What Causes Red Tide in the Ocean? Florida County Residents Warned of Respiratory Problems

A phenomenon known as a red tide has prompted health warnings from officials in Florida amid reports of respiratory irritation.

The red tide has also been linked to a significant number of dead fish found along the shores of St. Petersburg, Florida.

Nine tons of dead marine life was reported to have been removed within a 24-hour period on Friday, more than from the entire week prior.

On Sunday, the National Weather Service (NWS) issued a beach hazards statement for coastal southern Pinellas County bay regions from 7:50 p.m. EDT that night through to Monday evening due to the red tide.

The NWS said the tide was associated with symptoms including coughing, sneezing, and teary eyes. It added people with asthma, emphysema, or any chronic lung disease may be more sensitive.

The Florida Fish and Wildlife Conservation Commission (FWC) states: "For people with severe or chronic respiratory conditions, such as emphysema or asthma, red tide can cause serious illness."

The Florida Department of Health issued a similar statement on June 11 for Pinellas County, in which it warned of respiratory symptoms related to the red tide, and said people should stay away from beach areas or go into an air-conditioned space if they experience these symptoms, citing health officials.

It also said people should keep pets away from water, sea foam, and dead sea life. Algal blooms can be harmful to existing wildlife.

The FWC said in a July 9 update that there had been reports of respiratory irritation in Pinellas and Sarasota counties.

A red tide, also known as a harmful algal bloom, occurs where there is an unusually high level of microscopic alga—a plant-like organism—in the water.

The specific species usually associated with red tides along Florida's coast and in the Gulf of Mexico is called Karenia brevis, or K. brevis.

Red tides really do turn the water red when these algal blooms occur in high enough concentrations. Other algal species can turn the water other colors too, such as green or purple.

At other times, the water may appear normal even during a bloom, the FWC said.

How long a red tide lasts can vary and depends on a number of factors including sunlight, nutrients in the water, and the speed and direction of water currents. Red tides can last for just weeks, or potentially longer than a year.

There are multiple causes for red tides.

In order for the algal bloom to occur, the organism must first be present in the water and be able to out-compete other organisms. The water must also have the right chemistry, including salinity, temperature, and nutrients, according to Florida's MOTE Marine Laboratory and Aquarium.

It is also known that certain nutrients in human-caused pollution can help algal blooms grow, and this can occur once they are transported inshore.

However, the tides first develop up to 40 miles offshore, away from man-made nutrient sources, the FWC states, noting that red tides occurred in Florida "long before human settlement."

People can experience respiratory problems during a red tide because toxins within the algal species can be blown onshore by winds. Some people may also experience skin or eye irritation if they swim in red tide water.

The FWC states: "Use common sense. If you are particularly susceptible to irritation from plant products, avoid an area with a red tide bloom.

"If you experience irritation, get out of the water and thoroughly wash off. Do not swim among dead fish because they can be associated with harmful bacteria."

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Fish killed by red tide pictured on January 4, 2021 in Captiva Island, Florida. Red tides are caused by algal blooms in the water. Andrew Lichtenstein/Corbis via Getty Images