Atlantic Ocean off Florida Spawns a Giant Sargassum Blob Due to Climate Change and Nutrient Pollution

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 3,417,000 square miles.

The Sargasso Sea is to the north, bounded by currents to become a sea within the Atlantic Ocean. It is 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.  

The sargassum provides a habitat for diverse marine life, an oasis in a nutrient-poor sea. Sargassum shelters mahi-mahi, tuna, and young sea turtles and houses stay-at-home seahorses, fish, crabs, and invertebrates that have evolved to resemble Sargassum. 

Sargassum has recently flourished in a body of water separate from the Sargasso Sea.  The Azores Current flows south to 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, and 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.

This direct result of climate change dramatically increases the nutrient loading into this portion of the Atlantic Ocean from three directions. More nutrient-rich cold Arctic waters are flowing into the Atlantic Ocean due to the more open Arctic Ocean. When sea ice forms, it consists of fresh water. Salts left behind make the cold water around the ice denser. 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 beneath the less dense seawater. 

In Sahara, Africa, climate change has brought longer dry periods.  The land dries, and the prevailing Westerly winds carry more dust out over the Atlantic to seed the waters with more nutrients.  

Finally, more extreme weather events, the cutting of forests, and excessive amounts of fertilizer cause significantly more nutrients to wash down rivers and off the land into the sea. And the green sargassum grows all about.

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.  

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. 

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. 

Sargassum blooms coming to shore can negatively impact marine life, particularly sea turtles. When the seaweed accumulates in large amounts, it can trap baby sea turtles trying to make their way to the ocean. It can also smother and kill sea grasses, other marine plants, essential habitats, and food sources for many species.

As Sargassum breaks down, it releases organic matter that 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.

As we 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 the climate crisis. From not fertilizing your lawn to pressuring our lawmakers to take big, bold action to reduce emissions and increase the drawing down of carbon, our efforts will make a world of difference.  

Our Partners

Frontiers in Marine Science

ECO Magazine is a marine science trade publication committed to bringing scientists and professionals the latest ground-breaking research, industry news, and job opportunities from around the world.


8502 SW Kansas Ave
Stuart, FL 34997


Newsletter Signup

The ECO Newsletter is a weekly email featuring the Top 10 stories of the past seven days, providing readers with a convenient way to stay abreast on the latest ocean science and industry news.