MIAMI BEACH — As international visitors converge on South Florida for World Cup events and festivities, the region’s beaches are confronting an unwelcome natural phenomenon: extensive mats of sargassum seaweed that have coated shorelines from Miami Beach to Bal Harbour.

The brown seaweed, which washes ashore in thick piles, has been present on South Florida beaches since early spring, arriving earlier than usual with significant accumulations noted over the Memorial Day weekend. The decomposing seaweed emits an unpleasant odor reminiscent of rotten eggs and attracts swarms of flies, detracting from the typical beach experience for many tourists and locals.

Sargassum is naturally occurring algae originating in the Atlantic Ocean, but its proliferation has intensified sharply since 2011. A vast belt of sargassum now stretches from the west coast of Africa to the Caribbean Sea and Gulf of Mexico. Scientists investigating the cause are divided; some suggest that dust and wildfire particles from Africa or nutrient-rich runoff from agriculture may be contributing factors.

Researchers estimate that nearly 30 million metric tons of sargassum are currently floating in the Atlantic, sending what may be a record-breaking bloom toward Florida’s coast this year. Experts say this is likely to become a recurring challenge for the region.

Visitors share mixed experiences: some, like Kirsty Douglas from Scotland, found the seaweed “like walking on a sponge” while entering the water, and children turned collecting the algae into a game. Others, including Colombian tourist Eduardo Mujica, found the sargassum off-putting enough that their family members avoided swimming.

Local environmental and municipal authorities face constraints in managing the issue. Miami-Dade County staff are prohibited from removing sargassum at sea to protect marine life, notably endangered sea turtles, so their efforts are limited to clearing the algae once it reaches the shore. Daily operations involve surveyors checking for sea turtle nests and coordinated cleanup crews that use machinery to mix and remove the seaweed under environmental regulations.

“The sargassum situation is dictated by what Mother Nature brings us,” said Chris Bumpus, chief of conservation for Miami-Dade County’s parks department.

The cleanup process involves mixing sargassum into the sand using tractors and collecting heavier piles for transport to landfill sites. Despite these efforts, negative impacts are felt by tourism-related businesses, as fewer beachgoers are seen patronizing affected areas, with attendant Daniel Cook from Bal Harbour noting a decline in visitors and income.

Social media platforms have become informal channels for beachgoers to report sargassum conditions across the region. However, not all tourists are aware of these resources. Minisha Walker, visiting from Jacksonville, expressed frustration over the situation but indicated it would not deter her family’s beach visits.

Some visitors remain hopeful that the algae will clear soon. Lucela Perez, an Argentine tourist attending World Cup matches, described her surprise at the scale of the algae but said she planned to wait until the seaweed subsides.

The recurring sargassum blooms threaten to pose ongoing challenges for South Florida as it continues to attract visitors during peak event seasons. Local officials, researchers, and businesses alike are watching closely as the region balances environmental concerns with its economic dependence on tourism and recreational coastal activities.