What is sargassum and why does it reach Florida?

A seaweed that never touches the bottom

Sargassum is a brown seaweed. Unlike almost every other seaweed, it does not grow attached to a rock: it spends its whole life floating. Small gas-filled bladders, about the size of a pea, keep it at the surface. It reproduces by breaking into pieces that keep growing, so a small clump can turn into a mat several yards across.

Two floating species dominate in the Atlantic, and they give their name to the Sargasso Sea, a stretch of open ocean in the middle of the North Atlantic where there has always been plenty of it. Out at sea, sargassum is not a problem at all: it is shelter and food for small fish, crabs, young sea turtles and many other species.

Why more of it arrives now

For roughly the last fifteen years, satellites have seen far larger amounts than before in the tropical Atlantic, between Africa and the Caribbean. Scientists are studying several possible causes, such as nutrients reaching the ocean and water temperature, and there is still no single, settled explanation. The effect is clear, though: in some years a lot of sargassum reaches the Caribbean, the Gulf and the coasts of Florida.

How it reaches one particular beach

Sargassum in the ocean does not mean it will reach your beach. Three things decide whether the mats end up on the sand:

• Wind. When it blows from the sea toward the shore, it pushes sargassum onto the beach. With an offshore wind, it stays out at sea.
• Currents. They carry the mats for days before anyone sees them from the beach.
• The shape of the coast. Coves, piers and jetties make it pile up on some stretches while others barely see any.

That is why two beaches a few miles apart can look very different on the same day, and why we look at each beach on its own.

How we measure it

Our 0 to 100 index combines satellite images, wind, currents, tides and reports from people on the sand. The satellite sees sargassum at sea, but not always: clouds hide it, and close to shore it is hard to tell apart. Community reports fill that gap. We explain it in detail in How we measure.

If you are going today, check your beach before you leave home. And if you are already there, a ten-second report helps whoever is on the way.