Massive Sargassum Seaweed Bloom is Choking The Caribbean — Climate Change a Likely Culprit

According to Caribbean leaders, it’s a disaster that will take at least 100,000 people and 120 million dollars to clean up. And disaster may not be the best word to describe it — for an enormous Caribbean beach and water choking bloom of sargassum algae may be a new abnormal ocean condition. Yet one more dangerous upshot of a warming world.


(Great, sulfur-stinking mats of sargassum algae are now choking the beaches and near-shore waters of the Caribbean. In some places the mats are 10 feet deep. These great piles of seaweed can foul beaches, kill off native species, and result in ocean dead zones when they rob waters of nutrients and then die off — pulling life-giving oxygen out of the water by decomposition. Image source: Mission Blue.)

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A Legend of the Ancient Mariners

The story, in this case, begins with an enormous mat of algae called the Sargasso Sea. This vast collection of organisms has at its foundation two forms of algae that produce inter-connected floating masses of seaweed. The mats collect and link together in an Atlantic Ocean Gyre — forming a vast region off the United States Coast.

Ancient sailors crossing the Atlantic during the dawn of North American colonization often passed through the Sargasso Sea. It tended to be a notable feature of their travels as the floating mats were sometimes dense enough to halt the progress of vessels.

1891 Map of the extent of low and high concentration Sargasso seaweed

(An 1891 map proved by NOAA shows the regions of low and high concentration sargassum seaweed in the North Atlantic and Caribbean. Image source: NOAA — Teachers at Sea.)

For hundreds of years the enormous collection remained a mystery. But by the 20th Century researchers had found that the seaweed was transported by the Gulf Stream from the Gulf of Mexico and Carribean into an area just south of Bermuda. There it bloomed as it fed on nutrient-laden run-off spreading outward from the large estuaries of the North American Continent. The sargassum then efficiently recycled these nutrients to support a vital community of hundreds of sea creatures and birds.

The Sargassum algae that make up the Sargasso Sea are not only native to this region. It ranges the tropical and subtropical zones of the Atlantic — blooming wherever there is warmth and nutrients to support it. Lately, there have been signs that biodiversity in the Sargasso Sea is falling. Recent research expeditions are noting fewer and fewer of the species traditionally supported by the sargassum mats. It’s a potential sign of failing ocean health. One that is, perhaps, linked to the massive accumulation of sargassum in the Carribean during recent years.

(Big changes in the Sargasso Sea. MBARI expedition finds lower biodiversity in the sargassum mats. Video source: MBARI/Youtube.)

Ocean Sargassum Fertilization in a Warming World

Due to its highly efficient use of nutrients, pelagic sargassum thrives in warm, well-fertilized waters. And lately, as the Earth has warmed, run-off into the Atlantic Ocean habitats of the sargassum has increased. Added heat in the atmosphere has resulted in greater instances of heavy downpours. These downpours increase erosion — flushing more nutrients into streams and rivers.

In addition, fertilizer-based farming industry leaves soil laden with phosphates and nitrogen. So the heavier downpours are now raining over lands that are artificially loaded with nutrient. Adding to the fertilizer flush is a constant rain of nitrogen particle fallout from an immense and global burning of fossil fuels over the world’s waters — a third new source of nutrient that wasn’t there for the sargassum to access before. Finally, an added warmth in the surface waters due to greenhouse gasses forcing the world to heat up by 1 degree Celsius over the past 135 years creates a yet more ideal environment for the sargassum to grow and bloom.

Reports now indicate that much of the seaweed choking off the Caribbean’s beaches and waters is issuing from a region east of the Amazon River outflow. These reports hint that deforestation, a resulting increase in erosion of Amazon Rainforest soils, and the rise of industrialized farming in Brazil may also be playing a role in the current epic bloom. Finally, there is growing evidence that the Gulf Stream current — a transporter of sargassum out of the Carribean and Gulf of Mexico may be slowing down as thermo-haline circulation weakens. All these factors — the warming waters, the increased nutrient loading of the surface waters, and the reduction of sargassum transport due to Gulf Stream slowing — combined hint at a sargassum seaweed train wreck whose epicenter is the Caribbean Sea.

Caribbean Beaches, Ocean Life Under Threat

Over recent years, it’s thought that these factors combined to help generate a massive bloom of sargassum in the Caribbean. As early as Fall of 2014 reports had been trickling in of 3-4 foot thick mats collecting along Caribbean coastlines and piling up on beaches. By August of 2015 the mats have grown to as dense as 10 feet thick. Now vast swaths of beaches are covered in the sulfur-stink of this great pile of dying biomass.

Typically sargassum is a vital part of the life-giving system of the Atlantic Ocean. Numerous species of fish, including tuna and jacks, rely on the food provided by the prolific algae. Birds, turtles, and scores of invertebrates also rely on the algae in one way or another. But when the algae becomes too prolific it turns from boon into curse. Sea turtle nests become fouled with the stuff. New hatchlings often are unable to clamber through the dense piles to reach the sea. The dense tangles reduce the mobility of larger animals including sharks, rays, and adult turtles. And when the piles become too thick large sections of the sargassum are cut off from light and nutrients. The result is that the large masses can contain oxygen deprived zones where the dead matter decays. These little pockets host hydrogen sulfide and other sulfur producing bacteria — further toxifying the waters and resulting in the now prevalent reports of a ‘rotten eggs’ smell near the sargassum piles.

(“From the surface, it looks bad. But could you imagine if you were a fish?” Dave Eliot goes underwater to take a look at these climate-change enhanced algae blooms. Video source: YouTube.)

For Caribbean Island nations, who rely so much on their pristine beaches and ocean habitats as a source of economic stability, the amazing accumulation of sargassum is a disaster. Today Sir Hilary Beckles of the University of the West Indies called on the international community for aid saying:

“Herein is an endemic and systemic threat to the resilience and development of these nations and therefore we must have an international response to this… What you are looking at is maybe US$120 million . . . and probably we would have to deploy over 100,000 people to carry out a similar strategy across the Caribbean space to make our beaches available to those who wish to use them for their multiple purposes… We must show our children enjoying our beaches and give visitors the assurance that the weed is not killing us and that life goes on. We must let people know that we in the Caribbean are not sitting on our hands but trying to find solutions to the threat presented by the Sargassum weed.”

But, as with so many of the disasters cropping up these days — simply reacting to the symptoms (be it sargassum, or drought, or flood, or mass migration, or sea level rise, or wildfires, or species endangerment, or a thousand other issues related to human fossil fuel emissions and a great heating of the atmosphere and oceans) does not address the root cause. And for that you need a rapid cessation of fossil fuel burning.


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Hat tip to Andy in San Diego

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