Scientists were baffled when a band of seaweed longer than the entire Brazilian coastline sprouted in 2011 in the tropical Atlantic – an area typically lacking nutrients that would feed such growth.

A group of U.S. researchers has fingered a prime suspect: human sewage and agricultural runoff carried by rivers to the ocean. The science is not yet definitive. This nutrient-charged outflow is just one of several likely culprits fueling an explosion of seaweed in warm waters of the Americas.

Six scientists told Reuters they suspect a complex mix of climate change, Amazon rainforest destruction and dust blowing west from Africa’s Sahara Desert may be fueling mega-blooms of the dark-brown seaweed known as sargassum.

In June 2018, scientists recorded 20 million metric tons of seaweed, a 1,000% increase compared with the 2011 bloom for that month. “There are probably multiple factors” driving the growth, said oceanographer Ajit Subramaniam at Columbia University. “I would be surprised if there is one clear villain.”

Still, a recent study examining the chemistry of seaweed from the 1980s up to 2019 offers the strongest evidence yet that water coming from city and farm runoff has been a major contributor to expansion of the so-called Great Atlantic Sargassum Belt, which now stretches for nearly 9,000 kilometers.

Nitrogen runoff

That study, co-authored by biologist Brian Lapointe at Florida Atlantic University, found that sargassum collected recently in coastal waters from Brazil to the southern United States, and including several Caribbean nations, contained levels of nitrogen that were 35% higher on average than in samples taken more than three decades earlier. The findings were published in May in the journal Nature Communications.

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