This is the world’s largest patch of seaweed. And it’s growing in an unexpected place

It’s a mystery of oceanic proportions: Sargassum—a brown seaweed that’s common in the North Atlantic—started blooming in a stretch of open ocean between Africa and South America in 2011, a place where it had previously been rare. The tropical blooms have grown in size since then, repeatedly sweeping ashore on Atlantic and Caribbean beaches (pictured), trapping nesting sea turtles and disrupting local tourism industries. In 2018, the tropical bloom reached a record 20 million tons—roughly 10 times the size of the 2011 bloom. Now, researchers think they may have solved the mystery of the suddenly surging seaweed.      

Tropical Sargassum blooms appear to have flourished in years when the central Atlantic Ocean was rife with nutrients, satellite records indicate. Some of the nutrients came from natural sources, the result of upwelling of nutrient-rich water along the western coast of Africa, the team of oceanographers reports today in Science. But some of them were deposited into the ocean by the Amazon River—a nutrient source that may to be on the rise due to forestry and agricultural activities. 

The oceanographers didn’t have direct measurements of nutrient levels for all of the years of the study. But they were able to infer nutrient levels by looking at how much chlorophyll a was in the surface waters, a plant pigment that’s found in phytoplankton and other algae and that reflects a higher proportion of green light than the surrounding ocean. In 2009, chlorophyll a levels spiked in the Amazon River plume—the part of the Atlantic Ocean that receives outflows from the Amazon—and they stayed elevated for most of the years that followed, possibly due to rising deforestation rates and fertilizer use upriver. The team thinks those years of high nutrient outflows may have helped to trigger the growth of Sargassum in a part of the ocean where it had never proliferated before.

More years of observation will be needed to get a better handle on what set of conditions—sea surface temperatures, nutrient levels, salinity—foster extra-large Sargassum blooms. But for now, the research team fears that waves of tropical Sargassum could be the new normal.