Some Caribbean countries and some parts of Florida are having problems with Sargassum seaweed. Sargassum is that golden/brown seaweed that washes up onto northeast Florida beaches when we have prolonged periods of winds out of the east, or when hurricanes travel along our eastern coast. The seaweed grows in the center of the North Atlantic, in an area called the Sargasso Sea. It is generally kept away from our shores by the Gulf Stream. However, strong winds can push the seaweed across the Gulf Stream’s currents, and then onto our beaches.

Golden tides

Starting in 2011, unusually large amounts of Sargassum have been washing up in the Caribbean, south Florida, and parts of the Gulf of Mexico. Municipalities in those locations have had to deal with tens of thousands of tons of seaweed. To learn more about these “golden tide” events, check out the bite-sized science webinar from July 7, “Seaweeds and nuisance algae.”

For this article, I would like to focus on some of the really cool characteristics of Sargassum as well as the unique animals that call it home. There are two species of Sargassum that grow as free-floating seaweeds. They have small gas-filled floats (called pneumatocysts) that help them maintain their position at the ocean’s surface. Ocean currents and wind help keep the Sargassum in one general area, most of the time!

Growing up in Bermuda, I looked forward to those times of year when fresh Sargassum would wash into our local bays and onto our beaches. As even a very young marine biologist, I knew that the seaweed would bring with it many unusual creatures. National Geographic magazine featured some of these in a 2019 article. (A free registration is required to see some of the images.)

Unusual fish

Many of the fish that live in the Sargassum are camouflaged to blend in perfectly with the golden-brown seaweed. Pipefish and the Sargassum frogfish are two of my favorites. I learned the hard way that the frogfish can eat prey that are its own size! Hundreds of other species of fish seek refuge underneath the mats of floating seaweed. Juvenile fish are often very difficult to identify by sight. It was always fun to catch some tiny fish and raise it in my fish tank at home in order to figure out what it would be! One of my strongest memories was of raising a small (1/2-inch) fish and realizing that it was a greater amberjack (which can get to weigh about 40 lbs!) We ended up giving that particular fish to the Bermuda Aquarium, where it took up residence in their huge reef tank.

Camouflaged invertebrates
a small golden-brown sea slug called a sea hare
A Sargassum sea hare (about an inch in length) is perfectly camouflaged for living in the seaweed.

There are also many invertebrates that live on the Sargassum. Sea hares (slugs), shrimp and crabs can be extremely well camouflaged! Looking closely at the blades and pneumatocysts, you might notice white lace-like encrusting patterns. These are structures made by animals called bryozoans. Tiny white spirals are the housings for spirorbid worms. A magnifying glass will make it easier to appreciate some of these tiny animals.

There is so much more to this amazing community than I can cover in this article. I will close by cautioning that Portuguese man-o-war often wash ashore with Sargassum, so be a little cautious when exploring the seaweed looking for cool finds!

This blog post is based on an article submitted to the Flagler News Tribune for the June 15 edition.


Posted: July 14, 2020

Category: Coasts & Marine, Natural Resources
Tags: Sargassum

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