Sargassum at Grand Cul-de-Sac: today's lagoon report
Clear
All good
Grand Cul-de-Sac closes off the north-eastern coast of St Barthelemy behind a coral barrier running from headland to headland. Between the reef and the shore lies a shallow, calm lagoon, busy with wind-powered water sports for much of the year.
Every morning the 0 to 5 rating published here sums up sargassum at Grand Cul-de-Sac, alongside a map placing the lagoon and its opening. You get the picture for the day itself rather than a general seasonal trend, before you drive down to the water. The rating covers this lagoon specifically, not an average across the beaches of St Barth.
Plage de Grand Cul-de-Sac on the sargassum map
What to know about this beach
Two things drive conditions here. First the north-east aspect: the bay looks towards the source of the trade winds, on the Atlantic side of the island, so it ranks among the first spots to feel an episode moving through the Lesser Antilles. Second the reef, which breaks the swell and makes the lagoon usable, but does nothing to filter weed floating at the surface.
That pairing explains how the site behaves. Sargassum rides over the reef when the sea carries it, then sits in shallow water where exchange is slow, and it lingers. The easterly wind presses it against the inner shore, while a sheltered lagoon lacks the churn that would break a raft apart. Day to day swings can be large here, which is what the daily rating tracks.
Frequently asked questions
Does the reef protect the lagoon from sargassum?
It protects it from swell, not from weed. Sargassum floats at the surface and crosses the reef when the sea pushes it over. Once inside the lagoon it stays longer than it would in open water.
How do I check the lagoon before a kite or wing session?
Open this page in the morning: the 0 to 5 rating and the map are updated daily. A loaded lagoon shows in the rating before you have taken the gear out of the car.
Why does weed sit in the bay for so long?
The lagoon is shallow and largely enclosed, so water turns over slowly, and the easterly wind pushes anything floating towards the inner shore instead of carrying it back out to sea.
Is Petit Cul-de-Sac next door in the same situation?
It shares the same windward aspect, but the bay is smaller and the bottom rockier. Compare both daily ratings, since the two coves regularly come out differently on the same morning.