Wildlife

Massive Seaweed Bloom

For eight years, thick mats of seaweed have smothered coral reefs, trapped sea turtles and brought economic instability to coastal communities as reddish-brown gobs of foul-smelling sargassum wash onto beaches along the Caribbean Sea, Gulf of Mexico and tropical Atlantic.

These phenomena are symptoms of a massive seaweed bloom scientists are calling the Great Atlantic Sargassum Belt. Stretching up to 5,500 miles (8,850 kilometers) from the Gulf of Mexico to just off the coast of western Africa, the Great Atlantic Sargassum Belt appears to be the product of natural and human-caused factors.

The bloom began around 2009 when discharge from the Amazon River brought unusually high levels of nutrients into the Atlantic Ocean. Upwelling of nutrient-rich water off the west coast of Africa in the winter of 2010 further enriched surface waters with deep-sea nutrients; that upwelling also lowered temperatures of that surface water, allowing sargassum to thrive in the summer of 2011.

A similar combination of factors led to especially large blooms in 2014, 2015 and 2017. The largest recorded bloom occurred in 2018, when the Great Atlantic Sargassum Belt grew to a mass of more than 20 million metric tons. The high levels of nutrients from the Amazon River come from deforestation and fertilizer use in the Amazon basin.

Under normal circumstances, sargassum provides critical habitat for marine life. The seaweed oases attract fish, birds and sea turtles as well. Dolphins and sea turtles also benefit from the tiny patches of life floating in the open ocean, but thick mats of sargassum pose big problems for some wildlife and coastal communities. Sea turtles sometimes can't swim through the dense mats to return to open water after laying their eggs.