World’s Largest Macroalgal Bloom Discovered

Jul 5, 2019 by News Staff

Free-floating mats of brown macroalgae called Sargassum in the center of the North Atlantic were first reported by Christopher Columbus in the 15th century CE. Since 2011, these mats have increased in density and size to generate a 5,500-mile (8,850 km) long belt that extends from West Africa to the Caribbean Sea and Gulf of Mexico. According to a new study, it contains over 20 million metric tons of Sargassum biomass and represents the largest bloom of macroalgae on Earth. The study authors have dubbed it the Great Atlantic Sargassum Belt.

Sargassum seaweed. Image credit:: Maria-Jose Vinas / NASA.

Sargassum seaweed. Image credit:: Maria-Jose Vinas / NASA.

In patches of the open ocean, Sargassum contributes to ocean health by providing habitat for turtles, crabs, fish, and birds and producing oxygen via photosynthesis like other plants.

But too much of this seaweed makes it hard for certain marine species to move and breathe, especially when the mats crowd the coast. When it dies and sinks to the ocean bottom at large quantities, it can smother corals and seagrasses. On the beach, rotten Sargassum releases hydrogen sulfide gas and smells like rotten eggs, potentially presenting health challenges for people on beaches who have asthma, for example.

“In the open ocean, Sargassum provides great ecological values, serving as a habitat and refuge for various marine animals. I often saw fish and dolphins around these floating mats,” said Dr. Mengqiu Wang, a researcher at the University of South Florida.

In 2011, Sargassum populations started to explode in places it hadn’t been before, like the central Atlantic Ocean, and it arrived in gargantuan gobs that suffocated shorelines and introduced a new nuisance for local environments and economies.

Some countries, such as Barbados, declared a national emergency last year because of the toll the seaweed took on tourism.

“The scale of these blooms is truly enormous, making global satellite imagery a good tool for detecting and tracking their dynamics through time,” said Dr. Woody Turner, manager of the Ecological Forecasting Program at NASA Headquarters in Washington.

Monthly mean Sargassum density for the month of July from 2011 to 2018. Image credit: Wang et al, doi: 10.1126/science.aaw7912.

Monthly mean Sargassum density for the month of July from 2011 to 2018. Image credit: Wang et al, doi: 10.1126/science.aaw7912.

The researchers used environmental and field data to suggest that the belt forms seasonally in response to two key nutrient inputs: one human-derived, and one natural.

In the spring and summer, Amazon River discharge adds nutrients to the ocean, and such discharged nutrients may have increased in recent years due to expanded deforestation and fertilizer use.

In the winter, upwelling off the West African coast delivers nutrients from deep waters to the ocean surface where the Sargassum grows.

“Our measurements of nutrient concentrations in surface waters of the Western Tropical North Atlantic showed greater nitrate and phosphate availability in spring 2018 than in spring 2010, a pattern consistent with increased inputs from the Amazon River due to land use changes in the drainage basin,” said Georgia Institute of Technology’s Professor Joseph Montoya.

“The increase in nitrate concentration is particularly important since the growth of photosynthetic organisms like Sargassum is typically limited by nitrogen availability.”

“The evidence for nutrient enrichment is preliminary and based on limited field data and other environmental data, and we need more research to confirm this hypothesis,” added Dr. Chuanmin Hu, from the University of South Florida.

“On the other hand, based on the last 20 years of data, I can say that the belt is very likely to be a new normal.”

The study was published today in the journal Science.

_____

Mengqiu Wang et al. 2019. The great Atlantic Sargassum belt. Science 365 (6448): 83-87; doi: 10.1126/science.aaw7912

Share This Page