Strange Seaweed: The Rise of Sargassum

Sargassum seaweed has been called quite a few names in its forty million year existence: “the golden rainforest of the ocean,” a “haven of biodiversity,” the “Monster Seaweed” and most recently “the greatest single threat to the Caribbean economy.”

In the summer of 2011, large “wracks” of seaweed began to drift into the Caribbean, landing with much fanfare on its beaches for the first time. The New York Times documented this mysterious and unprecedented “invasion,” offering little clarity whether it would be a one-time phenomenon. Was this an exceptional event or did it represent a new regime shift in how ocean currents operate?

After continued landings in the Caribbean between after 2011, in the summer of 2015 researchers tracking sargassum seaweed by satellite described the coverage in the Caribbean and eastern Mexico as the largest in history. Chuanmin Hu, University of South Florida professor, calculated that there were 12,300 square miles of sargassum in the Caribbean, an area about the size of Maryland, nearly five times the recorded amount in 2011. Many experts are now convinced that the once sporadic landings have become the “new normal.”

What exactly is happening?

Theories explaining the sargassum landings are almost as abundant as the seaweed itself. According to different sources, sargassum landings are due, to varying degrees, to global climate change, changing ocean currents, El Nino weather events, Saharan dust clouds, nutrients from Brazilian-Amazon farms, or dispersants used during the BP oil spill clean-up. Each idea shares elements of modern science fiction but all are within the realm of scientific plausibility.

Sargassum seaweed is not just an unwelcome but relatively harmless visitor in the Caribbean and Mexico. The region’s tourism industry has suffered significant lost reservations, devastating publicity (Washington Post, Daily Mail, New York Magazine, Linkedin, Smithsonian), and hostile reviews on Tripadvisor and other online booking sites due directly to sargassum. Visitors have been surprised and more often horrified by unswimmable beaches decorated with mountainous piles of stinking, decaying seaweed that overwhelm all efforts to remove it. Governments and tourism professionals have described the seaweed arrivals as “a threat to our regional economy;” some countries have even officially declared it a national disaster.

Ironically, when it is contained in its normal range in the Sargasso Sea, and not being vilified as an invading monster terrorizing the Caribbean and Mexico, sargassum is a unique and fascinating floating habitat that has enormous economic, ecological and even global benefits. The Sargasso Sea Commission was even formed in 2014 to protect the vital habitat and to develop conservation management strategies to minimize human impacts on it.

The Sargasso Sea, at 2-million-square-miles, is roughly the size of the United States. Positioned in the North Atlantic gyre, the sea is a vast floating island defined by ocean currents rather than by land boundaries. Encircling currents form a “gyre” that traps water (as well as plastics and pollutants) at the core of the Sargasso Sea for estimated periods of up to 50 years.

The Sea is immensely productive, serving as home and spawning area to 10 endemic species (those found only in the floating masses of sargassum), 145 invertebrates and 127 fish species. While the incoming red slick of seaweed approaching touristic beaches can be alarming to hotel managers, when looked at under a microscope, many of the species found hidden in the mats of seaweed are intricately evolved to camouflage in their habitat and can be quite exquisite.

Photo courtesy LookBermuda

The Sea is also an important nursery or migratory feeding station for numerous endangered and/or commercially important marine species, wahoo, tuna, sharks, sperm and humpback whales, sea turtles, and numerous migratory bird species among them. Recent studies found that endangered American eels and flying fish both use the sea for part of their life cycle.

The Sargassum Sea is also suspected to be an important player in the global ocean sequestration of carbon. Owing to complex physicochemical processes, sargassum is considered a strong net carbon sink and may be a major factor in controlling the concentration of carbon dioxide in the lower atmosphere, impacting the global climate system. As human caused climate change accelerates, this type of sink could become increasingly important.

The two species of sargassum that have most troubled the Caribbean (S. fluitans and S. natans) are holopelagic, meaning they reproduce on the high seas, as opposed to other seaweed species that start their life cycle at the bottom of the ocean. Sargassum feeds on nutrients it finds in the water column and the massive floating rafts of seaweed continue to grow and expand.

This unique mobility and capacity to expand while on the move provides a distinct clue as to why sargassum has become such a problem in the Caribbean. However it is finding its way out of the Sargasso Sea, whether by wind, currents or storm events, sargassum is finding food in the form of abundant nutrients. This food source varies from agricultural or sewage run-off, sediment from rivers, fertilizers used on golf courses or chemical dispersants from the BP oil spill. As long as it finds nutrients for food, the seaweed will continue to grow. Researchers in Texas, for example, have documented masses of seaweed continuing to expand while floating in ocean currents and even suspended along the coast in front of local beaches.

The Mexican government has confronted the seaweed problem with an almost military response, deploying the navy to find seaweed in the open ocean and investing over $9 million dollars in an army of beach cleaners. With vast stretches of beaches dedicated to tourism, Mexico cannot afford to lose tourists to seaweed. The language the Mexican government uses publicly even evokes thoughts of a war in which they are combatting an “invading” enemy that is “terrorizing” their tourism and beaches that must be “aggressively confronted.”

The Caribbean has been invaded not only by seaweed but also by a cottage industry of equipment vendors, well-intentioned entrepreneurs, and instant experts, all armed with solutions to efficiently rake seaweed off the beaches, convert seaweed into some useful product, or protect beaches from sargassum with floating oil boom lines. Several Caribbean islands have called for the creation of a regional emergency commission on seaweed and the Caribbean Hotel and Tourism Association has published a guide for best practices to manage sargassum on the hotel beaches. Numerous resorts have installed floating barriers that minimize the arrival of sargassum to their beaches. All of the solutions proposed from Mexico to the Caribbean are time-consuming, labor intensive, expensive, and most likely, temporary.

What is perhaps most striking about the sargassum phenomenon is that this type of rapid change may in fact be the new normal. Sargassum arrived quickly and without warning. It has proven difficult to predict and control. It has exacted devastating impacts on the local economies and ecosystems of the Caribbean and Mexico. And most important, it is caused by a complex, wide-ranging and often shifting set of factors that make preventing it challenging or even impossible.

Whether the current landings will be a long-term problem related to global climate change or simply represent a short-term environmental challenge, the sargassum “invasion” is representative of other complex environmental challenges humans are currently facing, and will increasingly be forced to confront in the future. In the coming years we will be embarking on a great experiment learning to adapt to rapid global change, like strange seaweed, as part of the new normal.


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