Using a 19-year record of satellite observations from NASA, Brian Lapointe, Ph.D., co-author and a research professor at Florida Atlantic University's Harbor Branch Oceanographic Institute who has studied Sargassum for more than four decades, is among a team of scientists from the University of South Florida and the Georgia Institute of Technology, who have discovered the world's largest macroalgae bloom.
The "Great Atlantic Sargassum Belt" extends from West Africa to the Gulf of Mexico. Last year, the massive 8,850-kilometer bloom contained more than 20 million tons of Sargassum biomass. This recurrent bloom and beaching events may just become the "new normal," according to a study published in Science.
In addition to satellite data, the team used environmental and field data to suggest that the belt forms seasonally in response to two key nutrient inputs: one human-derived, and one natural. In the spring and summer, Amazon River discharge adds nutrients to the ocean, and such discharged nutrients may have increased in recent years due to increased deforestation and fertilizer use. In the winter, upwelling off the West African coast delivers nutrients from deep waters to the ocean surface where the Sargassum grows.
To unravel the mystery, the team analyzed fertilizer consumption patterns in Brazil, Amazon deforestation rates, Amazon River discharge, two years of nitrogen and phosphorus measurements taken from the western parts of the central Atlantic Ocean near the Amazon discharge, among other ocean properties. While the data are preliminary, the pattern seems clear: the explosion in Sargassum correlates to increases in deforestation and fertilizer use, both of which have increased since 2010.
The conditions that appear to be associated with massive Sargassum blooms at magnitudes comparable to those in 2015 and 2018, are large seed populations during winter as a result of the previous year's bloom, high nutrient supply from the West Africa upwelling in winter months, and higher nutrient supply from the Amazon River input but normal or lower sea surface temperatures during the current years. If these conditions are met, then a massive bloom is likely to occur in the central Atlantic, followed by severe beaching events in the Caribbean Sea in later months.
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