Coral reefs are in desperate danger, suffering losses on an unprecedented scale. A new report suggests hope may lie in deeper waters, with reefs on the shoulders of continental shelves providing places of refuge for species that might otherwise become extinct.
The beauty and astonishing diversity of barrier and fringing reefs grab our attention. At greater depths, there are the lesser known, slow-growing species known as “black corals." In between these, however, are what as known as “mesophotic reefs,” which are even more poorly understood.
In the last 10-15 years, the appearance of underwater robots and small submersibles has allowed researchers to start to address this ignorance, and now a report commissioned by the United Nations has brought what we have learned together and highlighted how mesophotic reefs could help threatened shallow reefs.
The exact definition of mesophotic reefs is vague, but they are usually taken as being in water more than 30 to 40 meters (100 to 130 feet) deep. Black corals can live with minimal sunlight, but the species that inhabit mesophotic reefs cannot. However, where surface waters are clear, mesophotic reefs have been found at depths of up to 150 meters (500 feet), often perched precariously near the tops of the slopes that join continental shelves with the deep ocean floor.
Many mesophotic corals survive holding on to steep slopes. Sonia J. Rowley.
Dr. Jody Webster of the University of Sydney told IFLScience that “until recently, it was thought that the species that inhabited these reefs were very different from those in shallow reefs, but in the last 10 years we have been finding many familiar species at increasingly deep locations.” Webster is not an author of the report, but his work leading voyages that used sonar to locate mesophotic reefs off the Great Barrier Reef was one of the things that made possible the discoveries reported there.
Mesophotic reefs are just as vulnerable to ocean acidification as their shallower counterparts, Webster said, but live in far more stable temperatures. Consequently, they are protected against the high temperature-induced bleaching that is pushing so many of the world's reefs over the edge this year. Since most are far from shore, they are often less affected by overfishing or pollution than their shallower equivalents.
The deeper the water corals inhabit, the more restricted the light they receive is to short wavelengths. GRID Arednal
Therefore, the hope expressed in the report is that species that may become extinct on shallow reefs could survive in deeper waters, eventually recolonizing shallow reefs when circumstances improve. “Mesophotic coral ecosystems are a seed bank for some organisms,” said Professor Elaine Baker, one of the authors of the report, in a statement. "They aren't a silver bullet but they may be able to resist the most immediate impacts of climate change.”
In order for mesophotic reefs to fulfill this function, they need to be protected from other threats such as bottom trawling. As Webster told IFLScience: “Many of these reefs were unknown when marine park boundaries were created and have been left unprotected.” Many probably remain undiscovered.
The report also contains information about the diversity of mesophotic reefs and how depth affects the population structures.
Mesophotic reefs have been found at many different depths, and there may be more still to come. Anhildra-Brown et al