Miami-Dade county spends $3m per year on sargassum removal, more than any other county in Florida, Bumpus said. The Florida Keys, however, can see heavier accumulations of sargassum because of the inlet-like shape of their beaches, he said. For centuries, sargassum has grown in the Sargasso Sea, a region of the north Atlantic encircled by ocean currents that keep the seaweed concentrated there. But in 2011, scientists saw the Great Atlantic Sargassum Belt form for the first time, stretching from west Africa to the Gulf of Mexico. Scientists say several factors contribute to the blooms, including nitrogen and phosphorus from rivers – such as the Amazon, Congo, and Mississippi – warmer water temperatures and ocean circulation.