Expert Q&A: The Summer of Sargassum?

A 5,000-mile-wide mass of sargassum seaweed is drifting toward Florida's beaches. It could arrive, with its brown, slick leaves and rotten eggs smell, around July as the state's Atlantic beaches are filled with locals and tourists.  

Florida Tech oceanographer Kevin Johnson offered his insights into this looming arrival. Is it dangerous? Will it spoil the beaches? Here are his answers to some key questions.

QUESTION: Is this situation really a big deal?

Kevin Johnson: I wouldn’t say it is being overhyped, but it is yet to be determined whether this year will have greater amounts of sargassum in the Atlantic offshore, (visible by satellite) or coming ashore and covering the beaches, a connected but separate phenomenon requiring the cooperation of onshore winds. 2018 was a record-setting year, and this year could exceed what was seen in both locations in 2018. If the winds contribute strongly to sargassum coming ashore, it could make it smelly and hard to find an area of clean sand to set up on the beaches.

QUESTION: Is there a health risk if we head to a beach where sargassum is present?

Kevin Johnson: Sargassum doesn’t produce aerosolized organic toxins, but as it rots on the beach, it will smell and give off hydrogen sulfide gas, or H₂S. That's the stuff that smells like rotten eggs. When concentrated in enclosed areas, this gas is toxic and can be harmful. However, in the open air, diluted on an exposed beach, it is more of a smelly nuisance than a genuine health hazard for most people.

But there could be a minor irritation, like itching, after coming into contact with sargassum. There are symbiotic species living in and amongst this seaweed when it is healthy and floating at sea. (These intricate relationships are part of what makes sargassum ecologically significant as a food source, nursery and habitat for many populations.) One of the symbionts is a tiny branching colonial hydroid, related to corals. it grows on sargassum fronds. Hydroid colonies have a sting like corals and jellyfish, but it is generally not severe. Some people may experience itching around their feet or ankles (or any body part that contacts the sargassum hydroids) if they brush up against sargassum in waves or swash, or step on freshly deposited sargassum on the sand. Many people will not be bothered or only mildly troubled by this, but some people may be more sensitive to the hydroids.

QUESTION: I was planning to visit the beach. Should I reconsider now? 

Kevin Johnson: If the winds collude with the large bloom already underway to deliver huge mats to the beach, it could be hard for beachgoers to find a place to sit on the sand, and the smells could make the beach untenable. I emphasize “could” with the hydrogen sulfide smell because it depends on how the sargassum is deposited and how wet it is as it breaks down. In some cases, beached Sargassum can be very dry and in such cases it may not smell too bad. People who are more sensitive to the hydrogen sulfide or to the hydroids on the fresh sargassum may find the experience even more unpleasant. Westerly winds would be helpful from the perspective of keeping sargassum away from beaches, but unfortunately easterlies are pretty common this time of year.

If you're a reporter looking to know more about this topic, let us help with your coverage. Dr. Johnson can be available for phone, Zoom or, depending on scheduling, in-person interviews.