Coral reef discovered at mouth of Amazon

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The Amazon deposits three million tonnes of sediment every day into the Atlantic Ocean. This outpouring of water and sediment is so vast that the salt content and colour of the Atlantic Ocean are altered for a distance of several hundred kilometres from the mouth of the river. – ESA

Scientists have found a huge coral reef off the mouth of the Amazon River. The discovery was a great surprise as usually there are usually gaps in reefs at the mouths of rivers as coral cannot live in the outflow of fresh water.

What’s more, coral reefs typically like pristine clear conditions and the waters of the Amazon delta are muddy with the silt deposited by the river in the Atlantic Ocean. 

The reef, which ranges from 30 to 120 metres deep, is 1,000 kilometres long, covering an area of 9,300 square kilometres – about the size of the island of Cyprus – stretching from French Guiana to Brazil’s Maranhão state.

Scientists, who wrote about the discovery in Science Advances,  say that the reef survives beneath the plume of fresh water.

Looking for the reef wasn’t even their first priority.

“Our expedition into the Brazil Exclusive Economic Zone was primarily focused on sampling the mouth of the Amazon,” said Patricia Yager from the University of Georgia and principal investigator of the project.

But a colleague had an article from the 1970s that mentioned catching reef fish along the continental shelf and said he wanted to try to locate these reefs.

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An image, derived from European Space Agency data, shows the plume of fresh Amazon River water as it enters the Atlantic Ocean. – I. Corbella, UPC / Google Earth

The team used multibeam acoustic sampling of the ocean bottom to find the reef and then dredged samples to confirm the discovery.

“We brought up the most amazing and colourful animals I had ever seen on an expedition,” Yager said.

While describing the reef as “impoverished” compared to others around the  world, they still found more than 60 species of sponges, 73 species of fish and spiny lobsters.

Life on the reef system varies along its length from the dark waters beneath the river plume to the lighter areas to the south.

“The paper is not just about the reef itself, but about how the reef community changes as you travel north along the shelf break, in response to how much light it gets seasonally by the movement of the plume,” said Yager.

“In the far south, it gets more light exposure, so many of the animals are more typical reef corals and things that photosynthesise for food. 

“But as you move north, many of those become less abundant, and the reef transitions to sponges and other reef builders that are likely growing on the food that the river plume delivers. So the two systems are intricately linked.”

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An illustration from the paper shows the position of the reef and how it is affected by the freshwater plume of the outflowing Amazon.

But the reefs may already be threatened.

“From ocean acidification and ocean warming to plans for offshore oil exploration right on top of these new discoveries, the whole system is at risk from human impacts,” Yager said.

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