COCOA BEACH, Florida – A massive blob of Sargasso seaweed stretching from West Africa to the Gulf of Mexico that can be seen from outer space is heading for Florida beaches.
Scientists at the University of South Florida in St. Petersburg’s College of Marine Science used NASA satellite observations to discover and document the largest bloom of macroalgae in the world, dubbed the Great Atlantic Sargassum Belt.
In 2018, more than 20 million tons of it – heavier than 200 fully loaded aircraft carriers – floated in surface waters and became a problem to shorelines lining the tropical Atlantic, Caribbean Sea, Gulf of Mexico, and east coast of Florida, as it carpeted popular beach destinations and crowded coastal waters.
When seaweed piles up along the high tide line, it is called a ‘wrack zone’ or ‘wrack line’ which plays an important ecological role as a valuable food webs source to beach wildlife, especially shorebirds that eat small marine creatures tangled up in the seaweed.
This floating habitat provides food, refuge, and breeding grounds for an array of critters such as fishes, sea turtles, marine birds, crabs, shrimp, and more.
Some animals, like the Sargassum fish (in the frogfish family), live their whole lives only in this habitat. Sargassum serves as a primary nursery area for a variety of commercially important fishes such as mahi-mahi, jacks, and amberjacks.
Because of the ecological importance of Sargassum, many local governments do not clean up the seaweed.
“The scale of these blooms is truly enormous, making global satellite imagery a good tool for detecting and tracking their dynamics through time,” said Woody Turner, manager of the Ecological Forecasting Program at NASA Headquarters in Washington.
Chuanmin Hu of the USF College of Marine Science, who led the study, has studied Sargassum using satellites since 2006
In 2011, Sargassum populations started to explode in places it hadn’t been before, like the central Atlantic Ocean, and then it arrived in gargantuan gobs that suffocated shorelines and introduced a new nuisance for local environments and economies.
“The ocean’s chemistry must have changed in order for the blooms to get so out of hand,” Hu said.
Sargassum reproduces from fragments of the parent plant, and it probably has several initiation zones around the Atlantic Ocean. It grows faster when nutrient conditions are favorable, and when its internal clock ticks in favor of reproduction.
The team identified key factors that are critical to bloom formation: a large seed population in the winter left over from a previous bloom, nutrient input from West Africa upwelling in winter, and nutrient input in the spring or summer from the Amazon River.
Such discharged nutrients may have increased in recent years due to increased deforestation and fertilizer use, though Hu noted that the evidence for nutrient enrichment is preliminary and based on limited available data, and the team needs more research to confirm this hypothesis.
In addition, Sargassum only grows well when salinity is normal and surface temperatures are normal or cooler.