When there is a higher-than-normal concentration of microscopic algae (plant-like organisms) in the water, this is often referred to as a Red Tide.
In Florida, most red tides are caused by the species Karenia brevis (K. brevis), a type of microalgae known as a dinoflagellate. This organism produces a toxin known as a brevetoxin that can harm manatees and many other species of aquatic life. Red Tide is one of several events which scientists call Harmful Algal Blooms or HAB.
|
A
Close-up look at Red Tide |
 Karenia brevis |
| Photo Credit: FWC | |
The
term Red Tide is used because when there is a high concentration of this
algae (called a bloom), the water may be discolored, and it may be the
color red. They can actually look greenish, brownish, and even purple
in color, or the water can just keep its normal color. (Photo
Credit below: FWC
)
 |
During high concentrations, the water can be discolored. |  |
Do
Red Tide Levels Vary?
The toxic effects of Red Tide usually depend on the level of its concentration
in an area. And concentrations and effects can vary depending on physical
concentrating factors such as winds, tides, and currents. This chart from
the FMRI helps understand the differing health effects at various concentration
levels:
|
Key
for Result |
Karenia
brevis* cells/liter |
Possible
Effect |
| PRESENT |
normal
levels of 1000 cells or less |
None |
| VERY LOWa |
>1000
to <5000 |
Possible
respiratory irritation |
| VERY LOWb |
5,000
to 10,000 |
Possible
respiratory irritation and shellfish harvesting closures |
| LOWa |
>10,000
to <50,000 |
Respiratory
irritation but chlorophyll levels too low to be detected by satellites |
| LOWb |
50,000
to <100,000 |
Respiratory
irritation, maybe fish kills, and bloom chlorophyll probably detected
by satellites |
| MEDIUM |
100,000
to <1,000,000 |
Respiratory
irritation and probable fish kills |
| HIGH |
>1,000,000
|
As
above plus discoloration |
How Is Red Tide Measured?
During a Red Tide, FMRI officials will sample the water to determine Red Tide concentrations and effects, and then will publish maps to show those concentrations.
|
|
 |
How
Does Red Tide Affect Manatees?
Karenia brevis can create severe health problems for manatees,
and in fact it has been implicated in many deaths of the endangered Florida
manatee, in 1963, 1982, and 1996. During those years, it was found that
seven, 39, and 149 animals, respectively, died in southwest Florida during
the winter/spring. This spring in Southwest Florida, approximately 60
manatees have been found dead in a Red Tide zone.
It is believed
that there are three potential routes of how a manatee is exposed to the
toxins of Red Tide. First, the churning of the sea and waves creates aerosol
with toxins present, which the manatee breathes. Second, manatee can ingest
toxic food. Third, and finally, the manatees can take in some seawater
that contains the toxins.
What
Other Animals Are Affected by Red Tide?
Many other aquatic animals are affected by Red Tide as well, including
fish, invertebrates such as certain clams and oysters, other marine mammals
such as dolphins, and water fowl and other birds.
|
|
 |
| Credit: Mary Hosier | |
In addition, land dwellers can also be affected by being exposed to and breathing in the toxin laden aerosol. People can suffer from respiratory effects when brevetoxins become aerosolized by breaking waves, surf, or onshore winds. So too can reptiles, small mammals such as rats, and even "man's best friend", the dog, and cats too. It was recently reported that between 04/13 and 04/23, eight or more dogs on Little Gasparilla Island were taken to veterinary facilities with signs indicative of red tide-induced toxicity.
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