Monday, January 22, 2018

The mysteries of Sargassum


NATURAL OCCURRENCE: Mats of the brown seaweed, Sargassum, cover Pinfold Bay, Tobago, April 2014. —Photo and article submitted by Jahson Alemu I

Mark Fraser

 Over the last month many fishermen in Trinidad and Tobago have been wading through a cold thick mass of seaweed and saltwater in the early hours of the morning before hauling themselves onto their boats to head out to sea.  

Many of you may remember the washing ashore of large mats of the brown floating marine algae (called Sargassum) in mid-2011 blanketing our Atlantic coasts. This is a natural occurrence affecting the Atlantic shores of most islands in the Lesser Antilles, southern Caribbean and Bahamas; with some years known to have larger amounts of Sargassum washing ashore than others. Three years later, we are again seeing the same phenomenon—the arrival of large amounts of Sargassum on our shores. Several beach-goers have reported beaches being covered in Sargassum up to 0.6m (2ft) thick. In Tobago these beaches include Pinfold Bay, Barbados Bay, Speyside, Kilgwyn, King’s Bay, Goldsborough, Hope Beach, Little Rockley Bay and even Scarborough. In Trinidad, many popular beaches on the eastern coast have also been affected such as at Mayaro, Guayaguayare and Manzanilla.  

The main source of the Sargassum found throughout the Gulf of Mexico, Atlantic Ocean and Caribbean is the Sargasso Sea; a region in the middle of the Northern Atlantic Ocean with a mat of Sargassum approximately 3,520,000 km² in size; an area three times the size of South Africa! This expanse is called the Sargasso Sea; originally named by Christopher Columbus as “Sargaco”, due to the resemblance of the air bladders on the seaweed that keep it afloat, to little grapes. 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.

These prevailing currents surrounding the Sargasso Sea work to keep the Sargassum centrally located in the Sargasso Sea. However, during this time of year, strong prevailing winds, storm activity and spiralling currents disperse the weed throughout the region. As currents intersect with the edges of the Sargasso Sea, parts of the larger Sargassum mat are broken away. The currents carry the seaweed along, eventually sweeping it westwards towards the Caribbean islands where local currents wash it ashore. This is a natural phenomenon that occurs cyclically. Unfortunately, once ashore Sargassum is slow to decompose resulting in foul smells and it is aesthetically unpleasant to beach-goers.  

Several types of Sargassum exist, but the species washing up on our shores is a brown, macro-algae called Sargussum fluitans (sargassum seaweed), often found in association with Sargasso weed (Sargassum natans), both native to the Caribbean. Unlike most other Sargassum species, they have never been attached to the seafloor and spend their entire life afloat drifting with ocean currents. These floating mats of Sargassum serve as homes to a wide variety of animals, including species of crabs, nudibranchs, shrimp and fish, some of which are endemic to the floating mats. It also serves as a rich nursery area for over 100 species of fish and protection for juvenile sea turtles. Even sea birds feed and rest on the mats, while this floating ecosystem also attracts important commercial and game fish exploited by humans, such as dolphinfish or mahi-mahi, marlin and tuna. 

In summary, the mats of Sargassum weed washing up on Caribbean shores are a natural phenomenon and not a direct result of pollution; and the Sargassum ecosystem is one of the most unique and important in the world. The Sargassum will eventually break-down or be taken away by wave action, but in areas where it is aesthetically unpleasant, it can be trucked away to an appropriate landfill area.