Brazil is home to the world's largest area of rhodolith beds

A study led by Brazilians maps out the Abrolhos Shelf and reveals that the region has the largest known stretch of calcareous algae reefs, which are highly vulnerable to marine acidification

By Fábio de Castro

Agência FAPESP – The Abrolhos Shelf, situated along the Bahian coast, is home to the world’s largest area of rhodoliths, according to a study led by Brazilian researchers. Rhodoliths are algae that form structures similar to coral reefs.

The research, whose results were published in the PLoS ONE journal, concluded that the rhodolith reefs in Abrolhos cover more than 20,000 square kilometers, an area approximately the size of Sergipe State.

The study also reveals that these reefs, which produce some 25 million tons of calcium carbonate per year, are facing many threats and are especially vulnerable to oceanic acidification.

The study, which earned a comment in Science magazine, had contributions from the project “Mapping of benthic habitats of the Abrolhos Bank”, which is coordinated by Paulo Sumida at the University of São Paulo’s Oceanography Institute (IO-USP) and is financed by FAPESP through its Regular Research Support Program.

The scientists who contributed to the article were from USP, the Rio de Janeiro Botanical Gardens Research Institute (JBRJ), the University of Boston (U.S.) and the Federal Universities of Rio de Janeiro (UFRJ), Espírito Santo (UFES), Paraíba (UFPB) and Rural Rio de Janeiro (UFFRJ). The project was coordinated by Gilberto Amado Filho from the JBRJ Research Institute.

The group that performed the study also included researchers from Conservation International and the Instituto do Milênio Pro-Abrolhos Project, said Sumida, who currently coordinates the FAPESP-funded project “Acidification effects on benthic metabolism and trophodynamics”.

According to Sumida, rhodoliths make up nearly half of the total area of the Abrolhos Shelf, some 46,000 square kilometers. Given its dimensions, the study warns that there is a need for environmental conservation policy.

“It is an immense area of reefs that are home to very rich biodiversity. Moreover, these reefs are highly vulnerable to environmental threats, such as the acidification of the ocean. Rhodoliths are a factory of calcium carbonate, and their degradation could release enormous quantities of carbon into the environment,” Sumida told Agência FAPESP.

The research revealed the distribution, extension, composition and structure of the rhodolith reefs. Mapping for the project utilized sweep sonar, technical dives and robotic vehicles.

“The Abrolhos National Marine Park was the first of its type in Brazil. However, the area of the park, with depths of less than 20 meters, does not reach even 2% of the Abrolhos Shelf. We began to systematically map the shelf in deeper waters of up to 100 meters,” said Sumida.

The data collected with sonar instruments—which sweep an area of up to 400 meters around the ship—showed homogenic structures on the relief of the bottom. To identify these structures, the scientists used divers and remote-control vehicles.

“We discovered this immense area covered by rhodoliths, and when we consulted the literature, we found that it is the largest continual shelf of these structures in the world, over 20,000 kilometers square,” said Sumida.

Biodiversity Aggregators

Sumida says that because of its enormous stretch of rhodolith reefs, the Abrolhos Shelf could play an important role in global climate. Because these algae produce calcium carbonate in their structure, they sequester carbon from the atmosphere.

“Because they are composed of calcareous material, the rhodoliths are greatly affected by marine acidification. This breakdown could put all this carbon back into the marine environment. If this carbon is put into the water, the process will tend to increase the chemical imbalance of the ocean, increasing the effects of acidification even more in a vicious cycle of disastrous proportions,” he explained.

Another factor Sumida emphasizes is the need for policy to conserve the rich biodiversity in the area of these rhodoliths, which forms a complex and heterogenic environment full of ecological niches.

“Aside from being a carbon sink, the rhodoliths are important biodiversity aggregators, like reefs and corals. It is a critical preservation area. When these structures are broken down, the ecosystem is simplified, and some opportunistic species adapted to more homogenous habitats end up winning out. The result is a dramatic breakdown in local biodiversity,” said Sumida.

The scientist affirms that the study’s results are already being used as a basis for discussions on increasing the preservation areas in Abrolhos.

“Ideally, Abrolhos should be excluded from plans for oil exploration in the region, which has already been subdivided for the purpose. Oil exploration would have an extreme impact. The study also shows that limestone exploration would be unviable in the region,” he said.

After revealing the structure and area of the rhodolith reefs, Sumida said the IO-USP and JBRJ researchers will now most likely concentrate on studying the microinvertebrate diversity present in the ecological niches formed by the Abrolhos rhodoliths.

The article “Rhodolith Beds Are Major CaCO3 Bio-Factories in the Tropical South West Atlantic” by Paulo Sumida and others can be read in PLoS ONE at:
www.plosone.org/article/info:doi/10.1371/journal.pone.0035171.