Ten years ago, scientists were shocked when a huge amount of seaweed appeared in tropical waters in the Atlantic Ocean. The area where it was found usually lacks the nutrients to feed such growth.
Years later, a group of U.S. researchers has identified one of the leading causes for the event: city and farm runoff carried by rivers to the ocean. This nutrient-charged outflow is just one of several likely suspects fueling an explosion of seaweed in warm waters of the Americas.
Scientists around the world say they suspect the cause involves a complex mix of climate change, Amazon rainforest destruction and dust blowing west from Africa’s Sahara Desert.
"There are probably multiple factors" driving the growth, said Ajit Subramaniam. He is an ocean expert at Columbia University.
"I would be surprised if there is one clear villain," he added.
Seaweed is a kind of plant that grows in the sea. The dark-brown seaweed is also known as sargassum.
A recent study examined the chemistry of sargassum from the 1980s up to 2019. The results of the study offered the strongest evidence yet that water coming from sewage and agricultural runoff played a major part in the expansion of the so-called Great Atlantic Sargassum Belt. The area now stretches for nearly 9,000 kilometers.
The study found that sargassum recently collected in coastal waters from Brazil to the southern United States contained levels of nitrogen that were 35 percent higher than in samples taken more than 30 years ago.
The study's findings appeared earlier this year in the publication Nature Communications.
Nitrogen is found in human and animal waste and in fertilizers. The results suggest that runoff flowing into rivers throughout the Americas and into the ocean is feeding sargassum growth. Ocean flows, or currents, carry much of this seaweed to the Caribbean Sea.
The researchers found that changes in the Amazon River have played a big part in the seaweed growth.
Brian Lapointe is one of the writers of the recent study. He is with Florida Atlantic University. He told Reuters that stronger rainstorms over the Amazon have led to more extreme flooding. This, he said, is likely pushing more nitrogen-rich runoff out to sea.
Experts note that extreme Amazon River flooding pushes a large amount of nutrients hundreds of kilometers out to sea during March and April. That is also when sargassum blooms appear. From there, currents push the seaweed around the coast of Venezuela into the Caribbean Sea. Sometimes, the seaweed makes it farther north into the Gulf of Mexico.
Climate change is also fueling stronger hurricanes. The ocean storms can pull more nutrients up from the seabed to possibly fertilize sargassum.
African dust and ash
Scientists have also suggested that dust from the Sahara Desert, along with smoke and ash, could play a part in the seaweed boom. As the particles are blown west over the Atlantic Ocean, they run into clouds. They then get rained down as iron and phosphorus deposits in the water.
It will take many more years of research to prove exactly how much each of these conditions may be contributing to sargassum's growth. But scientists say there are still things governments can do to make a difference.
"This phenomenon will continue until there is a change in public policy," said Carlos Noriega. He is an oceanographer at Brazil's Federal University of Pernambuco. Brazil, for example, could slow deforestation, which has led to a rise in cattle ranching. Raising cattle leads to loose soil, waste and fertilizer washing into rivers.
Noriega also noted the rising human population in Brazil's Amazon area. The five largest cities in the area have grown by nearly 900,000 people since 2010. Much of the region lacks effective sewage treatment.
"Treating sewage and stopping deforestation, Noriega said, “that's the only way to control it.”
I’m Bryan Lynn.
The Reuters news agency reported this story. Ashley Thompson adapted it for VOA Learning English. Susan Shand was the editor.
Words in This Story
tropical - adj. of, relating to or happening in the tropics
multiple - adj. more than one, several
villain - n. someone or something that is blamed for a specific problem
sewage - n. waste material
sample - n. a small amount that is taken to get information
fertilizer - n. a substance that is added to soil to help plants grow
hurricane - n. an extremely large and powerful storm
phenomenon - n. something interesting that can be observed and studied
region - n. part of a country or world that is different than others in some way