Record Sargassum influx inundates Caribbean shores, impacting tourism and ecosystems

A record 37.5 million metric tons (41.3 million short tons) of Sargassum seaweed was detected in the Atlantic basin in May 2025, the largest amount since monitoring began in 2011.

Excessive Sargassum has inundated coastlines from Puerto Rico to Guyana, impacting tourism. This brown algae blocks sunlight to coral reefs, smothers seagrass beds, and releases a foul odor and harmful gases. In Martinique, gases containing hydrogen sulfide and ammonia led to the temporary closure of a school.

The Sargassum mass grew from 31 million metric tons (34.2 million short tons) in April to 37.5 million metric tons (41.3 million short tons) in May 2025, an increase of nearly 21 percent. Every monitored area except the Gulf of America saw seaweed levels surpass previous records.

Most of the Sargassum is concentrated in the western tropical Atlantic, near where the Amazon River flows into the ocean. While the exact cause of the increase is unclear, factors such as nitrogen and phosphorus from agricultural runoff, rising water temperatures, and shifts in wind, currents, and rainfall may contribute.

Two primary species of Sargassum, with several variants, are found in the Caribbean and nearby waters, and they stay afloat using small air-filled bladders and reproduce asexually. Their growth depends on factors such as sunlight, nutrient levels, and water temperature.

Large mats of Sargassum support an important habitat for marine animals, such as shrimp and sea turtles, in open ocean waters. However, when the seaweed drifts toward the coast, it blocks sunlight that coral reefs rely on. If it sinks, it can smother reefs and seagrass beds. When it washes ashore, the marine life within it either dies or is consumed by birds.

Communities are taking different approaches to address the increasing Sargassum influx. In Punta Cana, Dominican Republic, barriers have been set up to prevent the seaweed from reaching the shore. In Sint Maarten, cleanup crews with heavy equipment were deployed from May 20 to 31 after locals reported a strong odor of ammonia and hydrogen sulfide. French Caribbean officials plan to deploy storage barges and a specialized vessel capable of collecting several metric tons (several short tons) of seaweed daily.

Cleanup efforts are costly, and with limited government resources, much of the burden falls on hotels. Some have started offering refunds or free transport to cleaner beaches to avoid losing guests and business.

Sargassum is expected to continue washing up through June 2025 on many Caribbean islands and along Florida’s southeast coast.

Sargassum is a genus of brown seaweed that floats in large mats on the ocean’s surface. While it naturally occurs in the Sargasso Sea of the North Atlantic, since 2011, unusually large blooms have been observed drifting into the Caribbean Sea and the Gulf of America.

Sargassum mats serve as habitat for marine organisms in open water, but when they accumulate along coastlines, the decomposing seaweed releases hydrogen sulfide and ammonia, causing foul odors, and can damage local ecosystems. Sargassum influxes are known to disrupt tourism, fisheries, and coastal infrastructure.