Scientists recently discovered a coral reef in an unexpected place: where the Amazon River meets the Atlantic Ocean. The reef lives on the shallow seafloor off the coast of Brazil, which is host to six previously known large reefs, all rich in species. Why is the new finding surprising? That’s because the conditions where rivers outlet into the ocean are not known to support reefs. The area just off the coast has little light, and water that is acidic and not salty enough to favor coral growth carbonate precipitation. On top of that, the sediments are unstable and the waters move quickly, making it difficult for a structure like a coral colony to anchor itself. River outlets are usually known for dividing habitats, rather than nurturing them. It was previously assumed that a reef could not survive at the outlet of the Amazon River, which has the highest freshwater output in the world.
Keep in mind that this particular reef is different from the tropical coral reef you might be imagining. The river outlet reef is dominated by sponges, which are known to provide nutrients. Also common on the reef are rhodoliths, red algae that resemble corals. Rhodoliths, pictured above, are abundant off the east coast of Brazil, and they don’t need much light to survive. The reef has a rich community of bacteria that don’t need light and instead rely on nutrients like nitrate. The number of species increases to the south, as you move away from the river outlet.
The research team that discovered the reef spent the last few years learning about the variation of species close to and away from the river outlet. They hope to continue their study until they map the entire extent of the reef. Studying this reef will give us insight about how reef communities are adaptable to different ocean conditions now and in the future.
An extensive reef system at the Amazon River mouth, Science Advances
Surprising, Vibrant Reef Discovered in the Muddy Amazon, National Geographic
Shining Light on Brazil’s Secret Coral Reef, Smithsonian Mag
Managing Correspondent: Anna Waldeck