Written by Jahson Alemu I

Research Officer

Coral Reef Ecology

“What is it?” Where did it come from?”, “Why so much of it?”, “What can we do to get rid of it?” “Is this pollution?” Since May 2011, the IMA has been receiving calls about large amounts of a brown smelly mass that has been washing ashore the beaches of Tobago.  This is Sargassum, a brown algae (seaweed) which comes from the Sargasso Sea.
In the middle of the Atlantic Ocean floats a 3,520,000 km.² mat of floating seaweed.  This is the Sargasso Sea.  It was originally named “Sargaco” by Christopher Columbus, because of the resemblance to little grapes, of the air bladders that keep the seaweed afloat.  Sargaco eventually became Sargasso, the area - the Sargasso Sea, and the weed Sargassum.

Sargasso Sea, Northeastern Atlantic

The Sargasso Sea is bounded on the west by the Gulf Stream; on the north, by the North Atlantic Current; on the east, by the Canary Current; and on the south, by the North Atlantic Equatorial Current. This system of currents forms the North Atlantic Subtropical Gyre.
The main source of the sargassum found floating throughout the Gulf of Mexico, Atlantic Ocean and the Caribbean, is the Sargasso Sea.  The species being observed on our shores is a brown, macro-algae called Sargussum fluitans, often found in association with Sargassum natans, both native to the Caribbean and both commonly called sargassum seaweed. Unlike most other Sargassum species, they are pelagic (free-floating) and have never been attached to the seafloor during their lifecycle.

Sargassum seaweed (Sargassum sp.)

Even though the prevailing currents surrounding the Sargasso Sea work to keep the seaweed centrally located, winds, storms and spiraling currents help disperse sargassum throughout the world's oceans.  Generally, this is a natural phenomenon that occurs cyclically, but the right combination of winds, waves and currents bring the sargassum closer to shore.  It is not unusual to see small pieces of sargassum float up on Caribbean beaches particularly in July or August.
This year, however, the amount deposited was unusual - clumps of sargassum at densities not previously recorded washed up on shores throughout the Lesser Antilles, southern Caribbean and the Bahamas.  The hypothesis is that in May/ June 2011, currents were unusually strong due largely to several storms in the region.  As currents intersected with the edges of the Sargasso Sea, parts of the sargassum mat broke off.  The currents then carried the weed along with them, eventually sweeping it eastwards into the Caribbean islands where local currents carried it ashore.  Most beaches on the Atlantic coast of Tobago looked similar to the picture featured here of Little Rockley Bay.  Unfortunately, sargassum is slow to decompose resulting in foul smells and aesthetically unpleasant to beach goers.  
The sargassum ecosystem is one of the most unique and important in the world. Floating sargassum mats provide food, shelter and protection for a wide diversity of animals and plants, including species of algae, nudibranchs (sea slugs), shrimps, crabs, tubeworms, fish and other invertebrates, some of which are endemic (not found elsewhere) to the sargassum ecosystem. The mats also serve as rich nursery areas for a large variety of juvenile sea turtles and fishes that are associated with the mats.  This diverse nursery ecosystem also attracts important commercial and game fish. Ashore, the inhabitants of the sargassum mats serve as an important food source for many seabirds and crustaceans.  The decomposing algae also add nutrients to the beach.  This decomposition is also responsible for the foul odour that has offended many a beach stroller.
The mats of Sargassum weed washing up on Tobago’s (and other Caribbean island) shores is a natural seasonal phenomenon, and not a result of pollution. It should not be cause for alarm. This seaweed will eventually break-down (a life span of ~1 yr) or be taken away from shore by wave action. In areas where it is aesthetically unpleasant, it can be trucked away to an appropriate landfill.

Sargassum-covered Little Rockley Bay, Tobago
Photo: Dave Elliot

This article was published by the Trinidad Express Newspapers on Thursday 22nd September 2011.