Scientists discover the biggest seaweed bloom in the world

The record-breaking belt of brown algae stretches from West Africa to the Gulf of Mexico—and it’s likely here to stay, says a team led by the USF College of Marine Science.

ST. PETERSBURG – Scientists led by the USF College of Marine Science used NASA satellite observations to discover the largest bloom of macroalgae in the world called the Great Atlantic Sargassum Belt (GASB), as reported in Science.

They confirmed that the belt of brown macroalgae called Sargassum forms its shape in response to ocean currents, based on numerical simulations. It can grow so large that it blankets the surface of the tropical Atlantic Ocean from the west coast of Africa to the Gulf of Mexico. This happened last year when more than 20 million tons of it – heavier than 200 fully loaded aircraft carriers – floated in surface waters and some of which wreaked havoc on shorelines lining the tropical Atlantic, Caribbean Sea, Gulf of Mexico, and east coast of Florida.

The team also 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 increased 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.

“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,” said Dr. Chuanmin Hu of the USF College of Marine Science, who led the study and has studied Sargassum using satellites since 2006. “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,” said Hu.

Hu spearheaded the work with first author Dr. Mengqiu Wang, a postdoctoral scholar in his Optical Oceanography Lab at USF. The team included others from USF, Florida Atlantic University, and Georgia Institute of Technology. The data they analyzed from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) between 2000-2018 indicates a possible regime shift in Sargassum blooms since 2011.