What exactly is a red tide—and how does it affect humans?

Red tides are a type of harmful algal bloom, and they occur worldwide. There are over 300 species of red tide, and within the United States, these blooms appear in three main coastal areas, although researchers are studying bigger blooms in regions where they were once mitigated before. Red tides are caused by several different microorganisms, all phytoplankton that use light energy to grow.

In the U.S., red tides predate Spanish settlers; history shows Native Americans were aware of the algae’s existence. But with human influence and climate change, the nutrient-hungry algae is exacerbated, inflicting harm on animals and people alike—and ruining spring break plans.

What is a red tide?

At least three species of dinoflagellates and one diatom species are responsible for the toxic mess of red tides in the United States. These microscopic forms of algae produce toxins that can sicken humans and be fatal for marine animals. And despite the name, not all red tides are red. The algae color ranges from rusty orange to green to bioluminescent, depending on the pigment of the cells and local atmospheric conditions.

Within the United States, red tides occur most commonly in the Gulf of Mexico, off California, and in the Gulf of Maine. In each of these locales, a different microorganism—present at low levels normally—generates the algal bloom when conditions align. (Read more about red tides in Australia.)

Gulf of Mexico

A type of toxic algae, Karenia brevis, lives in the Gulf of Mexico throughout the year at low concentrations. When environmental conditions, like temperature, nutrient levels, and wind, are right, the algae population explodes and huge blooms form. These blooms release massive amounts of brevotoxin into the ocean. With their characteristic red color, these red tides affect Florida, Louisiana, Alabama, and Texas.

Richard Stumpf, an algal blooms expert at the National Oceanic and Atmospheric Administration (NOAA), says red tides tend to occur in Florida every year. In Texas, they used to bloom every ten years, but now appear every three years. Louisiana and Alabama cases tend to appear more rarely, often only when hurricanes push blooms northwest from southwest Florida.

Even at relatively low levels, K. brevis can interact with other types of algae, worsening the overall red tide. In particular, a cyanobacterium, Trichodesmium, blooms in the Gulf of Mexico after iron-rich dust drifts over from Africa. The cyanobacteria consume nitrogen from the atmosphere and when they die, they provide K. brevis with a food source of crucial elements: phosphorous, nitrogen, and iron.

As of March 2023, Florida is in month five of a red tide bloom post-Hurricane Ian, which has been interfering with residents and spring breakers who chose the state’s Gulf coast as their getaway destination. While a typical Florida bloom lasts from five to seven months, Cynthia Heil, director of the Red Tide Institute at Mote Marine Laboratory & Aquarium in Sarasota, said Ian’s 20-inches of rainfall has played a role in the current bloom, which is impacting areas along the entire coast.

Gulf of Maine

In the Gulf of Maine, Alexandrium catanella and Alexandrium fundyense are the main culprits for red tides. The result tends to only turn the characteristic red color when it blooms and concentrates in a front, the boundary between two pooling water sources.

Alexandrium has a fascinating life strategy, feeding, aggregating to reproduce, and then resting on the ocean floor as a dormant hard cyst over the winter. When the water temperature exceeds 50°F, generally in late spring, the cysts “perk up,” says Stumpf, and swim up to the surface to start growing. Alexandrium produces the toxin saxitoxin, which remains in the gulf in low concentrations during the year and explodes during blooms.

During a bloom, filter-feeders like clams inadvertently consume the toxin as they filter the water for food (algae), and the toxin accumulates inside their tissues. At these much higher concentrations, the toxin poisons shellfish and if consumed, can lead to paralytic shellfish poisoning in people.

California

The Pseudonitzchia species of algae, forms of diatoms, more commonly create red tides off the coast of California. Some of these blooms can be harmless, because not all produce the toxin domoic acid. In contrast to dinoflagellates, which swim to access nutrients and light, diatoms float in the ocean and if nutrients abound, the algae bloom. If not enough nutrients are in the water, they die. Generally, these algae bloom in spring and summer, though the more toxic blooms often occur during the spring.

Alexandrium species are also found in California, first discovered there in 1927 after an outbreak of paralytic shellfish poisoning.

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Red tide effects on animals

The neurotoxins produced by harmful algae blooms can cause massive fish kills, with dead fish washing up on shores by the thousands. The toxin works its way through the food web, with other species preying on the sick fish. Die-offs of many marine species, like whales and sea turtles, have been linked to red tides. The toxins can even produce a foam that causes seabirds to lose the waterproofing on their feathers, resulting in their death.

In Florida, manatees can be affected by brevotoxin, so much so that it can lead to respiratory problems or even death.

It’s a vicious cycle: Either from oxygen deprivation or direct exposure to the toxins, red tide kills marine life — including sea turtles, manatees, and goliath grouper. When those creatures die and decay, the nutrients released from their carcasses are a significant nutrient source for blooms, allowing the red tide to grow, and more fish to die. 

Red tide effects on humans

The effects of red tides don’t stop at animals. People can be sickened from the toxins in the water or in tainted shellfish. K. brevis, the dinoflagellate causing red tides in Florida, can cause respiratory problems, particularly for people with asthma or allergies. Before humans see it, they’ll feel it: itchy eyes, sneezing, coughing, and shortness of breath are all clues to a present bloom.

Red tides cause an estimated $82 million in economic losses each year due to beach and fishing closures, including associated drops in tourism and seafood revenues.

Human Influences on red tides

Climate change complicates the frequency and occurrence of red tides. Wind and temperature are expected to increase with a warming planet. With more wind stirring up nutrients, dinoflagellates can get food easier. But diatoms prefer calmer conditions.

Scientists predict that climate change will increase the frequency and intensity of hurricanes, potentially moving blooms to new locations. In Florida, a red tide lasted for 17 months after big hurricanes blew through in 2004 and 2005. And in 2017 after Hurricane Irma, another giant red-tide imperiled the state. “That's one of the predictions of climate change in Florida,” Heil said. “We may not see more hurricanes, but they're going to be stronger and [provide] more rainfall, and that can potentially impact red tides.”

Additionally, more nutrients, running into the ocean from excess fertilizer on farms or lawns, can contribute to blooms. K. brevis is increasing in Florida’s waters, more abundant year-round now than in the 1950s, most likely because of higher nutrients. If the algae don’t have enough nitrogen, it dampens the bloom, says Stumpf.

Potential Solutions

Treatments of aluminum sulfate are sometimes used to fight algal blooms in small lakes or canals, but is not effective in the much larger ocean. In the 1950s, scientists tried using copper sulfate. That killed the algae, but also everything else in the water.

The best “fix,” says Stumpf, is mitigation through forecasting. “Right now, red tides in Florida are a crisis. We are trying to make it an inconvenience,” he says.

Stumpf and a team of scientists have created red tide forecasts that are released biweekly to let people know how safe an area in Florida (and elsewhere in the Gulf of Mexico) is for recreation. The forecasts take advantage of satellite images to estimate the size and impact of a bloom. The NOAA team has also introduced a new tool, called HABscope, for citizen scientists to help collect data on blooms. A miniature microscope, the HABscope, , attaches to a smartphone and allows a user to capture short videos of water samples collected from different locations in Florida. Counting the number of microalgae cells in the water sample lets scientists determine the risk or severity of a bloom.

In the Gulf of Maine, crews in the late fall collect sediment samples to count the number of Alexandrium cysts. They incorporate that level into biological models to predict the number that will survive until the next season. Then, scientists combine that predicted number with salinity and ocean current data in physical models to help forecast how bad a bloom will be in the next year. Usually if they find a lot of cysts, the Gulf of Maine will have a larger bloom. But that’s not always the case, and scientists are still trying to figure out why the forecasts were off this year.