Researchers in Florida have identified the world's largest seaweed bloom, a massive expanse of Sargassum visible from space.

Though expanses of the brown algae have increasingly blanketed beaches in Florida and the Caribbean, scientists know relatively little about what fuels their growth and how they impact marine ecosystems.

In moderate amounts, algae and seaweed are beneficial, vital even, to marine food chains. But extreme quantities can harm ecosystems, sucking the oxygen from swaths of the ocean, synthesizing dangerous toxins and simply get in the way of marine life.

To better understand the growth patterns of Sargassum, a genus of brown macroalgae, or seaweed, scientists at Florida Atlantic University, the University of South Florida and the Georgia Institute of Technology surveyed nearly two decades of satellite images of the so-called Great Atlantic Sargassum Belt, which stretches from West Africa to the Gulf of Mexico. Their survey -- published Friday in the journal Science -- revealed the largest Sargassum bloom in history.

The massive bloom peaked last year. At its maximum, the belt of seaweed stretched 5,500 miles and featured 20 million metric tons of seaweed.

Scientists used field study observations and environmental data to provide context for their satellite survey, determining that the belt of seaweed blooms seasonally and responds to two main nutrient inputs. In the winter, upwelling of deep ocean water off the coast of West Africa provides the seaweed with nutrients, fueling growth. In the spring and summer, the belt's expansion is accelerated by nutrients carried into the ocean by the Amazon.

Researchers found preliminary evidence that the belt has swelled in recent years in response to nutrient overloads fueled by deforestation and increased fertilizer use in the Amazon.

"Severe floods have recurred in the Amazon basin since 2009, which would result in extensive freshwater runoff and nutrient enrichment in the western Atlantic Ocean," Brian Lapointe, a research professor at FAU, said in a news release. "It is reasonable to suggest that the 2011 massive bloom is therefore the result of nutrient accumulations since 2009, resulting from stronger upwelling in the eastern Atlantic and excessive Amazon River discharges in the western Atlantic."

When conditions are right, the belt swells in the central Atlantic before drifting westward, covering beaches in the Caribbean several months later.

"The significant biomass accumulations along the pathway of the Great Atlantic Sargassum Belt underscore the need for multidisciplinary research to better understand their ecological and biogeochemical impacts, as well as their impacts on coastal environments, tourism, economies and human health, especially when the role of Sargassum changes from an essential habitat to a significant and perpetual nuisance," Lapointe said.

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