Sargassum seaweed floating in the Caribbean Sea.

(Zen Rial via Getty Images)

This year’s giant bloom of sargassum seaweed, which is estimated to be the largest ever, has started washing up on beaches in Florida and the Caribbean. Curious to learn more about why this is happening, I talked with Dr. Stephen Leatherman, aka “Dr. Beach,” a professor of Earth & Environment at Florida International University. Here’s our conversation.

Can you explain where sargassum comes from?

The sargassum seaweed that is washing up in mass amounts on South Florida beaches is carried from off the coast of Brazil by oceanic currents, finally arriving via the Gulf Stream to be blown ashore by the prevailing onshore winds. You can find the seeds (termed sea beans) within the sargassum seaweed that washes up on the beach as a drift line—these seeds came from trees in the Amazon River basin.

What's not so good about giant blobs of sargassum washing up on the Florida coast (and other coastal areas)?

Sargassum has always been found on South Florida and Caribbean beaches, but at manageable amounts until 2011. At Miami Beach, the seaweed was plowed under the sand and buried in the beach, where it naturally decayed out of sight and without any bad smell. Now that there is such a massive amount of seaweed arriving with the peak in June and July, this methodology no longer works.

Is there data that shows how coastal intrusion of sargassum has changed over the past decades?

The first year that the sargassum was arriving in massive enough amounts to be visible on NASA satellite imagery was 2011—the data are quite clear.

In your opinion, what might be causing the sudden increase in floating seaweed, and why? What do other scientists think is causing this problem?

Increased nutrients are being flushed into the ocean that feeds the sargassum seaweed, causing it to bloom (explode in amount). This article provides detailed information. The second reason is African dust, which also carries nutrients. Human pollution is the culprit.

What sort of ideas are scientists working on to address the sargassum blooms?

A number of scientists are trying to find some use of sargassum, but it can contain heavy metals such as arsenic and cadmium, making it unusable as livestock feed or human consumption.

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