What is Sargassum Anyway?

So what's up with all of the Sargassum littering the Gulf of Mexico, the Caribbean islands, and even the west coast of Africa? Why is there so much and where does it come from? Ask locals and they'll contend that alongside an event in 2011, the continuous coastal onslaught of Sargassum piling high on beaches and in bays from the summer of 2014 to the current date is the most they have seen in their lifetimes.  With 2014 being the hottest year on record, algal biomass has seen increases in productivity previously unobserved that has spread south to Trinidad and even to Sierra Leone in west Africa.  Normally, higher summer temperatures spark moderate blooms of Sargassum that are distributed by summer current and wind patterns to islands in the northern Caribbean. So why has the current event spanning the past year been a stark contrast to those before and what are the larger impacts?

Algae are defined as photosynthetic organisms other than embrophyte land plants, fungi, or lichen and encompasses three groups: the green algae (Chlorophyta), red algae (Rhodophyta), and the brown algae (Phaeophyceae), the latter of which Sargassum is a part of.  Sargassum, sometimes referred to as gulfweed, can exist as attached coastal plants, that during growth and decay can be cast ashore under certain conditions, or can be free-floating and form extensive mats.  The two species found on beaches, S. fluitans and S. natans, are holopelagic, never attaching to a hard substrate in their entire life cycle and are the only algae to do this.  Enabled by gas-filled bladders, those little bubbles that are fun to pop, these species are able to float on top of the water to take in more light to enable photosynthesis. The body of the algae is robust, yet flexible to withstand strong currents for transport.

Sources of Sargassum

The incorrect, but common perception of the source of the piles of Sargassum on beaches is that algal growth occurs just offshore due to pollution and runoff, in a similar manner to how the algal blooms that shut down beaches after a big rain do.  While this does cause trouble on coasts all over the planet, these "red tides" and other events are caused by blooms of diatoms and dinoflagellates, which are not themselves dangerous, but are indicators of pollutants and excess nutrients in the water. However, the source of Sargassum is not local and is therefore not indicative of pollution in the areas it is found.

Scientists from the University of Southern Florida and the University of Southern Mississippi are tracking the movement of large algal bio masses.  Satellite data supports that for both the 2011 and 2014-2015 events, a portion of the Sargassum comes from the Sargasso Sea and a portion from the tropical Atlantic Ocean outside the mouths of the Amazon and Orinoco Rivers offshore of Brazil.

The Sargasso Sea exists in the Atlantic Ocean and is the only sea in the world that exists without land borders, contained by surrounding currents within the North Atlantic subtropical gyre.  This sea contains the two free-floating species of Sargassum, S. fluitans and S. natans, that form a mass of varying depth and connectivity that serves as a critical habitat for many marine species.  The algae itself is a food source for many herbivorous animals that are fed on by carnivores. It also serves as a nursery and shelter habitat for juvenile sea turtles, especially loggerheads, and fish such as marlin. For other organisms, the mat formed can be used as a spawning area and a refuge for migratory species.  The Sargasso Sea is a critical habitat for crabs, molluscs, shrimp, seahorses, European and American eels, and sea turtles.  It attracts species such as mahi mahi, amberjacks, jacks, tuna, dolphins, flying fishes, and triggerfishes.  A layering effect is present when examining the community structure of the algal mat formed: small fish such as triggerfish and filefish within the Sargassum, larger predatory fish just under the algae, and yet farther down even larger predatory organisms.   There are even 10 endemic organisms including the Sargassum Angler fish, Histrio histrio, that camouflages perfectly into the algae and has modified fins to overtake prey.  Notably, the Sargasso Sea plays a disproportionately large role in global carbon sequestration performed by oceans.  Portions of the Sargasso Sea are pulled out by the Coriolis Effect and transported to the Caribbean and Gulf of Mexico by the Gulf Stream and local currents.

The other portion of Sargassum distributed throughout the Caribbean comes from just outside the mouths of the Amazon and Orinoco Rivers off the coast of Brazil. Excess sediment and nutrients are causing blooms of Sargassum in the area that are then carried northward by the North Brazil, Guyana, and Antilles Currents. Once the Sargassum is brought to this region, local oceanic and meteorological conditions facilitate the spread in the same manner as the algae brought from the Sargasso Sea.

Effects of Excess Sargassum

An excess of Sargassum in coastal bays and on beaches has both positive and negative repercussions.  For fishermen, there are difficulties in launching boats from the shore over piles of algae and starting motors until clear of the seaweed.  However, it is beneficial that the Sargassum draws large predatory fishes that feed on the smaller species that live within the algae.  There is a large impact on tourism because tourists expect pristine white-sand beaches characteristic of the Caribbean, so hotels are responsible for disposing of large quantities of algae daily, which requires more personnel to remove.  The alternative is to allow for decomposition, but there is hydrogen sulfide released in the process that smells quite bad.  Additionally, trash circulating in the same currents as the algae accumulates on beaches and bays creating another task of sorting the organic and inorganic materials when disposing of the algae.  A significant positive effect is that Sargassum has the capability of cementing sand grains together, creating sand dunes that are more resistant to tropical storms and coastal erosion.  Additionally, algae can be used to create compost, mulch, and support existing vegetation.

Read more on the current state of global Sargassum at Sargasso Sea Alliance.