August 30, 2001
It sounds like a story
from the Old Testament: Without warning, the sea turns a
shade of reddish brown, killing scores of fish and other
marine life -- and making the water an unwelcome place
for humans.
Such "red
tides" have, from time to time, plagued coastal
communities for centuries. Now a new study, partially
funded by NASA, has revealed a surprising connection
between red tides in the Gulf of Mexico and giant dust
clouds that blow across the Atlantic Ocean from the
distant Sahara Desert. NOAA and NASA satellites can spot
such dust clouds en route from Africa to the
Americas, raising hopes that space-based data could help
scientists predict when red tides will strike the Gulf
coast.
"The West Florida
shelf is a hot spot for fishing, aquaculture and
tourism, all of which can be drastically affected by a
surprise visit from a red tide," said Jason Lenes,
a graduate student at University of South Florida's
College of Marine Science, and the lead author in the
study.
Red tides, which are
actually blooms of toxic algae, have in the past killed
huge numbers of fish, shellfish, marine mammals, and
birds. They can also trigger skin and respiratory
problems in humans.
Storm activity in the
Sahara Desert region kicks up fine particles from the
arid topsoil there, generating vast clouds of dust.
Easterly trade winds carry the dust across the Atlantic
Ocean and into the Gulf of Mexico.
The new study shows
that these clouds fertilize the water off the West
Florida coast with iron. Plant-like bacteria use that
iron to set the stage for red tides. When iron levels go
up, these bacteria, called Trichodesmium, fix
nitrogen in the water, converting it to a form usable by
other marine life. The addition of biologically usable
nitrogen in the water makes the Gulf of Mexico a
friendlier environment for toxic algae.
"This is one of the first studies that
quantitatively measured iron from the dust and [linked]
it to red tides through Trichodesmium," said
Lenes.
The research was
partially funded by a NASA grant as part of
ECOHAB:
Florida (Ecology and Oceanography of Harmful Algal
Blooms), a multi-disciplinary research project designed
to study harmful algae.
The study used
satellite and ground based measurements to track large
dust clouds leaving Africa on June 17, 1999. Lenes and
his colleagues followed the clouds using data from the Advanced
Very-High-Resolution Radiometer
(AVHRR), an imager aboard the NOAA's Polar Orbiting
Environmental Satellites (POES).
The Saharan dust reached the West Florida shelf around
July 1st, increasing iron concentrations in the surface
waters by 300 percent. As a result, Trichodesmium counts
shot up to 10 times what they had been prior to this
event. Through a complex process involving a special
enzyme called nitrogenase, the Trichodesmium used
the iron to convert nitrogen in the water to a form more
usable for other marine life.
In October, after a 300
percent increase of this biologically-accessible
nitrogen, a huge bloom of toxic red algae (Karenia
brevis) had formed within the study area, an 8,100
square mile region between Tampa Bay and Fort Myers,
Florida.
Scientists have labored
for several years to develop a reliable method to
predict red tides, particularly because the results of
these blooms can be both physically and economically
devastating to a region.
Humans who swim in the Gulf during a red tide can
experience respiratory problems by breathing toxins from
K. brevis that get in the air. Also, eating
shellfish poisoned by red tides can lead to paralysis
and memory problems. Around the Gulf of Mexico,
scientists and others have recorded fish kills totaling
in the millions and manatee deaths in the hundreds
resulting from a single red tide bloom.
By using satellites to monitor dust arrivals and Trichodesmium
blooms, Lenes said this research could lead to
forecasting of red tides. "If you could predict
when a red tide is coming, you could close beaches and
fisheries ahead of time," Lenes said.
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