Sargassum is a genus of brown macroalgae that are commonly known seaweed. The genus is best known for its drifting, free-floating species.
First reports of sargassum in the North Atlantic
Christopher Columbus was the first to report floating mats of Sargassum in the North Atlantic in the 15th century. With the help of their air-filled sacks that look like berries, the brown seaweed forms floating island-like masses in the ocean, sometimes covering entire sections of the Atlantic.
The seaweed blanket provides a specific habitat for many sea creatures such as fish, crabs, turtles, and even sea birds. Additionally, like other plants, the brown macroalgae produce oxygen via photosynthesis.
Lines of Sargassum can stretch for miles across the water surface. The algae are so impressive that the western edge of the central Atlantic Ocean – the Sargasso Sea– was named after them.
However, the floating algae do have a dark side. Sargassum mats can suffocate corals and seagrass, and the ultra-thick layers of rotting algae can cause smelly chaos on the Caribbean and the Gulf of Mexico’s beaches.
Although the blooms have been getting more impressive for a while, June 2018 brought a new Sargassum record. The algae formed a belt that extended for 8,850 kilometers – from the west coast of Africa into the Gulf of Mexico – weighing at least 20 million tons.
In 19 years of satellite tracking, this is the largest bloom yet. The massive bloom of 2018 is a part of a pattern.
A sudden and dramatic increase in seaweed first occurred in the summer of 2011, and with a 2013 exception, it has been repeating ever since.
How are the Seaweed Blooms Measured?
The study, published in the journal Science, was conducted by a diverse team lead by Chuanmin Hu of the University of South Florida (USF) College of Marine Science and Dr. Mengqiu Wang, a postdoctoral scholar in his Optical Oceanography Lab at USF.
They analyzed the 2000-2018 data from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS) and found that a possible shift in seaweed blooms had occurred around 2011.
Using remote sensing to map sargassum in the open ocean
The satellite tracking of algae works in an interesting way. The MODIS instruments capture the ocean in visible and infrared wavelengths.
Since they are plants that photosynthesize, despite their brown color, Sargassum algae contain a lot of chlorophyll. Chlorophyll shows as really bright at infrared wavelengths, and thus creates a sharp contrast to the darker water it floats on.
Why do the seaweed blooms occur?
The Sargassum blooms are one thing that can’t be blamed on the soaring ocean temperatures. The extreme ocean temperatures of 2013 led to a decrease in size of the algae mat, suggesting that the algae do not tolerate heat well. Still, that doesn’t mean that climate change and anthropogenic factors are not responsible for the blooms.
The team identified the factors essential for bloom formation: a large seed population in the winter, leftover from a previous bloom, and sufficient nutrient inputs- the natural upwelling along the coast of West Africa in winter, and the Amazon river’s discharge in the spring or summer.
Before 2011, Sargassum was primarily found in patches around the Gulf of Mexico and the Sargasso Sea. However, the proliferation of seaweed seems to be caused by nutrient influx from the two sources mentioned above.
The Amazon is increasingly deforested and exploited for the production of soy and cattle, which means hefty amounts of nutrients (organic waste, fertilizers, etc.) will enter the nearby streams and rivers, the Amazon river’s tributaries. It seems entirely plausible that the anthropogenic increase in available nutrients leads to an increase in algae mass.
“The ocean’s chemistry must have changed in order for the blooms to get so out of hand,” states Hu, and later warns that the results are still preliminary – more research is needed to confirm or disprove the nutrient hypothesis. Yet, according to the leading expert, it is quite certain the massive Sargassumblooms are here to stay.
Wang, M. et al. The great Atlantic Sargassum belt. Science 5 Jul 2019: Vol. 365, Issue 6448, pp. 83-87. DOI: 10.1126/science.aaw7912 https://science.sciencemag.org/content/365/6448/83
Gramling, C. The largest seaweed bloom ever detected spanned the Atlantic in 2018. Science News. 4 July 2019. https://www.sciencenews.org/article/largest-seaweed-bloom-ever-detected-atlantic-ocean-2018
NASA Satellites Find Biggest Seaweed Bloom in the World. NASA. 8 July 2019 https://www.nasa.gov/feature/goddard/2019/nasa-satellites-find-biggest-seaweed-bloom-in-the-world/