What is red tide – and does it get worse?

Katherine Hubbard was off Florida’s Gulf Coast near Sarasota when dead and dying fish began to surface in the sea around the boat. She was collecting samples to analyze for an infamous type of algae that scientists know as Karenia brevis in winter 2014-2015. For most of the morning the sea had been calm, but around noon dozens of dying fish – some as long as 14 inches – began to appear as far as the eye could see.

Most of the general public refers to K.short like red tide, seaweed known to ruin beach vacations with irritated skin, burning eyes, and even bigger problems. Sometimes red tide can indirectly cause neurotoxic poisoning when it enters shellfish that humans later consume. For marine life, that can mean masses of dead and rotting fish washed up on shore for miles.

But while scientists have learned a lot about periodic blooms of this toxic algae, there is still much to understand, including questions about whether the situation is worsening due to factors such as climate change or pollution.

“It’s a really interesting phenomenon to work on,” says Hubbard, director of the Florida Fish and Wildlife Conservation Commission’s Center for Red Tide Research.

First reports of red tide

While the red tide may seem terrible for beachgoers, or for the thousands of fish that die dead during a bloom, the dinoflagellates are part of the native ecosystem, as far as scientists can tell. Some of the earliest reports of red tides can be traced back to Spanish explorers – according to Hubbard, the fish kills they recorded in the 1500s may have been the result of the proliferation of red tides.

“It’s definitely something that’s been going on in the Gulf for a long time,” Hubbard says.

As proof, an 1881 report published in the Proceedings of the National Museum of the United States describes a massive fish kill.

“It seems that this misfortune is not a new experience in the eastern part of the Gulf of Mexico,” wrote US Fish Commission scientist Ernest Ingersoll. “One of the oldest inhabitants of the Florida coast, Mr. Benjamin Curry, of Manatee, told me, which others have confirmed, that as early as 1844 a widespread destruction of all kinds of water animals salt has occurred, apparently due to causes precisely similar to those which produced the desolation recently noted.

Ingersoll and others he corresponded with did not elaborate on the cause of these deaths, except noting that people blamed “poisonous waters”, but these reports are consistent with later red tide reports.

Red tide in modern times

Modern science has revealed that even though the red tide seems to appear out of nowhere, there are still algae cells from K.short in the deep waters of the Gulf. There may also be other reservoirs in the sediments, although scientists are not yet clear on the life cycle.

“Red tide doesn’t just happen on the surface, it happens throughout the water column,” says Hubbard.

Red tide blooms can occur closer to shore when offshore currents push algae into shallower water. “As they move towards the coast, they access different levels of light and different sources of nutrients,” says Hubbard, adding that this can promote algal blooms.

Although large algal blooms can be unpredictable, they also exhibit a degree of seasonality. Blooms typically begin in late summer or fall, then fade in late winter or spring in the Gulf and surrounding waters. But when big blooms do occur, they can last all year round – even during the period when a new blooming season would normally begin.

These larger blooms are less predictable and can last for years. The worst on record happened between the late 1940s and early 1950s.

More recently, the Gulf Coast experienced a poor bloom between 2017 and 2019, with a very large red tide area and lots of dead fish.

It is difficult to determine the size of a bloom because it cannot be detected as easily in deeper water. But Hubbard says the kinds of fish that go up can give us clues – when bottom-dwelling species are floating, for example, we know the bloom is pretty big. Algae typically move up the water column during the day to access energy from sunlight through photosynthesis. And the fish killed can be massive. During some blooms, single counties in Florida reported over 1,000 tons of dead fish.

Long term trend tracking

Red tide is difficult to track and assess for a number of reasons. It seems to work well in a wide variety of conditions and doesn’t consistently show up in the same areas. As a result, it is difficult to track trends and determine whether algae is worsening due to climate change or other large-scale regional changes.

Hubbard and his colleagues circumvent these difficulties by examining sediment samples from the past 25 years to see if blooms might leave biomarkers. Researchers in China have found traces of a related red tide species found around the world, Karenia mikimotoiand resting stage markers of this species dating back 50 years.

What does the future hold for us?

Agricultural runoff has caused blooms of blue-green algae in freshwater bodies across North America, but red tide is also difficult to track once it enters estuaries, so that scientists still do not know how these chemical pollutants contribute to large blooms.

Since K.short seems to thrive in different temperatures and conditions, Hubbard says it’s also unclear whether the climate might exacerbate or dampen red tide blooms. A study found that warmer waters lead to increased growth rates, while another study found that warmer climates may lead to increased toxins released by algae.

How it will happen in different areas is unclear, but research indicates that red tide will be able to thrive in a variety of future conditions and currents, Hubbard says.

The fact that K.short is different from the red tide species that occur in Asia and is not found elsewhere in the world, which probably means that it evolved in the Gulf ecosystem. This means that although algae kill large amounts of marine life, they may not all be bad for the ecosystem. No research has been conducted on the potential ecological benefits of these blooms, but Hubbard speculated that it could be likened to wildfires important to forest regeneration. Red tide blooms can kill many species, but it can provide new growth opportunities for other marine species.

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