Researchers link seaweed blooms to pollution in ocean water

Massive mounds of seaweed piled up along Florida’s east coast beaches in October 2017, a smelly mess that made it difficult to walk on the beach, much less enjoy the stroll.

Was it coincidence those piles of seaweed showed up after a very busy hurricane season, including a flux of rainfall from Hurricane Irma out of the rivers and inlets along Florida’s East Coast?

Maybe not, says Brian Lapointe, a research professor with Harbor Branch Oceanographic at Florida Atlantic University.

With more and more of the seaweed, known as sargassum, piling up in places like Cancun and Miami Beach, Lapointe and a group of researchers are finding some of the same factors behind the increasing appearances of algae blooms in the Indian River Lagoon and blue-green algae outbreaks in Florida waters are at least partially to blame. As the seaweed becomes more abundant, the tangled and stinky piles could show up more often and in greater amounts on beaches, even as far north as Volusia and Flagler counties.

Working with researchers at the University of South Florida and Georgia Institute of Technology, Lapointe has been looking into why so much golden brown sargassum, a type of macroalgae, has covered beaches along the Caribbean Sea and the Gulf of Mexico since 2011, plaguing many popular tourist locations. And they want to know if they can forecast future blooms.

In June 2018, a blanket of sargassum extended 5,499 square miles across the Central Atlantic from West Africa into the Caribbean Sea and the eastern Gulf of Mexico, the Florida Straits and the waters off South Florida's east coast. A study the researchers recently published in the journal Science dubs the vast and growing expanse of seaweed the “Great Atlantic Sargassum Belt,” and states it's now the largest macroalgae bloom in the world. The USF researchers estimated the grass in that belt weighed an estimated 20 million tons.

In June 2019, the seaweed belt across the Atlantic covered an area five times bigger than it did in the years between 2011 and 2017, but not as big as in June 2018.

People who live in the Caribbean told researchers they've never in 50 years seen such a mass of seaweed, said Mengqui Wang, a post doctoral researcher in the Optical Oceanography Laboratory at USF and co-author on the study.

The belt is different than the Sargasso Sea, the area of seaweed in the Atlantic Ocean off Florida's East Coast that provides nursery grounds for sea turtles that hatch on Florida beaches.

Sargassum floats on the surface and provides important habitat and foraging for sea turtles, birds and other marine life. Once it reaches the sandy shoreline it becomes a foraging area for creatures that live on the beach, and it captures drifting sand to form new dunes. But when it piles up in massive amounts on beaches, it can smother turtle nests and attract pests, in addition to the putrid aromas that waft along the beach.

After studying 19 years of satellite data to find out where the seaweed comes from, where it goes and what feeds or suppresses it, Lapointe and the other researchers said nutrients, such as nitrogen, in the water may play a bigger role than imagined in the expansion and growth of seaweed in general. The study area extended to about 50 degrees north, into the Gulf of Mexico, and from West Africa to the Amazon, including the eastern coast of Florida, said Wang.

Local observers said they haven't yet seen the kind of piles showing up in South Florida this summer, but seaweed did make an appearance in Flagler Beach in June, not typical for that time of year. The piles appeared in North Peninsula State Park and Gamble Rogers Memorial State Recreation Area, said Matt Bledsoe, who manages two Florida state parks.

Jennifer Winters, who oversees habitat conservation on Volusia County's beaches, finds the study interesting but not surprising, given what is known about the way nutrients cause plants to grow. Locally, Winters said she knows of no one who formally monitors the frequency or proliferation of sargassum on local beaches.

Beth Libert, president of the Volusia Flagler Turtle Patrol informally tracks the seaweed on her daily walks. She and others said seaweed historically shows up on local beaches in the fall, at the end of hurricane season. That's when volunteers check the seaweed wrack line, to rescue any wayward sea turtle hatchlings that swam out to the ocean and were then washed back in with the seaweed.

But, in addition to the natural events, such as storms and nor'easters that wash seaweed in from the Sargasso Sea to local beaches, Lapointe said it's probable the sargassum belt they're researching could reappear along beaches in Volusia and Flagler counties bringing seaweed in more often and in greater volume.

That's what they've already been seeing in South Florida. Just this week the largest sargassum influx every reported as seen in Palm Beach and Key West, he said. It multiplies as small fragments break off and continue to grow. And the more nutrients it receives, he said, the more it grows.

Researchers are looking to try to improve forecasts for where and when the seaweed blooms appear as they learn more about it.

In the recent study, Lapointe and his collaborators, funded by NASA, focused on how nutrients flowing from the Amazon River in Brazil fuel the sargassum blooms in a combination of natural and human causes.

The "Great Atlantic Sargassum Belt" gets it start in the nutrient-rich waters moving out of the mouth of the Amazon, fed by  increasing deforestation and fertilizer use in the basin, he said, and also from an upwelling off the African Coast that churns deep, nutrient-rich waters from the bottom of the ocean up to the surface.

However, from Brazil, the sargassum circulates through the Caribbean, then into the Gulf, and then around the Florida Keys and up the East Florida coast, said Lapointe. That’s much the same way the red tide algae bloom moved last year from the Southwest Florida coast around the state and up along the coast to the southern end of Volusia County.

All along the way, runoff, agriculture, fertilizers, sewers and septic tanks, and flooding rainfall push nutrients out into large plumes that carry nitrogen into the water. Lapointe said the water — because fresh water is less dense than seawater — forms a buoyant plume offshore that enriches the sargassum and encourages it to grow. 

The seaweed continues to feed on that runoff as it circulates, he said. For example, Irma's heavy rainfall sent storm water surging into rivers across the state, sending a pulse of polluted water out into the ocean through inlets along Florida's coasts. Along with nutrients from six sewage outfalls in South Florida, he said the elevated coastal nutrients in 2017 nourished the sargassum.

“Our research shows when these plants become enriched like this, they can double their biomass in 10 days to two weeks," he said. “It’s just like the blue-green algae coming from Lake Okeechobee into the St. Lucie Estuary.

“All of these blooms we’re talking about here are feeding off increasing nitrogen, primarily from human activities, " he added. “We’ve been kind of sloppy housekeepers. We aren’t controlling our human nitrogen footprint very well.”

Anyone who has ever tried to keep a clean aquarium at home knows the challenges with keeping the water clean to control algae growth, said Lapointe. And, he added, it's the same problem causing algae blooms in Florida’s springs and estuaries and also causing problems for coral reefs in the Florida Keys.

The researchers said the dramatic increases in seagrass underscore the need to understand its ecological and chemical impacts on the coastal environment, tourism, local economies and human health.

USF researchers are studying how the seaweed blooms affect fish and other marine life and whether their arrival can be forecast in advance, Wang said. "There's so much sargassum out there, it must have a huge impact to the ocean chemistry."