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.
As sargassum decays, it releases hydrogen sulfide, which causes the rotten-egg smell. Florida DOH says the gas can irritate the eyes, nose and throat, especially for people with asthma or other breathing conditions. Leave the affected area if it causes irritation or breathing trouble.
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."
Peak season is not a steady climb. Wind, currents, tides, and beach cleanup can make the shoreline look much cleaner than it did a few weeks earlier even while the broader summer season remains active. Use the Village beach cam for a current visual check of that shoreline and the NOAA map for the regional risk; neither one should be treated as a report for every Key Biscayne beach.
Skin irritation and stinging larvae
Tiny organisms living in seaweed can irritate skin. Florida DOH recommends avoiding contact with or swimming near sargassum. If you suspect stinging larvae, remove the affected swimsuit before showering and wash it before wearing it again.
Is the water safe to swim?
Sargassum and bacterial advisories are separate things. A beach can have seaweed without a bacterial advisory, or look clear while a water-quality advisory is in effect. Neither the seaweed map nor a Good bacteria sample establishes that swimming is safe. For the official Florida Department of Health sample readings — Crandon North, Crandon South, Cape Florida, and the Village beach — see the Key Biscayne water quality today page. We pull it straight from the FL DOH dashboard and show the enterococci count, the Good/Moderate/Poor bucket, and any active advisory. The lifeguard beach flag color today (green / yellow / red / purple) is also worth a glance before walking down.
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 cache checks of NOAA's latest published report for up to an hour. For reports less than four days old, we read the map colors around Key Biscayne to estimate coastline risk. Older maps stay visible with their date and an “Outdated” label, but do not drive the current-risk badge. If we cannot verify the report or read the map, we leave NOAA risk unknown. A fresh Village beach-camera reading is shown separately and can raise concern, but a clear view of one beach cannot establish low risk for the whole island.
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. A regional seaweed signal cannot establish that another shoreline is suitable for swimming. Check dated water-quality results and posted beach flags, or choose an activity away from the water.