Many of America’s favorite beaches in Florida and the Caribbean have been piled high with foul-smelling seaweed again this summer, often in record amounts. What disappointed tourists may not know is that the inundations, which also smother coral reefs, mangroves, and seagrasses, are just the most visible sign of a vast new marine ecosystem — an expanse of floating sargassum that since 2011 has seemingly come from nowhere to spread in a 5,500-mile-long belt from Mexico to West Africa and Florida to Brazil.

The dense mats of offshore weed grow up to 20 feet thick, and this summer reached a record weight of 38 million metric tons — more than the total weight of all Americans.

The seaweed scourge is wrecking coastal economies in countries heavily dependent on tourism and fishing. The coastlines of 30 countries across the Caribbean and beyond are being visited by the brown mats piling up on beaches, where the weed decomposes, releasing gases such as hydrogen sulfide, with its distinctive rotten-egg smell, and ammonia, which trigger asthma attacks and exacerbate other respiratory conditions. Thousands of victims have been reported by doctors in the Caribbean, especially among workers clearing beaches.

The tropical sargassum belt now forms “the largest interconnected floating biome on Earth,” says an ecologist.

The weed also blocks water intakes and prevents fishers from taking their boats to sea, as well as clogging their nets, engines, and propellers. It has caused mass deaths of marine life on coastlines, while preventing sea turtles from laying their eggs on beaches and blocking hatchlings trying to return to the ocean. The weed also accumulates heavy metals such as arsenic, which leaches out as the weed decomposes on land and can poison water supplies.

For such reasons, most ecologists regard the belt as a menace.

But, while acknowledging its onshore harms, some researchers are now recognizing the Great Atlantic Sargassum Belt as a significant new habitat for marine life in the open ocean, and a valuable carbon sink for the planet. According to Jonathan Jung, a marine ecologist at the Max Planck Institute for Chemistry in Mainz, Germany, the weed and its growing army of inhabitants now form “the largest interconnected floating biome on Earth.”

Read the full article about the sargassum superbloom by Fred Pearce at Yale Environment 360.