Hakai Magazine

Coastal science and societies

Amazon River Delta
Near the mouth of the Amazon River, deep beneath the sea, lies a vast reef of corals, sponges, and rocky rhodoliths. Photo by Universal Images Group North America LLC/Alamy Stock Photo

The Amazon Reef Is Alive, Growing, and Under Threat—Again

A few years ago, scientists weren’t even sure the reef existed.

Authored by

by Richard Kemeny

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The mouth of the Amazon River, which yawns out of Brazil’s rainforested north, must be a stressful place to call home. Each year, six trillion cubic meters of water—roughly enough to fill the Grand Canyon one and a half times—surge from the river into the Atlantic Ocean. It brings 1.1 billion tonnes of sediment with it—equivalent to the weight of 1.5 million Christ the Redeemer statues. This plume blocks light, lowers the levels of oxygen and salt, increases the acidity of the water, and makes everything somewhat gritty. Yet in 2016, researchers confirmed what had been suspected by scientists since the late 1970s: below this plume lies a reef. Further research has shown the long-lost Amazon Reef to be vast and ecologically critical. Yet it is now under threat from oil exploration—and not for the first time.

The Amazon Reef is thought to cover an area roughly the size of Nova Scotia, stretching from the fringes of French Guiana to the Brazilian state of Maranhão. Much of it is in darkness, with the lowest areas lying more than 200 meters below the surface in the twilight zone. The Amazon Reef is a patchwork of sponges, corals, vast walls of crusted coralline algae, and rhodoliths—rock-like nodules of red algae that cluster into giant beds and are increasingly recognized as wellsprings of biodiversity and as carbonate deposits with an important role in the global carbon cycle.

The deep, dark reef is different from tropical coral reefs, where the resident algae, zooxanthellae, need abundant sunlight to survive.

The Amazon Reef sustains large populations of fish and lobsters, which many artisanal fishermen from nearby coastal communities and larger marine animals depend on. Underneath the reef, in the Foz do Amazonas Basin, however, lies an estimated 15.6 billion barrels of oil.

In 2018, some scientists—supported by politicians and oil industry professionals—began publicly questioning whether the reef even exists, and if so, whether it is still alive. They argued that the reef was a dead relict, and it would be impossible for anything to grow under such punishing conditions—particularly in the northwest where much of the sediment plume flows.

New research, conducted by a team of Brazilian scientists, now suggests otherwise. The team, led by Michel Michaelovitch de Mahiques, an oceanographer from the University of São Paulo, used radiocarbon dating on samples from throughout the reef to reconstruct its evolutionary development.

“We needed to convince deniers that the reef is full of living organisms,” says Mahiques.

According to their results, the reef began to develop at the end of the Pleistocene, between 14,700 and 12,100 years ago, at around 120 meters below the current sea level. At the end of the last ice age, the ocean rose dramatically and the river’s sediment plume became particularly prominent. The reef paused its growth for around 5,000 years. Around 7,100 years ago, however, it began to grow again, first in shallower areas at the southern end of the reef. This, the authors argue, means the reef is still building. Importantly, they found relatively young rhodoliths and sponges from northern, central, and southern sections of the reef, meaning that it is very much still alive and growing.

“The reef is a corridor for fauna from the Caribbean to the Brazilian margins,” says Mahiques, adding that the system houses hundreds of species.

The research shows that “the presence of sediments and darker waters does not necessarily represent a limitation for reef growth,” says Carla Elliff, an oceanographer at the Federal University of Bahia in Brazil who was not involved in the study.

Nevertheless, oil exploration threatens the reef. Last year, amid strong public pressure, Brazil’s environmental agency denied the oil company Total the environmental license needed to start drilling in the Foz do Amazonas Basin, citing concerns about the irreversible impacts of an oil spill. But now, BP is in the final stages of securing just such a license, with political winds more in its favor. Brazil’s new government, Thiago Almeida, a climate and energy campaigner at Greenpeace Brazil says, “has such an anti-environmental agenda that we are worried that it will give the license no matter the size of the threat.”

“Without knowing and understanding the full potential of this reef, we are missing out on valuing important ecosystem services that it could provide,” Elliff says.

“People must understand that the destruction of the habitat is very dangerous not only for the species that live there, but for humans also,” Mahiques says. “There is no plan B for the Earth, and if we destroy this planet, we will destroy ourselves.”