Studying the surging seaweed that is sargassum

With beaches from Mexico to northern Florida overrun by smelly sargassum, University of Miami researchers are investigating everything from the seaweed’s movement to its chemical composition.
Sargassum seaweed covers a beach in Mexico earlier this summer. Photo: Victor Ruiz/Associated Press

Sargassum seaweed covers a beach in Mexico earlier this summer. Photo: Victor Ruiz/Associated Press

The field samples of sargassum seaweed come from waters throughout the Caribbean, arriving by boat direct to the dock at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, where researchers Peter Swart and Amanda Oehlert quickly scoop them up.

Back at Swart's lab, they analyze and compare the chemical composition of the samples, trying to determine, for example, if sargassum collected from the waters off Florida has a different carbon-to-nitrogen ratio than a sample of the seaweed collected off the coast of the Cayman Islands.

Such analysis will help establish a “fingerprint” for the seaweed, giving the scientific community a better idea of where the stuff is coming from.

From studies on the ocean currents that drive it to the nutrients that feed it, Swart is one of a handful of UM researchers who are currently studying the brown macroalgae, which continues to invade beaches from Mexico to Martinique, covering shorelines with massive, thick brown matts that can block light from reaching corals, seagrasses and other organisms.

“There’s still a lot that we don’t know about sargassum, like the fundamental elements of its biology,” said Swart, a professor of marine geosciences at the Rosenstiel School, who has obtained samples of seaweed from waters throughout the Caribbean, off the coast of Mexico, and from Florida beaches.

Oehlert, an assistant professor of marine geosciences who only recently joined the Rosenstiel School, is analyzing sargassum samples for the presence of heavy metals to determine if the seaweed could be a potential source of ground contamination.

Beaches in Florida, Mexico and the Caribbean have been under siege by sargassum for the past few years, with the source of the mass beachings becoming a keen area of research. One study, using satellite data and ocean and wind models, has suggested the beaching events are the result of sargassum accumulation in the equatorial Atlantic, while other research has shown them to originate in the Sargasso Sea.

But whatever the source, it is Josefina Olascoaga’s goal to determine how much of a role ocean currents and wind play in moving the seaweed across the ocean. A professor of ocean sciences at the Rosenstiel School, Olascoaga is using GPS-equipped ocean drifters to accomplish that task.

“Water is still the most important factor for moving seaweed,” she said, “but we’re trying to figure out what percentage wind contributes—1 percent, less than 1 percent, 3 percent. We just don’t know yet.”

The drifters, which simulate the flow of seaweed and transmit their data to satellites, will help find the answer. Last year, Olascoaga served as the co-principal investigator of a National Oceanic and Atmospheric Administration study that deployed nearly 60 of those drifters in the North Atlantic, producing data that the UM researcher is still analyzing. She and her team are planning more drifter experiments related to sargassum in the near future.

Kathleen Sullivan Sealey, an associate professor of biology in UM’s College of Arts and Sciences, is in the second year of a two-year Florida Keys Nearshore Water Quality Monitoring Project funded by Monroe County in partnership with the Florida Department of Environmental Protection. She and her team are monitoring some 65 sites from Key Largo to Key West, documenting changes in water quality after recent county upgrades such as improved storm-water management and the conversion from residential septic to central wastewater treatment. Her study will also take into account the role sargassum seaweed has played in the water quality of the region.

It isn't the first time Sealey has studied at least one aspect of the seaweed. In a 2012 Schmidt Ocean Institute-funded investigation, she and a team of scientists examined how sargassum circulation patterns and amounts from Bermuda to the Bahamas have changed over the past 40 years.

For all the negative press it has received as of late, sargassum does have its benefits, Sealey said.

“A lot of it is important because it stays out in the open ocean, sinking into the deep sea. And so it’s really connected to very deep-water ecosystems,” she explained.

The only seaweed in the world that doesn’t begin life attached to the seafloor, sargassum is also a floating habitat, supporting an incredible diversity of organisms—the larvae and juveniles of everything from hatchling sea turtles and seahorses to crabs and shrimp that find shelter in and feed off the seaweed.

It is not until the floating macroalgae begins to decompose near shore that it is a problem, creating an unpleasant odor, preventing turtles from reaching the shorebreak, and impacting local tourism industries.

“I don’t think the patch itself is bad,” said Sealey, referring to the largest seaweed bloom in the world, the 5,500-mile-long patch of sargassum extending from the Gulf of Mexico to the African coast. “What’s bad is what’s been happening in the Caribbean over the last eight years. The seaweed has come out of the North Atlantic Gyre and drifted into coastal areas in such a way that it’s ending up on beaches. In the Middle Keys, it’s made coastal properties almost unusable because it’s built up so thick, and it’s entering canals, where it sinks and rots.”

Last year, the seaweed forced resorts in Antigua to temporarily close. This year, Mexico and islands in the Caribbean have been spending millions of dollars to remove the sargassum. And recently, Miami-Dade County brought in bulldozers, front-end loaders and dump trucks to remove massive amounts of the seaweed from Miami Beach.

Still, the question remains, what’s causing the uncontrollable blooms? Rising levels of nutrients associated with deforestation and the increased use of fertilizers in Brazil seeping into the Atlantic, perhaps?

Or maybe smoke from massive fires? “It’s possible,” said Cassandra Gaston, a Rosenstiel School assistant professor of atmospheric sciences.

An expert in atmospheric chemistry, specifically aerosols, trace gases and their interactions, Gaston is the senior author of a recent study that found smoke from African fires burning wild or to clear land may be an important source of a key nutrient—phosphorous—that acts as a fertilizer in the Amazon rainforest, tropical Atlantic and southern oceans.

But while that nutrient, found in atmospheric particles transported by winds and deposited to the ocean and on land, can stimulate the growth of marine phytoplankton and terrestrial plants and lead to the sequestration of carbon dioxide, it could also be contributing to the growth of sargassum.

“The smoke is certainly contributing nutrients that are readily available once it is deposited into the ocean, so it’s definitely possible that it’s playing a role,” said Gaston. “And African dust is certainly a potential contributor as well because it provides other nutrients that the seaweed needs to grow.”

Gaston and her team primarily looked for phosphorous in the aerosol samples of smoke and dust they collected in the Amazon region last year. “But there’s also iron in both particle sources that could also be playing a role,” she said.

But she cautioned that there is not enough information to establish a link between sargassum and fires in Africa.

Inside a UM College of Engineering laboratory, researcher Helena Solo-Gabriele and doctoral student Afeefa Abdool-Ghany are conducting tests to discover a link of different sort—that sargassum seaweed could be causing elevated levels of a bacteria called enterococci in local beach waters.

The bacteria normally reside in the gastrointestinal tracts of animals, including humans, entering the environment via feces. High levels of enterococci at the beach indicate fecal pollution from stormwater, pets and human sewage, and if the bacteria are ingested while swimming, it can cause gastrointestinal disease. Studies have also shown that enterococci correlate with microbes that cause skin rashes.

So once a month since last April, Solo-Gabriele and her students have visited a section of beach off Biscayne Bay, collecting water, sediment and seaweed samples, and then testing their specimens for the presence of enterococci.

“We’re pretty certain that the sargassum is not a source of the enterococci, but as the seaweed decays near the shore, it is probably causing bacterial levels to spike,” said Solo-Gabriele, a professor of environmental engineering and associate dean for research.

Abdool-Ghany noted, for example, that a sample of rotting seaweed collected from the shore last July contained over 1,000 CFUs (colony-forming units) of enterococci.

Their study will continue into next year.

Photos: Evan Garcia/University of Miami