Study discusses origin of floating seaweed that invaded Brazilian coast

Findings published in the journal Phycologia suggest that the huge mass of Sargassum macroalgae found on beaches in northern Brazil in 2014 and 2015 did not come from the Caribbean, as previously thought.

By Karina Toledo  |  Agência FAPESP – Surprised by the first-ever appearance of an immense carpet of floating seaweed on the Brazilian coast in 2014 and 2015, researchers from several institutions and countries joined forces to investigate the causes, origins and possible impacts of the phenomenon.

Initial results of the study, which was supported by FAPESP, were first published on March 10 in the journal Phycologia. The print version of the article, featured on the cover of the journal, was published in early May.

According to the authors, this type of brown seaweed, belonging to the genus Sargassum, is very common in the North Atlantic, especially in an area of the Caribbean surrounded by ocean currents; here, floating masses of the macroalgae form the Sargasso Sea, a sanctuary for many species of plants and animals. However, their research findings suggest that the biomass that reached Brazil did not come from the Caribbean, as initially supposed.

“In Brazil, there are some species of Sargassum that live anchored to the marine substrate, but the floating kind had never been observed on beaches before 2014. A single sighting was recorded by the Navy far out at sea in 2011,” said Marina Sissini, a PhD student at the Federal University of Santa Catarina (UFSC) and the first author of the article. For more information on the Navy sighting, see checklist.org.br/getpdf?NGD002-12.

In April 2015, when the event began in the region, Sissini was on Fernando de Noronha, an island about 360 km from Touros, the nearest point on the coast of northeast Brazil, doing research for the project “Sustainable prospecting on ocean islands: biodiversity, chemistry, ecology and biotechnology (ProspecMar)”. After the first reports of the phenomenon in the media, other seaweed masses were sighted in the archipelago of São Pedro and São Paulo (about 940 km from Touros), at the Rocas Atoll (145 km west of Fernando de Noronha), and in Maranhão and Pará.

“A research network was set up to find out which species were present in these algal rafts and how they reached the South Atlantic, apparently traveling against the ocean currents,” Sissini said.

Besides UFSC, the network links scientists at the Federal University of Rio de Janeiro (UFRJ); Fluminense Federal University (UFF); the University of São Paulo (USP); the Federal University of São Paulo (UNIFESP); the Federal University of Pará (UFPA); the Federal University of Bahia (UFBA); the Chico Mendes Institute for Biodiversity Conservation (ICMBio); the US National Oceanic & Atmospheric Administration (NOAA); and the University of Ghana in Accra, Ghana.

The size of the network and its many nodes have led to not only scientific interest, but also fears that the phenomenon could damage biodiversity and wreak economic havoc across the region, especially if it recurs frequently. 

For example, many fish and other marine animals are believed to have died under a 1.5 m-thick layer of seaweed in the vicinity of Fernando de Noronha.

“On Atalaia beach in Salinópolis, Pará, over 30 tons of algae were observed in 2014, and 121 tons in 2015. The municipal government opted to remove part of this biomass in trucks in order to mitigate the damage to tourism,” said Mariana Cabral de Oliveira, a professor at the University of São Paulo’s Bioscience Institute (IB-USP) and one of the article’s co-authors.

Although macroalgae of the genus Sargassum are not toxic, Oliveira explained, they tend to scare bathers when they pile up on beaches and eventually decompose, releasing an unpleasant smell and potentially preventing people from bathing in the sea. These macroalgae may also hinder fishing with cast nets and make fish and shellfish farming impossible.

Another concern is that some of the organisms that cross the ocean with the Sargassum algae may turn out to be invasive species and disturb delicate ecosystems, such as those of Fernando de Noronha and Rocas Atoll.

“In the rafts of seaweed that accumulated on Fernando de Noronha, we identified algae of the species Oscillatoria sp., Sphacelaria tribuloides, Cladophora sp., Ceramium spp., and Pneophyllum fragile,” Sissini said. “Invertebrates included Portunus sp., a crab genus, and Lepas sp., goose barnacles belonging to the subclass Cirripedia. Fish species included Canthidermis maculata, the rough triggerfish.”

According to Oliveira, none of the species found in the collected material is known to be invasive, but the species should be monitored to see whether they stay where they have landed and what impact they will have on the local flora and fauna.

Origin

One of the study’s main aims, Sissini explained, was to investigate the origin of the floating seaweed masses that arrived off northern Brazil.

“The first step, after identifying the species in the samples, was to see whether those present in the samples collected on Fernando de Noronha were the same as those collected at Atalaia and whether they were the same as the populations found in the Caribbean,” she said.

Analysis of the seaweed’s morphology showed that it belonged to two species also found in the Caribbean, Sargassum natans and S. fluitans. Molecular analysis was also performed to try to confirm this identification, but the results were not conclusive.

“We sequenced small fragments of DNA both from algae collected on Fernando de Noronha and at Atalaia and from species of Sargassum that live anchored to the marine substrate in Brazil. We used the internal transcribed spacer region, or ITS, as a marker. There are many sequences from this region among the genes deposited in public databases, so the ITS is used to distinguish between different species of Sargassum,” Oliveira explained.

However, she went on, the marker proved not to be sensitive enough to discriminate between species, let alone to separate populations of the same species, so the origin of the biomass could not be inferred using this technique.

Satellite images from NOAA suggest that the algae almost certainly did not originate in the North Atlantic because no movements of large seaweed rafts were recorded in the weeks leading up to the bloom’s arrival in Brazil.

For Oliveira, the most plausible hypothesis is that, in the Central Atlantic, not far below the equator, there is a matrix of floating Sargassum similar to the mass found in the Caribbean and that this was the origin of both the algal rafts sighted off the coast of Africa in 2014 and those that arrived in Brazil in 2014 and 2015.

Another possibility that is being investigated by the group is that the events observed in 2014 and 2015 were linked to an abnormal rise in ocean temperatures, which boosted the growth of the macroalgae already present in smaller quantities in the South Atlantic.

“Permanent monitoring is important because similar events could occur again,” Oliveira said. “If they become frequent, they could have a significant economic and environmental impact. More research is needed to measure their effects.”

Sissini also said that studies are needed to determine the best management strategy to implement if the phenomenon does recur. “Some of the seaweed was removed from Atalaia beach in Pará, whereas on Fernando de Noronha, the authorities preferred to wait until the tide bore the biomass away,” she recalled. “We don’t know what the best strategy is. The recommendation will probably be different in each case, depending on the characteristics of the environment.”

The article “The floating Sargassum (Phaeophyceae) of the South Atlantic Ocean – likely scenarios” can be read at phycologia.org/doi/pdf/10.2216/16-92.1.