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Atlantic Ocean off Florida spawns a giant Sargassum Blob

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By Rob Moir

· 6 min read

The Sargassum Blob, a vast expanse of sargassum weed the size of Texas, has been discovered far out in the Atlantic Ocean below the Sargasso Sea. Sargassum weed was not expected here for lack of nutrients. Climate change has played a part in nutrients coming from three directions to turn blue water into a golden floating rainforest threatening Florida beaches.

In the central Atlantic Ocean, there is a belt characterized by massive floating mats of seaweed called sargassum. The belt stretches from West Africa to the Gulf of Mexico, covering an area of approximately 3,417 square miles.

The Sargassum Blob

The Sargasso Sea is a sea unconstrained by land. To the north, it is bounded by currents to become a sea within the Atlantic Ocean. It’s known for vast floating mats of leafy marine algae with spherical float bladders that reminded Columbus of grapes, called Sargassum. This seaweed lives entirely away from land, unlike most other seaweeds with holdfasts that anchor it to the substrate.  

Sargassum has been discovered flourishing in a sea south of the Sargasso Sea. The Azores Current flows south where the African continent recedes from the Americas. Here, the current splits and goes in two directions. Some current flows West to encircle the bottom of the Sargasso Sea while some current flows Southeast to join the Westerly Equatorial Drift. Because the Earth spins to the East, the Equatorial Drift goes West. It is from this sea, formed by the Westerly flowing Azores Current on the north side and the Westerly Equatorial Current to the south, that the Sargassum Blob threatens Florida. 

Nutrient loading from three directions

This greensward on a sea below the Azores Current the size of Texas is the result of plant life finding nutrients in a previously barren seascape. In the tropical Atlantic Ocean with plenty of warmth and sunlight, climate change has dramatically increased nutrient loading from three directions. 

More nutrient-rich cold Arctic waters are flowing into the Atlantic Ocean due to the more open Arctic Ocean. In autumn, more sea ice is forming than ever before. When sea ice freezes, the ice is fresh so that the water may turn to ice. Salts left behind increase the density of cold water that is at the freezing temperature. This briny water sinks to displace water laterally. This is called thermohaline circulation. In the Denmark Strait between Iceland and Greenland, cold, nutrient-rich Arctic waters meet warm, nutrient-poor Atlantic waters and plunge down 11,000 feet to flow south into the Atlantic beneath the less dense seawater. As a result, Svalbard is suffering from more warm Gulf Stream water coursing back into the Arctic Ocean.  

From Africa, more nutrient-rich dust is blowing off the Sahara. The land dries as a result of long dry periods and more droughts. The prevailing Westerly winds carry more dust out over the Atlantic to seed the waters with more nutrients.  

Finally, there are more extreme weather events, more water sluicing to the sea along with the cutting of more forests, and more developments with excessive amounts of fertilizer. Sargassum does not discriminate between nutrient sources. There is more drawdown of carbon dioxide from the air and more carbohydrates for biomass and exudate to feed fungi, bacteria, and microbes. There are more Sargassum ecosystems flourishing in clear waters. 

A golden wildlife oasis

Frilly sargassum, a brown macroalgae, provides critical habitat and teams with life for a not-yet-fully-understood diversity of marine life. Sunlight streams through the lacey greens where there are no apparent shadows earning this place the name “golden floating rainforest of the Atlantic.” The sargassum fish, with the scientific name of Histrio histrio meaning stage actor, dresses the part with frilled appendages and the ability to change colors to match the seaweed. The planehead filefish with a flat head is, well, plainer. However, planehead fish are more agile, quick-moving, and more numerous than sargassum fish.

This is home for the sargassum swimming crab and the sargassum seahorse with a tail grasping leafy fronds to stay upright. In this sheltered bright green nursery, male seahorses brood eggs until they hatch, and young seahorses come out of the pouch.

In the “forest” dwell gold and emerald juvenile mahi-mahi, jacks, flying fishes, triggerfish, and juvenile loggerhead sea turtles. Larger predators patrol in the water below the drapes of seaweed including wahoo, amberjack, blue marlin, and sailfish

What’s good for the ocean may be bad for beaches

The good news is that the ocean is pulling more carbon out of the atmosphere than ever before, creating lots of biomass and entire ecosystems full of marine life. The bad news is when all that organic carbon arrives on Florida’s shores.  

When large quantities of Sargassum wash up on beaches, it can make the sand unsightly and cause a foul smell akin to rotten eggs, causing respiratory problems. The large mats of Sargassum can also make navigation difficult for boats and ships, particularly those with propellers. This can pose a safety hazard and increase the risk of accidents. Fishing derbies are canceled at great expense. 

Unfortunately, because it is always complex, there is more bad news. Corexit, a toxic forever chemical used as a dispersant to break up oil spills in the Gulf of Mexico, has been found in Sargassum weed. 

Sargassum coming to shore can negatively impact marine life. When the seaweed accumulates in large amounts, it can smother and kill sea grasses, other marine plants, essential habitats, and food sources for many species. Manatees in Florida are already starving due to the lack of seagrass smothered by harmful algal blooms and cut up by propellers. 

As Sargassum breaks down, it releases organic matter that rots and can lead to oxygen depletion in the water. This can create "dead zones" that harm fish and other marine life.

While Sargassum is a crucial component of ocean ecosystems, excessive amounts can negatively impact the environment and economy of coastal regions.

We will continue to see the unintended consequences of rising global temperatures and the use of harmful chemicals. It becomes even more apparent that everything is connected, from the Arctic Ocean, the Sahara Desert, and rivers to the Atlantic Ocean to the coast of Florida. 

Everyone must do their part to combat this climate crisis. From slowing water down, keeping water in our neighborhoods, not fertilizing lawns so the grass will pull down more carbon and build more soil, and pressuring our lawmakers to take big, bold actions, our efforts will make a world of difference on land, shore and sea.

illuminem Voices is a democratic space presenting the thoughts and opinions of leading Sustainability & Energy writers, their opinions do not necessarily represent those of illuminem.

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About the author

Rob Moir, PhD, is Executive Director of the Ocean River Institute and Director of Global Warming Solutions IE-PAC in Cambridge Massachusetts. He is an educator, scientist, and advocate with a proven history of institutional management and climate policy success.

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