There Is a Giant Reef Under the Amazon’s Muddy Waters
Oceanographer Patricia Yager has been studying the Amazon River plume—where nearly one-fifth of the world’s river-water discharge gushes into the Atlantic Ocean—for the past 15 years.
But even with her level of expertise, she was shocked by the discovery of a 600-mile-long reef below the murky waters off Brazil’s northern coastline, stretching from the French Guiana border to Brazil’s Maranhão state.
More than 73 species of fish and 60 species of sponges, as well as spiny lobsters, sea stars, and invertebrate species not yet identified were discovered living along the reef, according to a study published Friday in the journal Science Advances.
“Astounded is really the word,” said Yager, an associate professor of marine sciences at the University of Georgia and coauthor of the study. “It’s so muddy there, you just assume that it all ends up resting on the seafloor. There’s no way coral could survive down there, but it’s there.”
At the right time of year, the plume from the Amazon can stretch 1,000 miles offshore, pouring up to 11 million cubic feet of freshwater into the ocean every second. Huge amounts of sediment accompany that water flow and give the plume its telltale brown tinge. Inside the plume live millions of microorganisms that can break down carbon dioxide and create a carbon sink in the ocean.
Backed by the National Sciences Foundation, Yager in 2012 embarked on the research vessel Atlantis on a multi-day expedition to the river mouth to study how these tiny organisms might affect climate change on a global level.
Once aboard, she met Rodrigo Moura, a coral reef specialist and a professor at Rio de Janeiro’s Federal University.
“I asked, ‘Aren’t you a reef guy? What are you going to do out here? You know how muddy it is, right?’ ” Yager said.
Coral reefs as we think of them typically flourish in crystal-clear, sediment-free environments with ample sunlight to allow for photosynthesis. But Moura knew what he was looking for. Since the 1950s, fishers had been nabbing lobsters and reeling in reef fish at specific locations along the plume.
“It was scattered information,” Moura said. “Information on an abundant sponge fauna, and reef-associated species off the Amazon, and that ‘suspicious’ fisheries landing data. There should be a big reef somewhere out there.”
He also had a chart, which looked more like a treasure map than research coordinates. It came from a federal study conducted in 1977 that mapped a few locations where sponges and reef fishes had been identified.
“It looked hand-drawn, with x’s and o’s on it,” Yager said. “But I told him, ‘What the heck—let’s go check out the spots.’ ”
So on the way back to open ocean from the river mouth, the vessel stopped at a couple spots marked on Moura’s map and used multibeam acoustic sampling to map out the ocean floor. He identified several structure-rich spots and dropped a dredge to bring up samples.
“What came up was unbelievable,” Yager said. “Brilliant colors, brittle stars, bright colored fish, beautiful sponges—all surviving in this murky water.”
The findings prompted the team to come back in 2014 and conduct the largest survey ever done in the region, but it only covered about 10 percent of the estimated 3,700-square-mile environment.
The reef can exist, says Moura, because the freshwater and much of the accompanying sediment plume pouring out of the river is less dense and floats as a sort of veneer over the saltwater below.
But the plume blocks sunlight from reaching the reef below, which affects what animals and corals survive there. Wind and ocean currents typically push the plume northward, meaning little light penetrates to the hard bottom below, where only a few sponges could survive. Still, a number of species appear to have adapted to the darker environment—including 29 specimens collected but not yet identified.
But south of the river mouth, the plume’s extent is seasonal, allowing light to penetrate to the ocean floor 100 to 300 feet down.
“Southwards, there is an increasing presence of resistant reef builders that are still mineralizing calcium carbonate, including coralline algae and a few corals,” Moura said.
Mark Eakin, a coral reef watch coordinator with the National Oceanic and Atmospheric Association, who is not affiliated with the study, said the findings extend the range where coral community–based organisms might survive.
“Don’t get it wrong—these reefs they are describing aren’t the vibrant reef ecosystems we see in the Caribbean. But it’s still more than I would have expected for that environment,” Eakin said.
The 600-mile-long structure south of the Amazon River mouth has existed for at least the past 4,000 years, the authors found, and it could be an important connection point for reef-dwelling species that migrate from the coral reefs of the Caribbean in the north to reefs along Brazil’s southern coast.
“The structures function as biogeographic stepping-stones,” Moura said. “Species that are able to dwell under the freshwater and sediment plume may cross back and forth between the Caribbean and Brazil.”
But before the reef’s extent was fully mapped, it was already in danger. More than 80 exploratory blocks have been acquired for oil drilling along the shallow continental shelf, 20 of which are producing petroleum, the study’s authors wrote.
Moura is hopeful the findings will push the oil companies and Brazil’s Federal Petroleum Agency to complete more accurate assessments of the reefs in the region to avoid destroying sensitive habitat.
“Even our shallow continental platforms still need major mapping efforts,” Moura said. “We have better maps of the moon and Mars than maps of the ocean floor.”
Yager said the next move could be to designate the region as a marine-protected area.
“Fishermen have been exploiting these waters for decades, and we don’t know what impacts those have had on the long-term health of the reef either,” Yager said. “We need to get the news out there that these reefs exist and figure out what we need to do to keep them healthy.”