Scientists discover reef concealed by Amazon River's plume

Considered one of Brazilian oceanographers' greatest discoveries, the reef stretches at least 900 km from the coast and is teeming with species such as sponges weighing more than 100 kg.

By Peter Moon  |  Agência FAPESP – Approximately 200 km from the mouth of the Amazon, a huge, extremely rich reef is hidden under the thick plume of sediment transported by the world’s most voluminous river.

Huge because to date it is known to stretch at least 900 km from the coast, between the Brazilian state of Maranhão and French Guiana. And extremely rich because it is brimming with endemic species, many of them unknown, such as giant sponges with diameters of up to 2 m and weighing more than 100 kg.

“Finding this reef was a major surprise,” said Michel Michaelovitch de Mahiques, a professor at the University of São Paulo’s Oceanography Institute (IO-USP) and one of the researchers involved in the description of the new marine habitat. “The most important feature is its improbable location. Reefs or reef-like structures have never been detected in river mouths. It’s a paradigm shift.”

The discovery was announced in the article “An extensive reef system at the Amazon River mouth” published by Science Advances, a journal of the American Association for the Advancement of Science.

The research was led by Carlos Eduardo de Rezende, affiliated with Northern Rio de Janeiro State University (UENF), and Fabiano Thompson from the Federal University of Rio de Janeiro. Scientists from several other Brazilian universities also took part.

The side scan sonars and echo sounder that detected the reef were acquired with FAPESP’s support during the project entitled “Increase in research capabilities in oceanography and related sciences in São Paulo State, Brazil”, which was part of the FAPESP Research Program on Global Climate Change (RPGCC). The project received additional support from CNPq, CAPES, FAPERJ and other institutions.

Thompson also highlighted the unexpectedness of the discovery. “The textbooks say there are no reef systems in the mouths of rivers like the Ganges, Orinoco or Amazon because of the conditions prevailing there, such as lack of light, for example,” he said. 

There is little light because of the dense plume of sediment and organic matter that is incessantly deposited in the ocean by these huge rivers, so that the sun’s rays are unable to penetrate very far down. Without sunlight, there can be no photosynthesis, which is the primary process underpinning the entire food chain in the coral reefs found in tropical waters.

Without photosynthesis, the amount of oxygen in suspension diminishes drastically because photosynthesis releases oxygen into the environment, be it aerial or aquatic. Hence, the idea that reefs could not exist in the mouths of tropical rivers that carry large amounts of sediment, known as muddy rivers.

According to the researchers, the idea remains valid in the case of coral reefs. These reefs depend on photosynthesis, and their structures are formed by the accumulation and deposition of the calcareous skeletons of dead coral polyps. However, coral reefs are not the only type of reef system. Many reefs are also formed by sponges and calcareous algae. This is precisely the origin of the great reef that is flourishing at depths of 60-120 m some 200 km offshore the mouth of the Amazon.

Maritime currents sweep northwards most of the 300,000 m³ of muddy water that pours into the Atlantic every second from the Amazon, and according to the scientists this is one reason why the newly discovered reef is not homogeneous.

Corals and giant sponges

The researchers have divided the newly discovered reef into three sectors. The Northern Sector, which extends from the Brazilian state of Amapá to French Guiana and beyond (the researchers believe it may well continue on as far as Suriname), has emerged beneath the permanent plume, which reaches depths of some 25 m. Below this plume, light levels are only at 2%. Photosynthesis is practically impossible under such conditions. Nevertheless, there are corals, giant sponges, dozens of fish species, and lobsters.

The Northern Sector is the most interesting from the scientific standpoint, precisely because it is the most improbable. Given the absence of photosynthesis, the researchers know the food chain must be based on chemosynthesis, in which simple bacteria use nitrogen compounds and ammonia to produce energy. The bacteria serve as food for microorganisms, sponges and mollusks. 

The Central Sector, which is opposite the island of Marajó (Pará, Brazil), is characterized by a shallower plume than that of the Northern Sector. Its density is variable and declines in the southward direction. The thinner plume allows more sunlight to penetrate the water.

As a result, the Central Sector displays a transition between the sponge and calcareous reef structures that prevail in the Northern Sector and the coral reef systems typical of clear tropical waters that prevail in the Southern Sector between Pará and Maranhão. This area contains Parcel Manoel Luis, the largest coral reef in the South Atlantic. Its existence has been well-known for decades because it is Brazil’s largest ship graveyard.

“The newly discovered reef apparently began forming between 14,000 and 12,000 years ago,” Mahiques said. “In geological terms, it’s an infant.”

The explanation is simple. At the height of the last ice age some 21,000 years ago, sea levels were 130 m lower than they are today and Brazil’s entire continental platform was exposed. When the great ice caps that covered much of the northern hemisphere eventually thawed, sea levels rose and flooded the entire continental platform, creating a favorable environment for colonization by reefs. 

The area of the reef is approximately 9,500 km². “During our two expeditions, in 2012 with the oceanographic research vessel Atlantis and in 2014 with the Cruzeiro do Sul, we were at sea for a total of 15 days and mapped only 10% of the area. We would need another 100 days, or about three months, at sea to map the rest of the reef,” Thompson said. “We have a long way to go before we understand the system. Countless aspects of the reef need to be studied.”

The team plans to return soon, this time on board the oceanographic research vessel Alpha Crucis, although they might have to use the Cruzeiro do Sul, which belongs to the Brazilian Navy, or one of the ships operated by the Ministry of Science, Technology & Innovation.

The article “An extensive reef system at the Amazon River mouth” (doi: 10.1126/sciadv.1501252) by Rodrigo Moura et al., published in Science Advances, can be read at advances.sciencemag.org/content/2/4/e1501252.