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What is Red Tide?

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
normal levels of 1000 cells or less
>1000 to <5000
Possible respiratory irritation
5,000 to 10,000
Possible respiratory irritation and shellfish harvesting closures
>10,000 to <50,000
Respiratory irritation but chlorophyll levels too low to be detected by satellites
50,000 to <100,000
Respiratory irritation, maybe fish kills, and bloom chlorophyll probably detected by satellites
100,000 to <1,000,000
Respiratory irritation and probable fish kills
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.

Sampling for Red Tide
Credit: FWC

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.

You and "Man's Best Friend"
can be affected by Red Tide

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.