The golden seaweed piling up along the water line has a name, a history, and a season. It is genuinely unpleasant on the sand and genuinely vital offshore. Here is the whole story, with every source listed at the bottom.
Sargassum is a brown algae, and unlike almost any other seaweed it never attaches to the seafloor. The two species drifting through the Atlantic, Sargassum natans and Sargassum fluitans, live their whole lives afloat.1,2
Since 2011 it has arrived every summer in what scientists named the Great Atlantic Sargassum Belt, a band that stretches roughly five thousand miles from West Africa to the Gulf. It was discovered from satellite by researchers at the University of South Florida. 3
The oceanographer Sylvia Earle called it a golden floating rainforest, and the numbers back the phrase. More than 145 kinds of invertebrates have been recorded in the mats, and ten species live in sargassum and nowhere else on Earth, including the sargassum frogfish, which climbs the weed with limb-like fins and changes color to match it. 1,4
Young sea turtles ride it through what researchers call the lost years, the stretch after hatching when science loses track of them. Satellite tags on young green turtles confirmed it in 2021. 5
The fish you know by name shelter under it too: mahi, tuna, wahoo and jacks spend their first weeks hiding beneath the rafts. 1
Sandpipers and plovers come to eat the crabs and shrimp tangled inside. As the wrack sits, it traps windblown sand and feeds the sea oats, which is how dunes rebuild, the same dunes that take the hit in a storm. 6
Fresh sargassum is harmless. After roughly 48 hours on the beach it begins to decompose and releases hydrogen sulfide and ammonia. That is the rotten egg smell, and it can irritate eyes, nose and throat and cause headaches, nausea and dizziness. 7,8
People with asthma or other respiratory conditions should keep well back, as should children, older adults and pregnant women. The mats can also carry jellyfish larvae, which is what stings swimmers. Do not wade through heavy sargassum, and do not let children play in the piles. 7
Sargassum peaks in June or July and reaches its minimum in the winter months. In 2026, researchers expect amounts to keep decreasing in most regions through roughly November, with beaching events continuing but less severe than at the summer peak. 9
Which beach and which day is a different question, and a much harder one: the same researchers note that predicting beaching for an individual beach months ahead is not possible. That is why we check every beach, every day. 9
ShoreScore scores every Florida beach on sun, surf, wind, water quality and seaweed, and tracks which way the seaweed is heading at each one.
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