USVI · Sargassum Guide

Seaweed Season

What It Is, Where It Isn't

2026 is shaping up to be a record year for sargassum across the Caribbean. Here's what's actually happening in St. Thomas and St. John, which beaches are your best bet, and what to do if you run into it.

If you've seen headlines about record sargassum levels this year and started worrying about your St. Thomas trip, take a breath. The seaweed is real, 2026 really is a heavy year for it, and it really has closed some beaches elsewhere in the Caribbean. But St. Thomas isn't Cancún, and the beaches you're actually planning to visit are, for the most part, still fine. Here's the real picture.

01 · What Sargassum Actually Is

Sargassum is a free-floating brown algae, not a plant rooted to the seafloor. It's native to the Atlantic and has always existed here. The Sargasso Sea, where much of it originates, was named by Portuguese sailors back in the 1400s. Out in open water, it's genuinely valuable: it provides nursery habitat for mahi mahi, sea turtles, and dozens of other species.

The problem isn't sargassum existing — it's sargassum showing up in quantities the Caribbean didn't used to see. Since 2011, a second bloom originating near the mouth of the Amazon River has driven repeated, large-scale accumulations throughout the region, on top of the algae's traditional home range.

02 · Why 2026 Is Different

This isn't a one-off bad year — it's the continuation of a trend that's been building for over a decade. Researchers at the University of South Florida's Optical Oceanography Lab, which monitors sargassum across the Atlantic, Caribbean, and Gulf by satellite, reported that Caribbean sargassum volume tripled in a single month, from roughly 0.45 million metric tons in December 2025 to 1.7 million metric tons by January 2026. By May, total sargassum across all monitored regions had climbed to an estimated 28.9 million metric tons.

Scientists increasingly view the Great Atlantic Sargassum Belt less as a seasonal event and more as a self-sustaining feature of the ocean — nutrient cycling within older mats is now helping the blooms feed themselves, independent of the wind and current patterns that used to drive year-to-year variation. In plain terms: it's gotten worse nearly every year for the last decade-plus, and the underlying cause doesn't appear to be going away soon.

03 · Is It Actually Dangerous?

Sargassum floating in open water doesn't harm swimmers. The real issue starts once it washes ashore and begins to decompose.

The smell isn't just unpleasant

Decomposing sargassum releases hydrogen sulfide — the same compound behind that "rotten egg" smell. Beyond being unpleasant, it can irritate the eyes, nose, throat, and respiratory system, and the effects are more pronounced for people with asthma. The most effective precaution is simply avoiding beaches with significant piles of decaying sargassum, rather than pushing through it.

Marine life riding along in the mats

Sargassum itself won't sting you, but jellyfish and other small organisms that live within the floating mats can cause skin irritation on contact. If you're snorkeling through visible mats rather than around them, keep this in mind.

Boats and propellers

Thick sargassum can entangle motorized propellers. If it happens to you, boat operators are advised to shift into reverse to help clear the obstruction rather than forcing forward power.

04 · Which St. Thomas & St. John Beaches Tend to Stay Clear

St. Thomas and St. John benefit from the same geography: prevailing trade winds blow east to west across the territory, so sargassum tends to drift past north- and west-facing beaches rather than piling up on them. East- and southeast-facing beaches, sitting more directly in the wind's path, are the ones most likely to see accumulation.

Usually clear

Check before you go

This list reflects typical seasonal patterns rather than a live, day-of guarantee — conditions still shift with wind, tide, and current, so it's worth a quick check (Section 05) before you finalize plans, especially during peak months.

05 · How to Check Conditions Before You Go

Because conditions can change from one beach to the next and one day to the next, checking right before you head out is worth the extra two minutes:

• CARICOOS Sargassum Tracker — a live satellite map covering the Caribbean, including the USVI
• DPNR's Beach Advisory page — official Virgin Islands government updates
• Ask your hotel or villa — most beachfront properties clear overnight accumulation each morning, so front desk staff usually know current conditions better than any app

06 · What to Do If You Run Into It

• Small amounts: Sargassum can be removed manually by hand or rake — plenty of beachfront spots do this themselves each morning
• Large accumulations: These require permits from DFW and Coastal Zone Management for mechanical removal, so it's not a DIY situation — if a beach looks bad, it's usually a sign to move to another spot rather than wait it out
• Snorkeling: Floating sargassum generally stays at the surface and doesn't affect underwater visibility once you're in the water and below it
• If you have asthma or respiratory sensitivity: Give beaches with heavy decaying sargassum a wider berth than you might otherwise