An 'Impossible' Coral Reef System Discovered at Amazon River Mouth

Coral reef
Researchers dredged a region of the mouth of the Amazon, discovering a massive coral reef. Rodrigo Moura

Sometimes science is a treasure hunt, and the only clues you have to guide you are a hand-drawn map and a brief, 40-year-old scientific paper that suggests something amazing might be found somewhere under the ocean. And then you go look, and you actually find that something amazing.

A recent paper published in the journal Science Advances describes the discovery of a 600-mile-long (966-kilometer-long) coral reef system in the Atlantic Ocean, under the muddy plume of the Amazon River. It was overlooked for so long because corals reefs are generally known for preferring shallow, clear water, and conditions at the mouth of the Amazon have more commonly been described as "turbid" or even "goopy."


"You don't really look for things unless you think it's possible they could be there," says oceanographer Patricia Yager of the University of Georgia, one of the principal investigators on the study.

In 2012, Yager and her research team received funding to go to the mouth of the Amazon to study how the nutrient-rich plume of the world's largest river affects the ocean. In order to study the plume, she had to get permission from the Brazilian Navy, who required that she bring on several Brazilian scientists as collaborators. One of these was a reef ecologist named Rodrigo Moura, from the Federal University of Rio de Janeiro.

"At the planning meeting for this cruise we were going to take around the mouth of the Amazon, I sat next to Rodrigo," says Yager. "When he told me what he did, I was a little confused. 'You're a reef ecologist? What are you going to do on this cruise?' It's really muddy out there, you know.' He smiled and handed me a paper from 1977. It's only about four pages long with a hand-drawn map. It said they found reef fish and sponges out there. The dot on the map where this stuff was found turned out to be about 20 miles across when I put it into Google Maps, but we decided to just take a look around."

Moura spent the cruise reading the conditions of the sea floor until he found a spot hard enough to support a coral reef. There, the team put a dredge down in the water and pulled up a sample of what was underneath the boat.

"When they pulled up the dredge and brought it on board, I was flabbergasted," says Yager. "It wasn't anything you'd imagine was down there. We pulled up some of the most amazing things I've ever seen on an expedition: coral, colorful sponges, fish, brittle stars."

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Scientists examine specimens extracted from the ocean floor after dredging at the mouth of the Amazon.
Yager : Landrum : Oikimura

As it turns out, a lot of the reason this amazing reef system was never discovered has to do with what's in oceanography textbooks. Because coral needs light to grow, conventional wisdom says that a big, muddy tropical river like the Amazon or the Congo in Africa blocks so much light a coral reef would be prohibited from growing.

While it's true that coral reefs need clear water, the plume pouring into the Atlantic from the Amazon is freshwater, and therefore more buoyant than the salty ocean water. Think of a bartender adding a bit of cream to float atop a cocktail, or the way oil and water separate — the fresh, muddy water floats on the surface.

A plume of muddy Amazonian freshwater floats atop salty ocean water, occluding the coral reef below.
Lance Willis

So though the plume sometimes blocks light from reaching the reefs, the muddy section is only about 10 meters deep. The reefs sit between 160 to 295 feet (50 and 90 meters) below the surface in very salty, tropical — but clear! — water.

Due to the movement of the Amazon's plume during the year, the southern end of this reef system gets enough sunlight for corals to grow. The northern part of the reef is obscured by the plume almost year-round, which prohibits coral growth, but sponges, barnacles, and other filter feeders have attached themselves to an old coral substrate that grew 15,000 years ago. These animals and others use these reefs as protection and a place to feed. Some large, open-water fish even use it as a nursery ground for their young.

"The final piece of the puzzle has to do with human impact," says Yager. "We need to do a better job of describing what reefs are there before continuing with oil exploration and heavy-duty fishing, which is already happening."

Read this as an instance of science getting it wrong, or read this as an instance of science getting it just right by science being science: constantly searching for new knowledge, and readjusting when new revelations are discovered.