Scientists study how sargassum seaweed travels

Sargassum offers food, housing, for marine life

With the waves inside the 49-foot-long Air-Sea
Interaction Saltwater Tank (ASIST) looking like small ripples rushing to a shoreline, Katie Simi, a University of
Miami doctoral student, stood on a small step ladder and
dropped a clump of sargassum seaweed into the current.
Just a few steps away, Maria Josefina Olascoaga,
professor of ocean sciences, recorded all the action
with a high-definition camcorder, jotting down the
speed of the brown macroalgae after it traversed the
length of the tank.

The whole process would be repeated more than
a dozen times in June inside the Rosenstiel School of
Marine, Atmospheric, and Earth Science’s SUSTAIN

While the experiment did not have the stunning visual effect of the simulated Category 5 hurricane conditions
that are sometimes created in the tank’s bigger brother —
the 75-foot-long Surge-Structure-Atmosphere Interaction
wind-wave tank — it is of profound importance.
Using ASIST and other techniques, Olascoaga
and team members from the Nonlinear Dynamics
Laboratory, are studying how the combined action of
ocean currents, wind and waves affect the transport of
sargassum seaweed.

Every year since 2011, the free-floating seaweed
has inundated the Caribbean, Gulf of Mexico and
Florida coastlines during the hot summer months, emitting toxic fumes and wreaking havoc on coastal ecosystems as it decays.

Combining experiments in the laboratory
with drifters deployed in matts of thick
seaweed, scientist Maria Josefina Olascoaga
hopes to learn more about the oceanic
conditions that transport the
brown macroalgae.

“Understanding and learning more about its movement in order to predict where it will end up is challenging,” Olascoaga said. “Wave heights, current velocities,
and wave energy all come into play. And that’s where
ASIST is helping us.”

During three days of simulations, Olascoaga and
her team placed several samples of sargassum into the
ASIST tank, adjusting wave and wind velocities to determine the parameters for a mathematical model for
the motion of sargassum rafts, flexible stems kept afloat
by bladders filled with gas.

“The dynamical systems model envisions sargassum rafts as networks of floating particles of finite size connected by elastic springs,” said Francisco Javier Beron- Vera, a physical oceanographer and research associate
professor in the Department of Atmospheric Sciences.

He is also coprincipal investigator with Olascoaga on her
three-year National Science Foundation-funded study on
the transport of sargassum seaweed.

The results of their experiments are being analyzed. But they are just one component of the project.
Specially modified drifters deployed within thick
matts of sargassum, and an analysis of the biomechanical
properties of the seaweed, will complement what has been
learned from data gleaned from the tank experiments.
Cedric Guigand, a senior research associate in the
Department of Ocean Sciences, will lead a team of scientists up to 30 miles off the coast of Miami into the Gulf
Stream to deploy 24 drifters equipped with GPS trackers
into thick patches of sargassum, obtaining figures that
will help validate the experiments conducted in ASIST.
Guigand has attached strings from mops to the
bottom of each drifter to ensure they stay entangled in
the sargassum. “They look like Medusas,” he said.
Now, it’s just a waiting game. Olascoaga and Beron-
Vera are monitoring satellite ocean images, hoping to
locate large matts of seaweed that can accommodate
Guigand’s drifters.

Meanwhile, Nathan Putman and Taylor Beyea, scientists at LGL – Ecological Research Associates, which is
participating in the study, are studying the tensile strength
and stretching properties of sargassum seaweed, hoping
to learn more about the forces that break it apart.

Gage Bonner, a postdoctoral associate and member
of the Nonlinear Dynamics Laboratory, is working out
statistical problems related to forecasting the arrival of
sargassum at coastlines.

“It’s something where we would be able to say,
‘Okay, there’s so much sargassum in this area. What
times do we think it might start to arrive at various places?’ It’s a different aspect of the project that complements the other areas,” Gage said.

Gustavo Jorge Goni, a physical oceanographer with
the National Oceanic and Atmospheric Administration’s
Atlantic Oceanographic and Meteorological Laboratory,
who is also collaborating on the project, said the study
will help shed light on other areas of marine transport, such as the accumulation of debris pollution and
search-and-rescue operations at sea.

Olascoaga’s study comes after satellite images this
year revealed massive patches of sargassum, known as
the Great Atlantic Sargassum Belt, headed this way.
In some respects, the study is akin to storm forecasting, Olascoaga pointed out.

“Of course, sargassum has its benefits. It’s a vital habitat that provides food and refuge for fish, birds,
crabs, and many other marine organisms,” she said.
“But still, we need to know how it is affected by ocean
conditions to make any kind of predictions as to where
it’s going. And that’s were our research comes in.”