An unwelcome visitor is headed for Florida and the Caribbean: huge floating mats of sargassum, or free-floating brown seaweed. Nearly every year since 2011, sargassum has inundated Caribbean, Gulf of Mexico and Florida coastlines in warm months, peaking in June and July. This brown tide rots on the beach, driving away tourists, harming local fishing industries and requiring costly cleanups.
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According to scientists who monitor the formation of sargassum in the Atlantic Ocean, 2023 could produce the largest bloom ever recorded. That’s bad news for destinations like Miami and Fort Lauderdale that will struggle to clean their shorelines. In 2022, Miami-Dade County spent US$6 million to clear sargassum from just four popular beaches.
Sargassum isn’t new on South Florida beaches, but its rapid increase over the past decade indicates that some new factor – likely related to human actions – is affecting when and how it forms.
In my work as a coastal scientist, I’ve watched these invasions become the new normal, choking beaches and turning clear blue waters golden brown. Along with other researchers, I’m trying to understand why sargassum has proliferated into this new sprawling bloom, how to deal with such massive amounts of it, and how affected countries can predict the severity of the next influx.
A biological hot spot at sea
Sargassum grows in the calm, clear waters of the Sargasso Sea – a 2 million-square-nautical-mile (5.2 million-square-kilometer) haven of biodiversity that lies east of Bermuda in the Atlantic Ocean. Rather than beaches, it’s bounded by rotating ocean currents that form the North Atlantic Subtropical Gyre.
In the open ocean, islands of sargassum create a rich ecosystem that ocean explorer Sylvia Earle calls “a golden floating rainforest.” Suspended by round “berries” filled with gas, the seaweed offers food, sanctuary and breeding grounds for crabs, shrimp, whales, migratory birds and some 120 species of fish. Mats of it form the sole spawning grounds for European and American eels and habitat for some 43 other threatened or endangered species.
Sargassum also shelters sea turtle hatchlings and juvenile fish during their early life in the open ocean. Ten endemic species live nowhere else on Earth. The Sargasso is a valuable commercial fishery worth about $100 million per year.
But in recent years, large quantities of sargassum have drifted west, forming what researchers call the Great Atlantic Sargassum Belt. As of late March 2023, the sargassum belt was about 5,000 miles (8,000 kilometers) long and 300 miles (500 miles) wide.
The belt is actually a collection of island-like masses that can stretch for miles. It doesn’t uniformly cover beaches when it washes up: Some areas can be relatively clear or only mildly affected. But the overall mass this year is overwhelming.
What’s fertilizing huge blooms?
What can plausibly explain the sudden increase in this floating seaweed since 2011 – the first time that large aggregations of sargassum were detected from space? While climate change is warming ocean waters, and sargassum grows faster in warmer water, I believe it’s more plausible that the cause is a drastic increase in agricultural activity in the Brazilian Amazon.
Scientists have shown that huge brown tides that were observed in the Gulf of Mexico in 2005 and 2011 were linked to nutrients carried down the Mississippi River. Now, intensive cattle ranching and soybean farming in the Amazon basin are sending rising levels of nitrogen and phosphorus into the Atlantic Ocean via the Amazon and Orinoco rivers. These nutrients are key ingredients in fertilizer, and also are present in animal manure.
Another major source of nutrients is dust clouds from the Sahara, which can stretch for thousands of miles across the Atlantic Ocean, carried by trade winds. These clouds contain iron, nitrogen and phosphorus from dust storms in Saharan Africa and biomass burning in central and southern Africa. As they blow across the Atlantic, they help fertilize seaweed.