Mystery of the coral reef 'halos': Strange rings of light-colored material can be seen from space

After a decades-long survey, researchers have unlocked some mysteries behind the ‘halos’ of sand surrounding coral reefs, and what they might mean for the embattled habitats’ health.

The so-called ‘halos,’ observable from satellite imagery, are rings of bare sand nestled between the algae and seaweed that surrounds coral reefs. 

Since the 1960’s theories have posited that the halos are created by hungry creatures that live in the reefs. 

To avoid exposure from predators in the open water, it was thought that sea urchins and fish munch on the greenery selectively using the algae as shield and carving out paths while remaining shrouded by the patches that remain. 

After a decades-long survey, researchers have unlocked some mysteries behind the ‘halos’ of sand surrounding coral reefs, and what they might mean for the embattled habitats’ health

While that theory was proposed 60 years ago, it wasn’t until now that researchers were able to document the phenomenon. 

Elizabeth Madin, assistant research professor at the Hawai’i Institute of Marine Biology and her team set up a series of cameras surrounding a reef by Heron Island in Australia. 

What they observed was precisely what the had been predicted — the theory, dubbed the ‘grazer hypothesis’ was right. 

Researchers saw specific characteristics of the theory for the first time. Some varieties of fish at the algae and seaweed from the interior, and others — much larger and less scared of predators — munched on the outside.

Even with those mysteries illuminated, however, others have cropped up. 

If the halos were created solely by grazing fish, then halos should be more common where fishing is restricted. 

They should also be smaller since fishing preserves large predators, meaning more creatures would be forced to stay inside their algae hiding spots eating the algae.

Neither turned out to be true, say a tandem of studies carried out by researchers, who plan to study the reefs further in hopes of figuring the puzzle out.

The so-called ‘halos,’ observable from satellite imagery, are rings of bare sand nestled between the algae and seaweed that surrounds coral reefs. Since the 1960’s theories have posited that the halos are created by hungry creatures that live in the reefs

In addition to illuminating a long held theory, Madin says that the research can also be applied to help judge the health of coral reefs.

Since the halos are at least partially contingent on a healthy predator and herbivore population, they can be used to extrapolate the population of denizens around reefs which are adversely affected by over fishing and pollution. 

For Madin, the discoveries, though exciting, may just be the beginning. 

‘This will pave the way for the development of a novel, technology-based solution to the challenge of monitoring large areas of coral reef and enable management of healthy reef ecosystems and sustainable fisheries,’ she said in a statement. 

WHAT IS CORAL BLEACHING?

Corals have a symbiotic relationship with a tiny marine algae called ‘zooxanthellae’ that live inside and nourish them. 

When sea surface temperatures rise, corals expel the colourful algae. The loss of the algae causes them to bleach and turn white. 

This bleached states can last for up to six weeks, and while corals can recover if the temperature drops and the algae return, severely bleached corals die, and become covered by algae. 

In either case, this makes it hard to distinguish between healthy corals and dead corals from satellite images.

This bleaching recently killed up to 80 per cent of corals in some areas of the Great Barrier Reef.

Bleaching events of this nature are happening worldwide four times more frequently than they used to. 

An aerial view of Australia’s Great Barrier Reef. The corals of the Great Barrier Reef have undergone two successive bleaching events, in 2016 and earlier this year, raising experts’ concerns about the capacity for reefs to survive under global-warming