‘Idiot’s Guide’ to Red Tide: Understand the Phenomena

by SC Assistant Publisher, Photographer Chuck Larsen

Empty Sanibel Causeway Beach in September 2018

Following the red tide and blue green algae outbreak that scared tourists away and precipitated serious health concerns in 2018, it’s helpful to understand the phenomena and causes. SantivaChronicle.com decided to ask some common questions and discussed the possibility of hope for 2019 with City of Sanibel Director of Natural Resources James Evans for an “idiot’s guide” to red tide.

What Is It?

Red Tide is a common colloquial term for a worldwide phenomenon know as “harmful algal bloom.”

“Karenia brevis blooms” is the scientific term for those found in the Eastern Gulf of Mexico.

It refers to large concentrations of salt water aquatic microorganisms.

Red tides are not necessarily red..sometimes brown or no discoloration at all. Although called red tide, it is unrelated to movement of tides.

Red tide is in the background all year, but harmful blooms depend upon temperature, salinity, and nutrient availability.

While occurring around the world, the first red tide in Florida was 1844 according to the Marine Lab at the University of Miami.

Modern record keeping shows the three worst years in recent times as 1994-97 (30 continuous months), 2002-2004 (21 continuous months), 2004-2006 (17 continuous months), and 2017-2018 (15 continuous months).

Upswelling of nutrients from sea floor, often following massive storms, provided for the algae and triggers bloom events.

Our red tide begins somewhere near the continental shelf, about 200 miles off shore and floats our way to Sanibel and Captiva beaches with the currents.

What Harm Does It Cause?

No human deaths have been attributed to Florida red tide, but respiratory irritation (coughing, sneezing and tearing) occur when red tide organism is present along the coast and wind blows its aerosolized toxins.

However, red tide production of nuturaltoxins such as brevetoxins and ichthyotoxins are very harmful to marine life.

Scientists have studied the effects of the toxins on marine life.

Within minutes of receiving doses of the toxin, fish started to exhibit a loss of equilibrium and began to swim in an irregular, jerking pattern, followed by paralysis and shallow, arrhythmic breathing and eventually death after about an hour.

Rather than let nature take its course, the city cleaned up the dead marine life last year because decomposing marine life produces nutrients that exasperates red tide blooms and worsens the duration. Last year’s cleanup cost was $1.6Million which was funded by the State of Florida. 852,000 pounds of dead sea life was removed and disposed in landfills or buried.

So far in 2019, the waters around Sanibel and Captiva are considered clear of red tide blooms.

What Does the Caloosahatchee River Have to Do With It?

Blue-green algae exists in fresh water and originates locally in Lake Okeechobee.

As the blue-green algae flows down the Caloosahatchee River it breaks down after the Franklin Lock where it mixes with salt water. As it breaks down, it generates toxins that release nitrogen and phosphorous that feed the red tide algae making it much worse.

In July 2018, prior to the red tide outbreak in October, blue green algae covered 90% of the lake.

James Evans

What’s Different This Year?

The water discharge from the lake is controlled by the Army Corps of Engineers.

James Evans, Director of Natural Resources for the city of Sanibel reports that the Lake Okeechobee runoff is being increased now, when the blue green algae is very low in the lake to avoid larger runoffs in the summer and fall when the blue green algae are higher and can feed the red tide, which is basically off shore at this time of year.

Forecasting into the future is difficult, but MySanibel.com has updates on red tide and several links to other resources that show the current status in our area.

Hopefully this year will be much better.


Take Away

Red Tide salt water algae forms near the continental shelf 200 miles off shore. Currents bring the harmful algae to our beaches.Blue Green fresh water algae forms in Lake Okeechobee. Run-off down the Caloosahatchee River mixes with salt water after the last dam, releasing Nitrogen and Phosphorous into the bay and gulf. These chemicals nourish Red Tide which causes more harmful blooms.

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