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; theyjudged themselves to be near land.” To his dismay, Columbus was nowherenear land; more than 500 years later, we know that weed as seaweed calledSargassum, named by Portuguese sailors who thought the seaweed’s floatsresembled “little grapes,” or sargaço in Portuguese. The Sargassum we see inthe northwest Atlantic is actually two different species that are similar inappearance: Sargassum natans and Sargassum fluitans (Figure 1). This free-floating brown algae is holopelagic, meaning it spends its entire life cycle at seaand is never anchored to the seafloor. Large expanses of Sargassum can befound east of the Gulf Stream and west of the Canary Islands, a region knownas the Sargasso Sea
As many fishermen know, Sargassum converges to form dense, tangledmats that provide protection from predators for dozens of fish species.Some of these fishes are endemic to Sargassum, meaning they useSargassum as habitat their entire lives. Other species only associate withSargassum temporarily, typically as larval fishes. Perhaps the bestexample 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 theirlife cycles (Figure 4). In addition to the multitude of fishes, Sargassum iscritical habitat for numerous species of sea turtle, shrimp, crab, and evensea bird, who forage on Sargassum inhabitants.
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 ofMexico. However, starting in 2011, Sargassum started to appear in other partsof the Atlantic Ocean. Dubbed “The Great Atlantic Sargassum Belt,” thisunprecedented Sargassum bloom heralded the arrival of a regime shift, or alarge and sudden change to an ecosystem that persists for a long time
Nowhere has this sudden increase in Sargassum been more evident thanin the Caribbean. Since 2011, large regions of the Caribbean and CentralAtlantic have been periodically inundated with Sargassum (Figure 6). Thearrival of Sargassum in these areas is particularly problematic for islandnations that rely heavily on tourism. Not only do huge mounds ofSargassum block beach access, but they also have an unpleasant smelland are incredibly costly to remove (Figure 7). For many islands, theseimpacts have been crippling.
What caused the regime shift that led to these recent changes in Sargassumdynamics? 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 concentrationsin Amazon River discharge. Regardless of the mechanism, the Caribbean nowfaces new cultural and economic challenges. These challenges have spurredthe development of potential new uses for Sargassum, including animal feed,fertilizer, biofuel, and even construction material. Bricks formed from driedSargassum termed “Sargablocks” have been used to construct houses alongMexico’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 SargassumBelt, coupled with innovative ways to commercialize Sargassum, are vital tocontrolling blooms of this critical habitat.
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