Sometimes the evidence of a red tide is
hundreds of dead marine animals floating
in the water or washed up on a beach.
Sunshine State Standards
In “Red Tide” you’ll learn about algae—singlecelled or multicellular organisms that live in
water and use sunlight as a source of energy.
(SC.F.1.3.2)
When toxic algae build up, they interact with
other organisms in their environment, resulting
in harm to plants and animals. (SC.G.1.3.4)
184 Unit 2: Cells
This satellite image
from 2002 shows the
location where a red
tide is occurring
along Florida’s southwestern coastline.
area of
red tide
What Is a Red Tide?
Spanish explorers wrote in their logs about areas
of reddish-brown water along Florida’s gulf coast
in 1530. Fast-forward to 1947, when the people of
Venice, Florida, woke to the sight of thousands of
dead fish along the beaches. A stinging, choking
gas filled the air. Residents feared that they had
been attacked by nerve gas. Instead, both the
1530 and 1947 incidents were caused by the same
problem: poisonous algae. Today the event is
known as red tide.
Algae are organisms that live in water and use
sunlight as a source of energy. Some are singlecelled and some multicellular. Excess nutrients in
the water can lead to an algae bloom, the
extremely rapid growth of algae. These nutrients
can occur naturally, but they often come from
pollution due to farm and yard fertilizers, leaking
septic tanks, and storm-water runoff. Blooms provide abundant food resources for larger marine
organisms and are an important part of the
natural ecosystem.
Not all blooms are beneficial however. Red
tides in Florida are caused by higher than normal
concentrations of a type of single-celled algae
called Karenia brevis, or K. brevis. These algae
occur naturally in the Gulf of Mexico. The number
of organisms can increase to several million cells
in each liter of seawater.
Unlike many algae blooms, a red tide is a
natural event. The rapid growth of poisonous
algae such as K. brevis is known as a harmful
algae bloom (HAB). Where such a bloom has
occurred, the surface of the water sometimes
appears reddish-brown—leading to the red tide
name. However, HABs are not always red.
The water may appear green, brown, or even
purple—or it may not be discolored at all.
There has been an increase in red tides in
recent years. The blooms begin in the Gulf of
Mexico, moving toward the Florida shoreline as
winds and ocean currents carry them along.
Scientists think that warm water from the Gulf
Stream can carry the algae out of the gulf, around
the southern tip of Florida, and as far north as
the coast of North Carolina. Once the red tide
reaches a beach, toxins can also enter the air as
the waves crash against the shore.
The organism that causes red tides in Florida was first
identified in 1948. In 2000 it was renamed Karenia
brevis to honor the work of Dr. Karen Steidinger, a
well-known scientist at the Florida Marine Research
Institute in St. Petersburg.
Florida Connection 185
Harmful Effects
Marine Life K. brevis algae produce a neurotoxin,
a poison that damages or destroys nerve tissue.
This neurotoxin affects many marine food webs.
In cases of large blooms, fish die quickly from the
toxins, which enter their bloodstream through the
gills. High concentrations of toxins can also kill
turtles and manatees. Manatees can consume the
toxin while grazing on sea grass or breathe it in
when surfacing for air. Fish exposed to lower concentrations of K. brevis do not die, but the toxins
may build up in their bodies. Scientists suspect
that dolphins and other animals that eat them
may die as a result.
Some effects of red tide are not so easy to
see. For example, blooms kill underwater plants
by blocking sunlight. Animals that depend on
these plants lose their sources of food and shelter.
Humans People are affected when winds, currents,
or waves move K. brevis toward the shore. Most
people can swim safely in a red tide area, but some
may experience skin irritation and burning eyes.
No one should swim near dead fish, as these fish
might contain harmful bacteria. The toxins in
K. brevis can cause respiratory irritation, leading
to coughing, wheezing, tearing, itchy throat, and
tingly lips. Those with severe or chronic respiratory
conditions, such as asthma or emphysema, should
avoid red tide areas altogether. Fortunately, there
is no evidence so far of long-term harmful effects.
When people are no longer exposed to the toxins,
their symptoms usually go away within hours.
The Fish and Wildlife Conservation Commission
issues shellfish-harvesting bans during red tides.
People should not eat clams and oysters harvested
at that time. Crabs, shrimp, and lobsters are less
affected by K. brevis and can be eaten.
Florida’s Economy Can a tiny one-celled organism
affect the economy of a large state like Florida?
The answer is yes. Obviously, fishing businesses
are harmed by harvesting bans due to red tides.
But tourist businesses are affected as well—
including hotels, restaurants, and stores near
beaches—when a red tide arrives at the shore.
SPOTLIGHT ON Hurricanes and Red Tide
In August and September of 2004, Florida
was hit by a series of hurricanes that caused
extensive damage. One after another, hurricanes Charley, Frances, Ivan, and Jeanne
passed through various areas of the state.
People died, homes and businesses were
destroyed, and many were left without
electricity and water for weeks.
In the aftermath of the storms, scientists
tested the water at 70 locations from Tampa
Bay to Key West, checking whether the hurricanes’ rains flooded the gulf with excess
nutrients—the kind of sudden water influx
that can cause large algae blooms. Research
has shown that K. brevis needs a steady supply of
nutrients to form massive blooms that can cover
several hundred square miles. Where these nutrients
come from remains a mystery.
186 Unit 2: Cells
Hurricane Frances,
September 4, 2004
One consequence of the hurricanes in 2004
was a foul odor coming from flood waters, which
some people attributed to red tide. Instead, it
turned out to be the smell of sewage, rotting
food, and debris stirred up by the storms.
Some scientists point out that because red tides
are a natural process, they serve a purpose in the
ecology of Florida’s coastal regions. As K. brevis
photosynthesizes, it produces a basic food source
for the entire web of life in the marine ecosystem.
In 2002 a massive algae bloom on the west coast of South
Africa reduced oxygen levels in the water. Over 1500 tons of
rock lobsters were stranded on the beach and died.
Controlling Red Tides
Research is under way to determine what causes
algae to reproduce at very fast rates. Scientists
also want to learn how to control harmful algae
blooms. Methods of control are being studied in
confined areas such as fish hatcheries. However,
even if a way to control blooms is developed, it
would be difficult to apply to such a large area as
the Gulf of Mexico.
Scientists are working on developing a model
to predict where red tides will appear. Some studies connect algae blooms with dust storms in Africa
that deposit nutrients as far away as the Gulf of
Mexico. The excess nutrients create an environment that encourages harmful algae blooms.
Scientists can use satellite data and images to
monitor or watch for changes in ocean color like
those accompanying red tides. Scientists from the
National Oceanic and Atmospheric Administration
(NOAA) work on research vessels equipped with
portable spectrometers, which are tools for measuring different wavelengths of light. They collect
data about water in particular places when a
satellite is recording images in the same location.
Then the results are compared. Scientists also use
satellite pictures to note changes in chlorophyll,
which gives plants their green color. This, too, can
help predict blooms. These techniques allow
NOAA to anticipate and respond to problems such
as red tide and other environmental issues.
It becomes toxic only when its numbers increase
dramatically. Red tides may be important for
maintaining energy flow and cycling nutrients
within the world’s oceans. These scientists hope
to understand and lessen the harmful effects of
red tides without causing greater problems for
the ecology of the Florida gulf coast.
• What other questions do you have about
red tides?
• Which of your questions could be the subject
of a scientist’s research?
RESOURCE CENTER
CLASSZONE.COM
Visit
ClassZone.com for more information
on red tides.
Use facts from this article to create an
informative pamphlet to hand out at
beaches, warning of red tides and
their effects.
Writing Tips
Plan, draft, and revise your writing
using the tips below.
• List the signs of a red tide.
• Find and list tips from this article on
how to stay safe during a red tide.
• Think of visuals you might add that
would help people understand red tides.
• Share your draft with a classmate to
see if it includes all of the information
needed to help people learn about
red tides.
• If possible, use a word-processing or
page-layout computer program to
complete your pamphlet.
Florida Connection 187