New Discovery: A Massive Reef System at the Mouth of the Amazon River

Could coral reefs have anything to do with the Amazon River? Apparently so. In case you missed this new finding among the sea of reports about the impending demise of coral reefs (especially the Great Barrier Reef) across the world, here’s the lowdown: A team of scientists from Brazil and the United States have discovered a 600-mile long sponge and coral reef at the mouth of the Amazon River, where it empties into the Atlantic Ocean.

The reef stretches across more than 3,600 square miles in ocean floor off of the South American continental shelf, between the French Guiana-Brazil border and the Maranhão State in Brazil, according to the researchers whose findings were published in the journal Science Advances on Friday.

Here’s why this is unexpected and amazing: First, coral reefs are usually found in clear, briny water, off the continental shelf in tropical areas — waters where sunlight can penetrate. Yet the water where the Amazon meets the Atlantic is anything but clear. In fact, the Amazon plume — the area where freshwater from the river mixes with the salty ocean — is full of sediment and pollutants and is among the muddiest plume areas in the world. Then there’s the fact that plume areas are places where there usually tend to be gaps in the reef distribution along the tropical shelves.

Yet the researchers who discovered this novel reef system say that while it is “impoverished in terms of biodiversity,” it is pretty extensive. The finding has marine scientists revising their idea of how and where reefs can exist. “We found a reef where the textbooks said there shouldn’t be one,” study co-author Fabiano Thompson of the Federal University of Rio de Janeiro told National Geographic.

Scientists have been hunting for this deepwater reef system since the 1970s after some researchers caught reef fish at the mouth of the Amazon. But until now, no one knew for sure if it existed. The study’s lead author, Rodrigo Moura of the Federal University of Rio de Janeiro, uncovered the reef system in 2012 when he dredged some areas near the plume, some 75 miles offshore, based on the old report from 1977.

The Brazilian researchers then organized a full team and took a Brazilian Navy research vessel back to the site in 2014, when they were able to collect and fully describe the findings for the study. So far the team has located some 61 species of sponges, 73 mostly carnivorous reef fish species, a “low-diversity assemblage” of seaweeds (34 species), 26 species of octocorals or soft corals, and an assortment of sea stars. “The novel system presented here adds to the repertoire of ‘marginal’ reef types shaped by conditions deviating from those of the archetypal tropical coral reefs,” the researchers wrote.The difficulty of finding the old map coordinates with modern GPS notwithstanding, the team used multibeam acoustic sampling of the ocean bottom to find the reef and then dredged up samples to confirm the discovery. “We brought up the most amazing and colorful animals I had ever seen on an expedition,” said Patricia Yager, an associate professor of marine sciences in UGA’s Franklin College of Arts and Sciences and principal investigator of the River-Ocean Continuum of the Amazon project.

“In the far south, it gets more light exposure, so many of the animals are more typical reef corals and things that photosynthesize 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,” Yager said in a statement.

But as is the case with most new species or ecosystems discovered these days: this reef is already under threat. Brazil has sold 80 exploratory blocks for oil drilling in the reef region to domestic and transnational companies and 20 of these are being tapped for oil.

“These blocks will soon be producing oil in close proximity to the reefs, but the environmental baseline compiled by the companies and the Brazilian government is still incipient and largely based on sparse museum specimens. Such large-scale industrial activities present a major environmental challenge,” the study’s authors warn.