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New ‘visitor’ on our shores: Sargassum on Sanibel’s beaches

By Staff | Jul 16, 2009

There have been recent accumulations of brown algae on Sanibel beaches (it has also been reported in Boca Grande). SCCF’s Marine Lab and the City of Sanibel have identified it as Sargassum; it is not red drift algae. Sargassum moves in floating mats on the ocean’s surface.

The City of Sanibel surveyed the beach from Lighthouse Beach to Bowman’s this past weekend, noting the presence of Sargassum both in the water and on the beach. Both the Sargassum and this past June’s accumulation of manatee grass on Southwest Florida beaches may be related to storm events and larger regional issues.

The floating mats of Sargassum weed that recently made landfall on island beaches traveled by ocean currents until winds blew them ashore. Most Sargassum weed originates in the Sargasso Sea of the North Atlantic Ocean.

Pushed by four different currents, including the Gulfstream, the plants float with the help of small, air-filled structures. Along the way, waves break this algae apart, the parts multiply asexually, and the Gulf’s warm waters encourage rapid growth. As the beached weed gets pushed further up into the shoreline, it is covered by sand.

Sargassum weed is like a floating hotel and nursery to a myriad of other organisms. Small sea turtles swim near the seaweed looking for cover and an easy meal of the shrimp and crabs that live there. The loggerhead turtles now hatching on island beaches swim thousands of miles to the Sargasso Sea spending as many as 11 years feeding on jellyfish, snails, crabs and shrimps protected from predators in the Sargassum weed. It is breakfast, lunch, dinner and home to them.

A careful examination of the beached Sargassum may also reveal a nudibranch still in residence. Sea Hares live attached to floating Sargassum weed, feeding on small plant-like animals called hydroids that grow there. Many of the animals that live in this goldish green seaweed have evolved into shapes and colors resembling their host. If you visit the touch tank at SCCF, you will find the spider and hermit crabs hiding in and munching on the floating Sargassum weed.