Coral reefs are having their worst year ever, and widespread extinction looms, but it's not time to despair just yet. A vast and previously unseen reef has been discovered in the most unlikely of places, and it is surviving better than most.
Coral only grows in salt water, so river mouths are usually accompanied by breaks in barrier reefs, often the only place ships can cross. Moreover, rivers bring with them muddy water, which blocks the sunlight shallow water species need to grow. So the mouth of the Amazon, home to world's largest freshwater discharge, is almost the last place you would look for a coral reef. So no one did.
Yet Brazilian scientists found one there anyway. A paper in Science Advances reporting their findings could just as easily be titled “Coral Reefs, A New Hope.” It reportedly stretches for more than 960 kilometers (600 miles), and is between 30 and 120 meters deep (100 and 400 feet), but amazingly had not been discovered until now.
The Amazon pumps out so much water that it freshens surface waters far out from land. At greater depths, however, deep currents keep the water salty enough to support the formation of the calcium carbonate body structures that make up coral reefs.
If it wasn't for all that fresh water, the mouth of the Amazon would be a perfect place for coral, with a huge continental shelf covered in water shallow enough for reefs to thrive, which they have been doing since sea levels rose after the Ice Age. “At present, the high sediment load from the river settles relatively quickly in the inner and mid shelves,” the paper reports, preventing coral growth close to shore, but making the outer shelf much more inviting. Muddy waters interfere with growth for several months a year, but clear waters from August to January allow corals to flourish.
The map shows the location of reefs and the seasonal extent of muddy plumes that prevent growth. The schematics show the water column depending on the extent of the sediment plume. Moura et al/Science Advances
A report of coral reef fish in the area dates to 1977 but it wasn't until last year that the existence of a reef system in the area was reported. The latest paper estimates the reef is 9,500 square kilometers (3,700 square miles) in size.
Such a vast system will take years to survey, but the authors made a start on the upper slope. They report an abundance of “large sponges and other filter feeders,” occupying patches up to 300 meters (1,000 feet) long and up to 30 meters (100 feet) high at the northern end, while the south is richer in corals.
The Amazon reef is less biologically diverse than other tropical reefs, which house a quarter of the world's marine species. Nevertheless, the authors report finding 73 species of reef fish and 40 corals, some previously unknown in Brazilian waters. Among the many sponge specimens 29 have yet to be recognized, and may represent new species
The reefs are vulnerable to changes in sea level, ocean chemistry, and temperature, but structure dating indicates the southern corals are still growing, demonstrating an adaptive capacity that may prove applicable to corals elsewhere.
Disturbingly, however, oil production has started nearby. Resilient as these reefs are, they're unlikely to survive a major spill.
Rodrigo Moura of the Federal University of Rio de Janeiro with one of the corals dredged from the mouth of the Amazon where none were expected to live. Federal University of Rio de Janeiro
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