The mouth of the Amazon river isn’t the first place you’d look for a coral reef.
After winding thousands of miles, the river has the color and opacity of chocolate milk as it reaches the coast, where it disgorges four times more freshwater than the next largest river – the Congo – along with a daily load of 1.2 million metric tons of sediment. The plume is visible from space, and its effect on the the salinity and acidity of the ocean can be observed more than 160km (100 miles) from shore.
New research published in the journal Frontiers in Marine Science by scientists from several Brazil universities and the environmental group Greenpeace found the Amazon mouth area harbors a deepwater reef that covers as much as 56,000 square km (22,000 square miles) – six times larger than previous surveys suggested – and may extend to depths of 220m (more than 700ft).
In the past three years, the Great Amazon Reef System has been truly explored for the first time. Although multiple media reports have portrayed these recent surveys as a “discovery” of the reef system, its existence had been detected as early as 1975. What is becoming better understood, however, is the Amazon reef’s actual size and its role as an ecological link between the Caribbean and South American coral ecosystems. Now, as offshore oil development threatens the Great Amazon Reef, Greenpeace is using new estimates of its size and importance to try and block drilling plans that have been in the works since 2013.
Greenpeace says it’s important to protect these kinds of mesophotic reefs, which haven’t yet experienced the same rising ocean temperatures that have caused shallower reefs to bleach and die around the world. Scientists have theorized that deep reefs may act as refuges, or gene banks, for some imperiled coral species. While disasters like the Deepwater Horizon spill in the Gulf of Mexico in 2010 illustrate the damages that offshore oil drilling can have on deep coral reefs, other impacts are invisible. Many offshore drilling operations experience smaller-scale leaks, which can take a toll on coral larvae.
In 2017 and 2018, Greenpeace partnered with Brazilian scientists to conduct the first video surveys of the Great Amazon Reef System. Their efforts built off a study published in 2016 in the journal Science Advances. That survey, conducted by sampling sites off the coast of the Brazilian states of Amapa and Para, described a deep and diverse habitat hidden in a wedge of saltwater beneath the brackish plume of the Amazon river outflow, covering 9,300 square km.
The new surveys aimed to verify the findings using submersibles and remotely operated vehicles launched from one of Greenpeace’s research vessels, a repurposed Russian ship called Esperanza.
In the murk below the ocean’s top 30m of water lies the mesophotic (“half-light”) zone. There, 30–150m below the surface, reefs are sparser and stranger than their sunnier analogues. Without enough light for hard corals to grow their stony skeletons, other organisms dominate. In the images taken on the expedition, colorful mounds rise from the seafloor, spackled with bright coralline algae, big-bellied sponges and delicate soft corals.
“They don’t make the same formations as you see in the Great Barrier Reef, but they are very important nonetheless,” said Helena Spiritus, a campaigner for Greenpeace in Brazil who is working with the researchers on the ongoing surveys.
The Greenpeace-backed survey found that the reef was much larger than scientists thought in 2016, partly due to the more limited sampling of the prior study and the fact that the reef extends to greater depths than anticipated. The ecosystem was also more varied, according to the study. This was the first time that some species the scientists identified – including a kind of damselfish called a blue chromis – had been found outside of the Caribbean. The research supports another hypothesis: The Amazon reef system acts as a biological corridor between the Caribbean and Atlantic ecosystems.
While the surveys have produced interesting results, Luiz Rocha, a marine biologist and associate curator at the California Academy of Science who studies Brazil’s deepwater reefs, says the existence of a large reef in the area near the Amazon river outflow has been known for decades. Evidence of it has been available at least since 1975 when a Smithsonian Institution scientist named Bruce Collette took samples at 57 sites along the Brazilian coast, eventually identifying 43 species of Caribbean reef fish and 35 species of sponges. In a 1977 paper, Collette concluded “hard bottoms” – reefs – “are abundant in this region.”
In fact, Rocha said, researchers now believe the Amazon reef area connects a deepwater reef system that extends along much of the coastline of South America.
Rocha, who was not affiliated with the most recent studies, has studied some of Brazil’s deep reefs himself using specialized scuba technology to explore reefs as deep as 45m. Like Collette in the 1970s, Rocha collected many species of fish off the Brazilian coast previously thought to belong to Caribbean ecosystems, leading him to conclude in a 2003 paper that “larval exchange between Brazil and the Caribbean is small but constant.” Where once a break in this chain of connection was theorized to exist at the gaping mouth of the Amazon river, the cumulative work since 1975 reveals a living corridor stretching from the Caribbean along the Atlantic coast of South America, he said.
Surveying the reef recently and labeling it a new discovery, Rocha said, “is the equivalent to me going to the Mariana trench today, getting the most detailed 3D scan of it and saying that I discovered the Mariana Trench.”
Based on its surveys, Greenpeace found the reef overlaps the perimeter of the area where the French oil company Total is planning to develop oil wells, 120km off the Brazilian coast. Because Total’s environmental assessment failed to turn up evidence of the reef so close by, however, Greenpeace in April called for the project to be canceled by the company or the Brazilian government, which is still reviewing the license.
“If they didn’t find any reefs there … they either didn’t do [the assessment] or they did a very poor job,” Spiritus said.
Total and Brazil’s environmental agency IBAMA, which has been reviewing the project, did not respond to several requests for comment.
Recently, Brazil has been auctioning more of its coastal waters to oil companies, and other companies are also interested in developing oil in the same area around the Amazon’s mouth, which is estimated to hold vast fossil fuel reserves.
Rocha said that even if the Amazon reef area is eventually protected, oil companies are just likely to look elsewhere.
“The best thing to do is moving to alternative fuels,” he said, pointing out that any drilling along Brazil’s coast could have serious impacts on mesophotic reefs. But that is the challenge of protecting ecosystems that exist so deep, whose functions have been so little studied, he says: “Out of sight, out of mind, right?”