leave, they instead emit a range of bright neon colours. The phenomenon has been captured by underwater photographers, but the reason for it has historically confounded scientists.
A team of scientists from the University of Southampton’s Coral Reef Laboratory has now revealed the process behind these curious light shows. By conducting lab experiments, during which they exposed coral to heat stress, the researchers found that during the colourful events, corals produce what is effectively a colourful sunscreen layer. They believe that this process encourages the coral symbionts (the all important algae) to return.
Professor Jörg Wiedenmann, head of the University of Southampton's Coral Reef Laboratory explained further: ‘When corals lose their symbionts, the excess light travels back and forth inside the animal tissue – reflected by the white coral skeleton. This increased internal light level is very stressful for the symbionts and may delay or even prevent their return after conditions return to normal. However, if the coral cells can still carry out at least some of their normal functions, despite the environmental stress that caused bleaching, the increased internal light levels will boost the production of colourful, photoprotective pigments. The resulting sunscreen layer will subsequently promote the return of the symbionts.’
Using satellite data, the team reconstructed the temperature profiles for known colourful bleaching events. This revealed that they tend to occur after brief or mild episodes of heat stress. When corals are exposed to severe or prolonged temperature extremes, they tend to bleach white. Nevertheless, the findings do present some hope for bleached coral, particularly that living in more mildly affected areas. The researchers point to reports of colourful bleaching taking place during the most recent mass bleaching event at the Great Barrier Reef, suggesting that some areas have a better chance of recovery.