The sea in front of the Ocean Frontiers dive shop on Grand Cayman is usually crystal clear. But recently sparking blue has turned murky brown. Like much of the Caribbean, the island has been engulfed by sargassum, a floating seaweed. Sometimes the algae stretches 30 metres (100 feet) out from the shore. It jams propellers and mars white beaches. Worst of all says Evan Verreault, the shop manager, it stinks. Decaying sargassum releases a stench of rotten eggs.

Sargassum is native to the Atlantic. It was documented in the 15th century, when Christopher Columbus spotted patches of it in the Sargasso Sea. It usually stayed offshore, providing a floating habitat for marine life. But in 2011 satellites spotted a vast belt of the stuff stretching from the Caribbean to West Africa. The blooms have grown bigger every summer. Ever more weed washes ashore. In May alone an estimated 38m tonnes of sargassum was lurking in the Caribbean and neighbouring parts of the Atlantic. Scientists are still unsure what exactly is driving the surge. They suspect a warming climate and increased agricultural runoff.

The algae blights tourism, fishing and wildlife. In Mexico, removing the weed costs between $300,000 and $1.5m per kilometre of beach. The tough, spiny fronds damage fishing gear and reduce catches. Piles of sargassum block turtles from reaching nesting sites and ensnare hatchlings making their first perilous dash to the sea. The gasses created by rotting sargassum are also toxic: last month, several seaside schools in Martinique were forced to close after hydrogen sulphide and ammonia levels spiked.

Some companies want to help. One called Seafields has developed a system to catch the algal tide before it reaches shore, using satellite images and drones to track the sargassum’s movements. Before it makes landfall they corral it into flexible open-water “paddocks” that keep it alive (and odour-free) until it can then be hauled in using nets or special conveyor belts. Seafields says it is agreeing contracts with hotels on several Caribbean islands to protect their beaches. The paddocks could even be a selling point for resorts, says John Auckland, the company’s boss, as the floating weed attracts marine life. “They’re really good to go snorkelling around.”

There may also be money in harvesting and processing the sargassum. Farmed seaweeds like kelp are already used in bioplastics, biofuels and livestock feed. Some companies are betting sargassum could be just as useful. BioPlaster, a Mexican materials company, has developed a sargassum-based alternative to cling film and plastic bags. The company prefers algae collected at sea, but it also helps coastal communities find ways to profit from what’s already washed ashore.

For now, sargassum supply is too unpredictable to be a viable feedstock. But if the harvesting of Seafields and others like it can be scaled up, the Caribbean’s smelly problem could be transformed into a clean, green industry.■

This article appeared in the The Americas section of the print edition under the headline “Sand, sun and stench”