The Secret Life of Sargassum

Anyone who goes to the beach in Florida is familiar with the seaweed sargassum. Sometimes big piles of it wash ashore and rot in the sun. You might hold your nose, step over it quickly and run to the water, but sargassum deserves a closer look.

These brown algae are very interesting biologically. Most marine algae are anchored to the sea floor with a holdfast. The two species we see on our beaches, Sargassum nathans and Sargassum fluitans, are free floating. Though their ancestors were attached, they spend their entire life floating. Small, gas-filled balls hold the algae up to the surface where it has plenty of sunshine for photosynthesis. With nothing keeping the sargassum in one place, it travels with the currents. It starts off in the Caribbean, and if it doesn’t wash up on a beach first, it rides the Gulf Stream up the east coast and into the Sargasso Sea in the North Atlantic Gyre. Here the algae eventually sink to the bottom and die.

When sargassum broke away from the sea floor, it lost the ability to reproduce sexually. It only reproduces by fragmenting, which is similar to rooting a plant from a cutting. No sex means that each of the two free-floating sargassum species is like an enormous organism. They divide, grow and form large floating rafts but have the same genetic information.

When sargassum left the bottom, it took a community of associated plants and animals with it. Marine life that once lived near the bottom can now be found in floating arks of sargassum far out to sea. Blair Witherington, a research biologist from Florida, says: “The open sea is like a desert, and sargassum is an oasis in that desert.” Hundreds of different animals, including favorites like mahi mahi and baby sea turtles, depend on sargassum. They find food and shelter there. Some breed in the sargassum. Many are camouflaged to look exactly like sargassum. Some organisms are found nowhere else.

Sargassum was once harvested from the ocean for livestock feed. The damage to this diverse ecosystem was too great, and now harvesting is strictly regulated.IMG_4583 Some communities keep their beaches tidy by raking away the sargassum that washes ashore. This has harmful, unintended consequences too. Piles of sargassum on the beach catch blowing sand and help build up dunes. Decomposing sargassum provides nutrients for growing dune vegetation in a place where nutrients are rare. Sargassum and the life in it provide food for crabs, insects and shorebirds.

Next time you go to the beach, take a closer look at the sargassum. You might find a surprise, like a crab that looks just like the sargassum it lives in. You will definitely find plastic. The currents that move sargassum around the ocean also move floating plastic. Remove those bottle caps, plastic forks and bits of rope tangled up in the sargassum before they are buried under sand or washed out to sea again. Grab a bag from the BlueTube at your beach. Pick up plastic and throw it away. Removing plastic from the beach is easy. Untangling it from sargassum in the ocean is impossible.