Coral bleaching? Not black and white

Problems with how scientists communicate with the media and in how reefs’ health is assessed have created a skewed public understanding of coral bleaching, according to a new study.

Coral bleaching is a widespread phenomenon in which corals lose their vivid colours. It’s a major concern to conservationists, as it can be triggered by rapid environmental change and sometimes presages the death of whole reefs, along with the complex ecosystems they support.

But the researchers suggest we need to take a more complex view of the matter – bleaching isn’t always a bad thing. ‘We go out to Indonesia twice a year, and in spring when the waters are warmest the reefs are always bleached,’ says Dr David Suggett, a marine biologist at the University of Essex’s Coral Reef Research Unit and co-author of the paper, published in Global Change Biology.

‘By summer when water temperatures are coolest they’re always looking as vibrant as ever,’ he adds. ‘They bleach for their own benefit, to help them adapt to normal changes in their environment. This year we had the largest temperature anomaly on record in the region, but as far as we know the reefs still did incredibly well.’

Suggett and co-author Dr David Smith, also of Essex, point out that corals go through natural periods of bleaching, and that many aren’t fatal. While sometimes bleaching really does herald mass coral die-offs, far more often it’s just a precautionary response to natural changes in conditions. At other times, corals lose their colour due to unusual environmental stress, but recover as conditions return to normal – the researchers call this ‘sub-lethal’ bleaching.

Current methods of assessing coral bleaching may mean its severity is being over-estimated, and this could be limiting the effectiveness of conservation efforts – reefs may be written off as in terminal decline, when really they are naturally acclimating. While scientists are well aware of such ‘natural’ bleaching, it has not been communicated well to the wider world.

On top of this, the media’s love of tales of catastrophe and disaster mean that many members of the public are under the impression that coral bleaching is always a sign of impending doom – more routine cases of natural bleaching don’t tend to get reported.

A coral isn’t a single organism; it’s a composite of two. The stony structures many people think of as coral are made by tiny host animals, which live in cooperation with communities of algae. These algae are known as zooxanthellae, and corals’ calcium carbonate skeletons are beautifully adapted to channel sunlight to them. They use this light to photosynthesise, making food for their animal hosts. The pigment in the zooxanthellae gives corals their colour.

Bleaching happens in response to environmental stress, when the coral hosts’ tissue is lost from the stony skeleton or in extreme cases when the zooxanthellae are damaged or expelled.

Reports of coral discolouration have grown more common in recent years, largely due to climate change. But even without sudden environmental shifts, this happens regularly as a natural part of how corals adapt to changing conditions over days, weeks and seasons.

This means corals that look unusually pallid during periods of high light or unusually warm water may be wrongly seen as at high risk, when in fact they are showing that they are well-adapted to cope with changing conditions by changing their internal zooxanthellae populations. Suggett argues that more scientific study is needed into how these corals adapt, as well as large-scale investigations into the complex interactions between environmental effects such as temperature, light levels and water quality.