The Amazon Mouth Reef System (AMRS) is a recently discovered, non-conventional reef located off the coast of northern Brazil and French Guiana. Its existence challenged scientific expectations, as large river mouths were thought to prevent reef formation along tropical continental shelves. This extensive carbonate structure, estimated to be over 600 miles long and covering at least 3,600 square miles, was officially described in 2016. The discovery of this unique system provides new insights into how reefs survive under marginal conditions and establishes a potential biogeographic corridor connecting the marine life of the South Atlantic to the Caribbean.
The Physical Environment and Discovery
The Amazon Reef is situated on the South American continental shelf, extending from French Guiana to Brazil’s Maranhão state. It lies in waters 100 to 400 feet (30 to 120 meters) deep, placing it within the mesophotic zone. This location is profoundly influenced by the Amazon River’s plume, which discharges about 20% of the world’s total riverine flow into the Atlantic Ocean.
The massive freshwater outflow carries an enormous daily load of sediment, creating a turbid, muddy layer extending hundreds of miles out to sea. This sediment-rich plume blocks sunlight from reaching the deeper seafloor where the reef is located. The reef’s survival depends on a permanent, near-bottom wedge of denser, saltier ocean water that flows beneath the freshwater plume.
Initial evidence for the reef came from researchers in the 1970s who noted hard bottoms and reef-associated fish. However, the reef was not confirmed until oceanographic surveys in 2012 and 2014 used acoustic sampling and dredging to retrieve samples. The official publication of the discovery in 2016 revealed a complex hard-bottom mosaic hidden beneath the constant veil of Amazonian mud.
Defining Biological Characteristics
The Amazon Reef is biologically distinct from shallow, sunlit tropical reefs that typically rely on corals for structure. This system is characterized by extensive beds of rhodoliths, which are free-living nodules of calcareous red algae that form hard substrates. These rhodolith beds, along with large sponge communities, are the dominant structure-forming organisms in this low-light environment.
The ecosystem supports high diversity despite the marginal conditions, with researchers documenting over 60 species of sponges and more than 70 species of fish. The presence of enormous filter-feeding sponges reflects an adaptation to the high levels of particulate organic matter delivered by the river plume. Some organisms, including certain corals, utilize chemosynthesis, a process that produces energy without sunlight, allowing them to thrive in the dark depths.
The reef’s biology varies depending on the seasonal influence of the river plume. Northern sections are covered by muddy water for more than half the year and are dominated by sponges and carnivorous species. Southern sections receive more sunlight and exhibit a higher presence of colorful corals. Scientists suggest this novel system, which connects the Brazilian and Caribbean reef provinces, may represent a new type of biome.
Immediate Threats to the Ecosystem
The Amazon Reef faces immediate threats, primarily from industrial development in the Foz do Amazonas basin. The area is targeted for oil and gas exploration, with companies like the state-owned Petrobras seeking licenses to drill for estimated fossil fuel reserves. The primary danger is the possibility of an oil spill, which poses a severe risk to the delicate, filter-feeding organisms that make up the reef.
An oil spill would be devastating because strong currents could rapidly carry oil toward the coast, threatening the world’s largest continuous stretch of mangrove forests. A spill could also put endangered species at risk, including the Amazonian manatee and sea turtles that use the region as a migratory and feeding ground. The Brazilian environmental agency, IBAMA, has criticized impact studies submitted by oil companies for failing to adequately model how a leak might disperse.
A secondary threat is increased sedimentation resulting from deforestation within the Amazon basin. The massive influx of fine particles can smother reef structures, reducing the available hard substrate for organisms to attach to. Climate change also contributes to vulnerability through ocean acidification, which makes it harder for calcifying organisms like rhodoliths and corals to build their skeletons.
Efforts Toward Protection and Research
The discovery of the Amazon Reef sparked an international movement aimed at securing its protection. International non-governmental organizations (NGOs), such as Greenpeace, raised global awareness through expeditions documenting the reef’s biodiversity using submersibles. These campaigns mobilized public opposition to proposed drilling operations, leading to millions of signatures on petitions.
The scientific community advocates for the establishment of Marine Protected Areas (MPAs) to safeguard the ecosystem. Researchers emphasize the need for further exploration to gather baseline data on the reef’s species diversity before any industrial activity proceeds. The legal and governmental debate in Brazil surrounding drilling permits remains a central focus of protection efforts.
Brazil’s environmental regulator has repeatedly rejected drilling license applications due to insufficient environmental safety guarantees. This governmental pushback, combined with sustained pressure from activist groups, has stalled the immediate opening of the region to full-scale oil exploration. Global efforts to establish a Global Ocean Treaty also offer a pathway to protect sensitive areas like the Amazon Reef by creating international ocean sanctuaries.