When most people think of coral reefs, they think of sunlit shallow shelves, teeming with sea creatures and iridescent tropical fish that almost anyone with a snorkel and a swimsuit can see.

But much deeper in the ocean, 100 feet down and below, exists another type of coral reef. These deepwater reefs, many of which are unmapped and unexplored, need protection, several marine scientists wrote in a commentary published yesterday in the journal Nature Climate Change.

Such reefs are often called mesophotic, which translates to middle light, because they are located deeper in the layer of the ocean that still receives sunlight.

"You've got an entire ecosystem that is largely unprotected in the mesophotic coral reefs around the world," said John Guinotte, a marine biogeographer with the Marine Conservation Institute and a co-author on the commentary.

Mesophotic reefs are important because they can provide a crucial refuge for many species that are shallow reef dwellers. Shallow reef habitats are in decline in part due to factors like coral bleaching, which happens when the oceans get too warm for the corals, and ocean acidification, which is caused by the oceans taking up excess carbon dioxide released by humans into the atmosphere.

Deeper reefs, because they are in cooler waters, are less susceptible to bleaching events. They are also less vulnerable to damage from violent storms, Guinotte said.

Shallow reefs are also increasingly beset by local pressures like coastal development, overfishing and pollution; a World Resources Institute survey concluded that more than 60 percent of the world's reefs face immediate threats.

The Great Barrier Reef's World Heritage status is now under review due to a variety of risks it faces, including development of major coal ports on the Australian coast (ClimateWire, April 30).

An overlooked resource
For a long time, scientists overlooked the deeper-water reefs, from 100 to 500 feet deep, because they are difficult to explore, Guinotte said.

"The reason they've largely been ignored in the science community is that scientists aren't able to scuba dive on them because they are deeper than 30 meters," he said.

But technological advances like the availability of relatively inexpensive remotely operated vehicles have opened up these reefs to science, and some of the research shows they are connected in important ways to the shallower reef systems.

Work in Western Australia, for example, has shown that when shallow reefs experience bleaching, some of the coral larvae that recolonize the damaged shallow reef come up from the deep reefs.

Rafe Sagarin, a marine ecologist at the University of Arizona's Institute of the Environment who was familiar with the paper, said a key issue in the paper was that the authors highlighted deeper reefs' ability to act as an ecological refuge for shallower coral reefs.

Unmapped refuges need protection
"What is new is thinking of [mesophotic corals] as an adaptational strategy, not just to climate change risk but to the whole suite of threats that corals are facing now," Sagarin said.

The paper's authors call for an increased effort to map mesophotic reefs, as many of them have not even been cataloged.

One way to cheaply do this is to use a predictive habitat model to determine where deeper reefs are likely to be, said Guinotte of the Marine Conservation Institute, and then later verify areas the model predicts as highly suitable for such reefs.

Countries could potentially protect such deeper reefs by expanding their existing marine protected areas, as Israel did in the Red Sea after mesophotic reefs were discovered there.

The fact that deeper reefs are now easier to access due to cheaper technology also means they can be found more easily by fishermen and others who want to use them as a resource, Sagarin said.

This just highlights the need to initiate collaborative conservation efforts, he added.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500