Researchers were amazed to discover a 600-mile long stretch of coral reef beneath the muddy waters at the mouth of the Amazon River in Brazil.
The Amazon is the largest river by discharge of water in the world. As large rivers empty into the world’s oceans in areas known as plumes, they typically create gaps in the reef distribution along the tropical shelves — something that makes finding a reef in the Amazon plume an unexpected discovery.
The Amazon plume — an area where freshwater from the river mixes with the salty Atlantic Ocean — affects a broad area of the tropical North Atlantic Ocean in terms of salinity, pH, light penetration and sedimentation, conditions that usually correlate to a major gap in Western Atlantic reefs.
It's thought that micro-organisms thriving in the dark waters beneath the river plume may provide the trophic connection between the river and the reef.
The team used multibeam acoustic sampling of the ocean bottom to find the reef and then dredged up samples to confirm the discovery — some of the trawl and dredge casts are shown on the ship's deck in the image above.
"The reef community changes as you travel north along the shelf break, in response to how much light it gets seasonally by the movement of the plume,” says Patricia Yager, an associate professor of marine sciences at the University of Georgia, who spent two months in Brazil as a Science Without Borders visiting professor.
"In the far south, it gets more light exposure, so many of the animals are more typical reef corals and things that photosynthesize for food. But as you move north, many of those become less abundant, and the reef transitions to sponges and other reef builders that are likely growing on the food that the river plume delivers. So the two systems are intricately linked."
But the reefs may already be threatened.
"From ocean acidification and ocean warming to plans for offshore oil exploration right on top of these new discoveries, the whole system is at risk from human impacts,” she adds.
Carlos Rezende from the State University of North Fluminense, Fabiano Thompson from the Federal University of Rio de Janeiro and Rodrigo Moura, a reef ecologist from UFRJ who has written extensively about richness of reef corals south of the Amazon River mouth, led the reef discovery team.
The results of the resesearch was published online at