Issue 28, Winter 2022

What is Sargassum?

As Christopher Columbus sailed his way across the northwest Atlantic Ocean in1492, he wrote in his logbook that his crew “saw much weed and very often,and it was vegetation from rocks and it came from a westerly direction; they judged themselves to be near land.” To his dismay, Columbus was nowhere near land; more than 500 years later, we know that weed as seaweed called Sargassum, named by Portuguese sailors who thought the seaweed’s floats resembled “little grapes,” or sargaço in Portuguese. The Sargassum we see in the northwest Atlantic is actually two different species that are similar in appearance: Sargassum natans and Sargassum fluitans (Figure 1). This free-floating brown algae is holopelagic, meaning it spends its entire life cycle at sea and is never anchored to the seafloor. Large expanses of Sargassum can be found east of the Gulf Stream and west of the Canary Islands, a region known as the Sargasso Sea (Figure 2).

Why is it important?

As many fishermen know, Sargassum converges to form dense, tangled mats that provide protection from predators for dozens of fish species.Some of these fishes are endemic to Sargassum, meaning they use Sargassum as habitat their entire lives. Other species only associate with Sargassum temporarily, typically as larval fishes. Perhaps the best example of a species endemic to Sargassum is the sargassum fish(Histrio histrio) (Figure 3), whereas gray triggerfish (Balistes capriscus)and greater amberjack (Seriola dumerili) only use Sargassum early in their life cycles (Figure 4). In addition to the multitude of fishes, Sargassum is critical habitat for numerous species of sea turtle, shrimp, crab, and even sea bird, who forage on Sargassum inhabitants.

The Great Atlantic Sargassum Belt

Clearly, Sargassum has long served an important role for a plethora of species.Sadly, though, it’s possible to have too much of a good thing. Until recently, most free-floating Sargassum was found in the Sargasso Sea and the Gulf of Mexico. However, starting in 2011, Sargassum started to appear in other parts of the Atlantic Ocean. Dubbed “The Great Atlantic Sargassum Belt,” this unprecedented Sargassum bloom heralded the arrival of a regime shift, or a large and sudden change to an ecosystem that persists for a long time

Sargassum in the Caribbean

Nowhere has this sudden increase in Sargassum been more evident than in the Caribbean. Since 2011, large regions of the Caribbean and Central Atlantic have been periodically inundated with Sargassum (Figure 6). The arrival of Sargassum in these areas is particularly problematic for island nations that rely heavily on tourism. Not only do huge mounds of Sargassum block beach access, but they also have an unpleasant smell and are incredibly costly to remove (Figure 7). For many islands, these impacts have been crippling.

A new normal?

What caused the regime shift that led to these recent changes in Sargassum dynamics? Most scientists suggest a combination of factors, including 1)warming ocean temperatures more hospitable to Sargassum growth, 2) upwelling of currents off West Africa, and 3) increased nutrient concentrations in Amazon River discharge. Regardless of the mechanism, the Caribbean now faces new cultural and economic challenges. These challenges have spurred the development of potential new uses for Sargassum, including animal feed,fertilizer, biofuel, and even construction material. Bricks formed from dried Sargassum termed “Sargablocks” have been used to construct houses along Mexico’s Yucatan Peninsula (Figure 8).

Time will tell if the Caribbean influxes of Sargassum are the new normal.Continued tracking and satellite monitoring of the Great Atlantic Sargassum Belt, coupled with innovative ways to commercialize Sargassum, are vital to controlling blooms of this critical habitat.

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