Each summer, vast quantities of Sargassum seaweed spread across the tropical Atlantic and foul the coasts of the Caribbean Islands, the Gulf of Mexico, and northern South America. The Sargassum is not only bad for tourism, it also disrupts ecosystems by providing massive amounts of food for many marine species, upsetting the balance of those systems.
Unusually massive Sargassum blooms have been occurring since 2011. This year saw the largest mass yet, with about 38 million tons of Sargassum drifting towards Atlantic coasts.
There have been many theories put forth about the cause of the increase. It is associated with increased amounts of nitrogen and phosphorous that stimulated the growth of the brown algae. It was thought that this came about because of overfertilization runoff, rainforest deforestation, or perhaps iron-rich Saharan dust blowing in from Africa.
Recently, a study by researchers at the Max-Planck Institute has determined that the main driver of the Sargassum explosion has been nitrogen fixation stimulating the growth of symbiotic cyanobacteria that in turn benefits the Sargassum. The reason for the excess nitrogen is strong wind-driven upwelling in the equatorial Atlantic, driving phosphorous to the surface where it benefits the cyanobacteria that grows on the Sargassum. Studies of cores taken from corals revealed that this process had increased strongly at the time when the Sargassum blooms intensified.
Understanding the mechanism driving the massive Sargassum blooms can help to improve the prediction of the magnitude of future blooms.
