24/05/2016 5:38 PM AEST | Updated 15/07/2016 12:53 PM AEST

Mesophotic Coral Reefs Could Help Marine Life Survive Bleaching

They are one of the few remaining ecosystems on earth to remain largely unexplored.

Coral on the Great Barrier Reef suffering from bleaching.

The most devastating coral bleaching event the world has ever seen is currently underway on the Great Barrier Reef, while severe bleaching has also been observed in the central Indian Ocean, Maldives, Sri Lanka and the Lakshadweep islands of India.

However, according to a new report commissioned by the United Nations, mesophotic coral ecosystems (MCEs) may be able to offer hope in managing the impact of bleaching.

Shallow coral reefs -- the ones that we can swim, snorkel and dive near -- are only the tip of the iceberg when it comes to the ocean's extensive coral ecosystem. MCEs are intermediate depth reefs which start at a depth of around 40 metres and continue down to around 150 metres.

Among these deeper reefs, the incidence and severity of coral bleaching caused by global warming is reduced because temperatures and ambient light levels are lower than in shallow water.

Humans have left an indelible mark on the marine environment that has led to almost 20 percent of coral reefs disappearing.

MCEs are not only found at greater depths, but they are also located farther from land, meaning that they are often shielded from chronic pollution, sedimentation and overfishing, which are more prominent inshore.

In addition to this, storms and outbreaks of coral disease -- two significant causes of coral loss -- are also less prevalent where MCEs are found.

The report questions whether these largely unexplored ecosystems can provide a refuge for the species under threat from bleaching in shallower reef ecosystems and whether they can provide the stock to re-populate shallow reefs if they continue to decline.

"This report highlights that there is a whole other deep water reef system that has essentially been out of sight and out of mind," Associate Professor Jody Webster from the University of Sydney told The Huffington Post Australia.

"It is a unique, interesting diverse ecosystem in its own right which may have great importance in terms of understanding the connection between deep and shallow water systems."

Credit NOAA’s Hawai'i Undersea Research Laboratory

A Mesophotic Coral Ecosystem in the 'Au'au Channel, offshore of Maui, Hawaii at a depth of 70 m.

Professor Elaine Baker, the University of Sydney's UNESCO Chair in Marine Science, co-authored the report and has said that while MCEs may not be the solution to coral bleaching, they may be able to help re-seed damaged or destroyed surface reefs and fish populations.

"Mesophotic coral ecosystems are a seed bank for some organisms," she said.

"More research needs to be done to firmly establish the role of MCEs in preserving our reefs; they aren't a silver bullet but they may be able to resist most immediate impacts of climate change -- thereby providing a refuge for some species and potentially helping to replenish destroyed surface reef and fish populations.

The fact that we don't know where many of these reefs are means we could be wrecking them already

"It may be that the cooler, deeper water in MCEs could be more hospitable to many species than the warmer surface water."

The unexpectedly large extent of MCEs around the world has emerged within the last five years due to the advancement and use of new technologies such as autonomous underwater vehicles and multi-beam sonar. However, there are still many that are yet to be discovered, according to report editor Peter Harris.

"We need to know more about mesophotic reefs, and we need proper ways to manage the seas in which they're located," Harris said.

"The fact that we don't know where many of these reefs are means we could be wrecking them already -- from pollution, poor fishing practices or other activities. For that reason, mesophotic reefs should be included in management and conservation plans."

Ho New / Reuters
Marine activist Suzanne Kavanagh swims above coral suffering from bleaching on Australia's Great Barrier Reef.

Among locating where mesophotic reefs exist, the report also recommended that awareness be raised amongst policy makers to engage in measures to protect them.

Another key recommendation was to increase an understanding of how these ecosystems are connected to shallow reefs in order to fully ascertain the extent to which they can be used as a refuge for, or to reseed, shallow reefs affected by coral bleaching.

According to UNEP executive director Achim Steiner, there's no time to waste.

"Humans have left an indelible mark on the marine environment that has led to almost 20 percent of coral reefs disappearing," he said.

"But coral reefs are an invaluable natural asset we can't afford to lose."

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