Stretching from West Africa to the Gulf of Mexico, a massive bloom of sargassum seaweed is swamping beaches and killing coastal wildlife. But scientists have discovered the floating algae also shelters a vast array of ocean creatures, and some are now calling for its protection.

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…