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 the warm waters of the Americas. Scientists told Reuters they suspect a complex mix of 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 percent 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 …