An Atoll of Algae
The Atol das Rocas, or Rocas Atoll, is the only atoll in the South Atlantic Ocean, located approximately 260 kilometers northeast of Natal, Brazil. It is an elliptical ring of reef measuring 3.7 kilometers east-to-west and 2.5 kilometers north-to-south. The structure sits atop a volcanic seamount that rises from the ocean floor, which is up to 4,000 meters deep in the surrounding area. Unlike most atolls, which are primarily built by coral polyps, Rocas is constructed mainly by crustose coralline algae, particularly red algae, with secondary contributions from vermetid gastropods and corals like Siderastrea stellata.
The atoll's reef growth began more than 4,800 years ago, with an accretion rate measured at 1.5 to 3.2 meters per millennium. This biological construction encloses a shallow lagoon with a total area of 7.1 square kilometers and a depth of up to 6 meters. At low tide, the reef crest is exposed, forming a natural wall about 1.5 meters high around the lagoon's tidal pools. Two small sandy islets, Farol Cay and Cemitério Island, are located on the western side of the lagoon. Their combined land area is just 0.36 square kilometers, with the highest point being a sand dune on Farol Cay that reaches 6 meters in elevation.
A Sanctuary in the Open Ocean
The atoll's isolation has made it an important sanctuary for marine and avian life, earning it designation as Brazil's first Marine Biological Reserve in 1979 and a UNESCO World Heritage Site in 2001. The reserve is an important feeding ground and nursery for many species. The area is a reproductive site for lemon sharks (Negaprion brevirostris) and provides resting grounds for Atlantic nurse sharks (Ginglymostoma cirratum).
Rocas is the second-largest rookery in Brazil for the green sea turtle (Chelonia mydas). From December to June, hundreds of female turtles come ashore to nest, with an average of 335 nests recorded annually during a study from 1990 to 2008. The islands host immense seabird colonies, with populations reaching 150,000 individuals. Five species breed here, including masked boobies (Sula dactylatra), brown boobies (Sula leucogaster), and sooty terns (Sterna fuscata). The atoll hosts the largest breeding colonies of masked boobies and brown noddies (Anous stolidus) in Brazil. Due to its protected status, access to the atoll is strictly limited to authorized scientific researchers.