Red tide stretches from Tampa Bay to the Florida Keys: what is it?

Florida Public Radio Emergency Network (FPREN) Storm Center | By Irene Sans

Red Tide is a harmful algal bloom caused by a naturally occurring alga (a plant-like microorganism) called Karenia brevis or K. brevis. When K. brevis appears in large quantities – typically in the Gulf of Mexico – it can turn ocean water red, brown, or green.

Red Tide is not new to Florida. A prolonged Red Tide event in 2018 lasted over 10 months. During this period, the Red Tide was extreme, killing thousands of fish, manatees, dolphins and turtles. Once these toxins occur, there is not much we can do about them, at least in the short term.

When it is present in the water, its contaminants suppress its oxygen, making it a fatal habitat for its in-water and waterfront residents and visitors. Red Tide can have debilitating effects on marine life. If the microorganisms are concentrated at over 10,000-cell-per-liter rate, fish of all kinds can start dying.

The toxic algae start building at microscopic scales well offshore. Warm waters and nutrients feed these microorganisms, and they often grow to high concentrations in the spring and summer months. The winds and currents drive the micro-organisms to the coast, and when they grow to big blooms, winds and currents can drive them for miles along the coast. While Red Tide appears almost every year, the severity varies by year. This Red Tide is unusual because it is winter when Red Tides are typically absent or minimal. Warmer-than-usual temperatures are likely a significant factor contributing to toxic algae bloom.

Red Tide is also not limited to ocean waters. The toxic bloom has sometimes grown in canals, the Intracoastal, and other waterways. If the conditions allow the microorganism to survive and thrive, it will grow and impact the water… and surroundings.

Currently, the highest samples of Red Tide are found across Southwestern Florida, with high concentrations found in San Carlos Bay and Pine Island Sound and to the south near Fort Myers Beach. Although not high, many samples with low concentrations were found across Charlotte Harbor. Hundreds of dead fish have washed up in a canal near Sanibel, staying and coming up to canals. Residents have complained about the foul smells and how it affects their respiratory systems.

It impacts humans and nearby businesses, too.

Although not recommended, most people can safely swim in Red Tide. However, it can cause skin irritation and burning eyes. Inhaling the toxins can lead to coughing, sneezing, and teary eyes, though those symptoms are typically temporary. People with chronic respiratory issues like asthma are advised to avoid Red Tide. While the medical effects on humans generally aren’t serious, the Red Tide is affecting the local economy in Florida: some small businesses have temporarily closed in past years. In seasons where the algae blooms have been extensive in distance and time, many tourists have either changed destinations or canceled plans to visit the west coast of Florida.

How long is the Red Tide expected to last?

It’s hard to predict how long the Red Tide will remain; although the forecast calls for a temperature drop in the coming week, they won’t be cold. It is just a drop to closer to normal temperatures. Also, La Niña is present, bringing warmer-than-normal temperatures to Florida and its surroundings. Limiting harmful fertilizers across the state, even over inland areas, could also help limit the growth of the toxic algae. More nutrients and runoff can make these harmful algae blooms grow faster and more prominent.

The University of South Florida also has a model that predicts and tracks the Red Tide on the West Florida Shelf. You can see it here.

NOAA also has a Gulf Harmful Algal Bloom Forecast, which states that over the next 36 hours, some beaches may experience a moderate risk of respiratory irritations from Red Tide in Lee County. Along with other areas of the Southwest region, the onshore flow could bring moderate to high risks of respiratory irritations. See the latest algal forecast here.