"Spanning thousands of miles... the blob — a tangled, buoyant, mass of a type of seaweed called sargassum — is expected to come ashore in Florida..."

"... and elsewhere along the Gulf of Mexico.... [I]t will then begin to rot, emitting toxic fumes and fouling the region’s beaches over the busiest summer months....While floating sargassum can benefit marine animals by providing shade and shelter, the problems begin once it comes ashore. As the sargassum begins to die, it degrades the water quality and pollutes beaches, scientists say. It can also choke vital mangrove habitats and suck oxygen out of the water. The decaying algae also releases hydrogen sulfide, a colorless gas that smells like rotten eggs, and can cause respiratory problems in humans.... The most immediate threat, however, is to tourism."

 
From "A Giant Blob of Seaweed is Heading to Florida/The mass, known as the great Atlantic Sargassum belt, is drifting toward the Gulf of Mexico. Scientists say seaweed is likely to come ashore by summer to create a rotting, stinking, scourge" (NYT).