Most sargassum grows off the shore of the Gulf of Mexico. Like all plants, it needs nitrogen, phosphorous, and sunlight to survive. In recent years, the sargassum has had its fair share of nitrogen, thriving on large amounts of fertilizer from the runoff of the Amazon River. The seaweed can also increase when upwelling in the eastern Atlantic brings cooler water and nutrients from the bottom of the ocean to the surface.
As the sargassum grows, it starts moving along ocean currents, which transport it from the Gulf to the Florida Current via the Loop Current, and then on to the Sargasso Sea, Tropical Atlantic, and the Caribbean. If winds are strong, it can end up near the Sahara desert, the Amazon River, or here on the Florida coast, bringing problems when it arrives en masse.