What sargassum is
Sargassum is a free-floating brown seaweed. Two species — Sargassum natans and Sargassum fluitans — drift in massive rafts across the tropical Atlantic. Since 2011 these rafts have grown into the Great Atlantic Sargassum Belt, a 5,000-mile band that releases tons of weed onto Caribbean and South Florida beaches each spring and summer.
The seaweed itself isn't dangerous. When it rots on the sand, though, it releases hydrogen sulfide — the eggy, sulfury smell that hangs over Crandon and Cape Florida after a big landing. Unpleasant, not toxic.
When sargassum peaks at Key Biscayne
The Atlantic basin cycle is fairly predictable. KB's typical year looks like this:
- March–May: rafts build in the central Atlantic and drift west on the equatorial current.
- May–August: peak landings on Florida's east coast. KB usually catches its worst stretches in June and July, often timed with Memorial Day weekend.
- August–October: tapers off as wind and currents shift.
- November–February: calm, clean beaches.
NOAA and the University of South Florida publish a basin-wide Sargassum Outlook Bulletin every month — that's the closest thing to a long-range forecast. The daily SIR map above is more useful for "should I go to the beach today."
Sea lice and jellyfish larvae — the real problem
The seaweed itself is harmless. What hurts is what lives inside it: thimble jellyfish larvae (Linuche unguiculata) and other stinging plankton, commonly called "sea lice." Stings show up as itchy red bumps under swimsuit straps and in body folds. Annoying, not dangerous, gone in 1–3 days.
What actually helps:
- Rinse with fresh water immediately after swimming — the larvae get trapped in fabric.
- Skip the water entirely on days the map shows red, especially with kids.
- White vinegar dulls the sting; calamine or hydrocortisone helps the itch.
- Don't take a swimsuit off and put it right back on later in the day. Wash it first.
Is the water safe to swim?
Sargassum and bacterial advisories are separate things. Key Biscayne beaches can be sargassum-heavy and clear for swimming, or sargassum-clean and under a water-quality advisory. For the official Florida Department of Health sample readings — Crandon North, Crandon South, Cape Florida, and the Village beach — check the live beach water quality card on the home page. We pull it straight from the FL DOH dashboard and show the enterococci count, the Good/Moderate/Poor bucket, and any active advisory.
How we measure the risk
The map above is NOAA's Sargassum Inundation Risk (SIR) product. It combines daily satellite imagery (Sentinel-3 OLCI and MODIS) with surface-current models from AOML and USF to estimate the risk of coastal inundation along the Caribbean, Gulf, and South Florida coast. The four colors mean:
- Low blue — minor algal coverage offshore; little expected on shore
- Warning yellow — elevated, monitor
- Medium orange — moderate risk; visible landings likely
- High red — heavy mats expected on shore
We fetch the GOMF (Gulf-of-Mexico / Florida) image once per day, read the actual pixel sitting on KB's coast, and let that drive the badge at the top of the page. No human in the loop. When NOAA's satellite has insufficient data (cloud cover), the pixel turns black and we report the risk as unknown rather than guessing.
What to do when it's bad
The Village contracts Beach Raker to mechanically clear the sand on major landings — CBS Miami reported around 100 tons removed in a single recent stretch. On red days, swim on the lee side of the island (Hobie Beach, the Crandon marina side) rather than the Atlantic-facing shores. Or pick a tennis court instead — see the courts section on the home page.