In the north Atlantic, there is an enormous bloom of Sargassum seaweed. Of course, Sargassum in the Atlantic Ocean is nothing new. Christopher Columbus discovered floating mats of Sargassum in the northwestern Atlantic in the 15th century. These mats are an important part of the Atlantic ecosystem. They are hot spots of biodiversity, hosting fish, shrimp, crabs, birds, turtles and dolphins.

At the same time, something is changing. Since 2011, NASA satellite imagery shows that the annual bloom of Sargassum has gotten dramatically bigger. In the summer of 2018, mats of Sargassum extended more than five thousand miles in a Great Atalantic Sargassum Belt from western Africa to the Caribbean. This is the largest bloom of seaweed scientists have ever seen.

This humongous bloom of Sargassum is also causing significant harm. It is wreaking ecological and economic havoc along coastlines in the tropical Atlantic, the Caribbean Sea and the Gulf of Mexico. Vast quantities of the seaweed wash ashore and rot, releasing foul-smelling hydrogen sulfide gas.

While the usual patchy distribution of Sargassum mats benefits sea creatures, too much Sargassum, especialy in coastal areas, makes it hard for them to maove and breathe. When it dies and sinks it smothers coral and sea grass.

A 2019 study indentified increased nutrient runoff due to deforestation and fertilizer use in the Amazon river basin as one cause. Change in rainfall and ocean circulation due to human-caused global climate change is likely another. In other words, the seaweed bloom in the Atlantic is yet another facet of our growing environmental crisis.

Sources and Further Reading

• Gramling, C. The largest seaweed bloom ever detected spanned the Atlantic in 2018. Science News, July 4, 2019.
• Gray, E. NASA Satellites Find Biggest Seaweed Bloom in the World. nasa.gov, July 8, 2019.