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But the scale of recent coastal influxes is unprecedented. And since the 2011 event, they have occurred every year except for 2013. Gluts of coastal seaweed have a damaging influence on the coastal environment. In large quantities, the seaweed strips oxygen from the water, killing fish and seagrasses that offer key habitat for many species. It may reduce sunlight needed by ocean plants and smother shallow coral reefs, like those in the Florida Keys. In 2018 NASA satellites revealed the largest marine algae bloom in the world. A belt of sargassum that contained over 22 million tons of seaweed stretched some 5,500 miles across the Atlantic to West Africa. Satellite images are showing abnormally high amounts again in 2021. Warmer, overnutrified waters Data gathered over the past decade has revealed the likely causes of these seaweed invasions: Saharan dust clouds, warming temperatures and the growing human nitrogen footprint. Just as nutrients feed red tide blooms, they feed sargassum, which thrives in warmer water. Climate change also increases upwelling of nutrients from deep ocean waters at the other end of the sargassum belt in West Africa. The influxes of the past decade seem to have originated along Brazil’s Atlantic coast, not in the Sargasso Sea. Large amounts of fertilizer flow into the Amazon River and then to the ocean from industrial-scale agriculture and ranches. Nutrients also pour into the Gulf of Mexico from the Mississippi River. Climate change-driven downpours increase runoff. Saharan dust clouds that extend for thousands of miles across the Atlantic Ocean have also contributed to this explosion of sargassum seaweed. The dust contains iron, nitrogen and phosphorus that fertilizes plankton and seaweed blooms. These thick atmospheric dust plumes corresponded with a sargassum spike in 2015 and the worst incursion of sargassum in 2018. Researchers are also exploring changing in ocean currents, which may be another contributing factor. Economic and ecological harm Sargassum threatens tourism, a major economic engine for the Caribbean and Florida. Mexico has deployed Navy ships for cleanup operations in Cancun. Some Caribbean destinations have installed floating barriers, like those used in oil spills, to keep seaweed offshore. In 2019 Prime Minister Mia Amor Mottley of Barbados likened the scale of the economic fallout to that of a hurricane. There is currently no good way to dispose of such great volumes of seaweed. It’s labor-intensive and expensive. Removing sargassum from 15 miles of Miami-Dade beaches cost $45 million in 2019. Some communities plow seaweed under the sand. Others, like Fort Lauderdale, collect it, wash off the salt and convert it to natural fertilizer or mulch. In Mexico some entrepreneurs are compressing it into bricks and using it, like adobe, for building construction. In the long term, lasting solutions will come only through addressing climate change and nitrogen emissions from human activities.