Seaweed as far as the eye can see
Wang M, Hu C, Barnes BB, Mitchum G, Lapointe B, Montoya JP. 2019. The great Atlantic Sargassum belt. Science 365 (6448) 83-87. DOI: 10.1126/science.aaw7912
Sargasso Sea
Nearly every sea in the world is bordered by some landmass, all except one. The Sargasso Sea lies in the heart of the Atlantic Ocean, bordered by no land. The Sea is surrounded by four currents, creating what is known as a gyre.
In the 15th century, Christopher Columbus first documented the Sea and its defining feature: Sargassum seaweed. The sea is full of planktonic, or free-floating, brown algae. Two species in particular dominate the sea, Sargassum fluitans and Sargassum natans. Within the Atlantic Ocean these massive beds of seaweed act as a “hotspot for biodiversity and productivity” (Wang et al. 2019) attracting numerous species of fish, birds, and turtles.
Too much of a good thing
However, in the last decade distinctive ecological benefits of Sargassum are overshadowed (quite literally) by the tremendous amount of seaweed biomass in the Atlantic. Some years, the seaweed belt stretches from the shores of West Africa to the coastlines of the Americas. More and more frequently there are reports of beaching events of Sargassum from the Central Atlantic and Caribbean Sea.
The seaweeds wash up onshore and seize the beach. The Sargassum seizure of coastlines strikes with a foul smell, driving away tourists. Other species can also hitch a ride on the Sargassum such as invasive fish and cyanobacteria that may cause harmful algal blooms once the seaweed begins to decompose onshore. These beaching events have led to serious environmental, ecological, and economical problems.
To solve the problem, we must look back
Researchers led by Dr. Wang from the University of South Florida set out to determine what is causing the recent increase in Sargassum. By examining the mechanisms that drive the explosion of this brown seaweed, researchers aim to predict which years or seasons will have large blooms of the seaweed belt.
By using 20 years’ worth of satellite data, field measurements and predictive models, their findings suggest that the source of the recent blooms is not from the Sargasso Sea itself. Nor is it really connected to the North Atlantic or West Africa.
The best tools for predicting future blooms include details about development, previous year’s seed production, and temperature. Nutrient runoff from the Amazon River, agricultural practices, and sewage, promotes the growth of selective algae and cyanobacteria. Seeds, produced from the previous summer, quickly take up nutrients and grow early the following summer. High temperatures actually decrease seaweed production, especially on the Eastern Atlantic. Winter winds and temperature cause upwelling on the coast of Africa, promoting the nutrient uptake of Sargassum.
What’s next?
The blooms are caused by numerous factors and to solve the problem will more than likely take numerous solutions and numerous collaborations. The seaweed not only impacts the marine life, but also coastal environments, tourism, economies, and human health. By working together with multiple disciplines, Wang and colleagues hope to further understand how this seaweed went from a unique habitat to a troublesome pest.
