This article was originally published on March 21, 2022.
Katherine Hubbard was off Florida’s Gulf Coast near Sarasota when dead and dying fish began rising to the surface of the sea around the boat. She was collecting samples for analysis of a scandalous species of algae known as scientists Karenia brevis in the winter of 2014-2015. For most of the morning, the sea was calm, but around noon dozens of dying fish – some as large as 14 inches – began popping up as far as they could see.
What is Red Tide?
Most of the general public refers to K. brevis like red tide, the algae known to ruin beach vacations with skin irritations, burning eyes and even worse problems. Sometimes red tide can indirectly cause neurotoxic poisoning when it gets into shellfish that people later consume. For marine life, this can mean masses of dead and rotting fish washed ashore for miles.
But while scientists have learned a lot about the periodic blooms of this toxic alga, there’s still a lot to understand, including questions about whether it’s getting worse because of factors like 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 Red Tide Research Center.
First reports of Red Tide
Although a red tide may seem terrifying to beachgoers or the thousands of fish that wash up dead during a bloom, dinoflagellates are part of the local ecosystem as far as scientists can tell. Some of the earliest reports of red tide may go back to Spanish explorers—according to Hubbard, the fish kills they recorded in the 1500s may have been the result of red tide blooms.
“It’s definitely something that’s been going on in the Gulf for a long time,” says Hubbard.
As evidence, an 1881 report published in Collections of the United States National Museum describes a large-scale fish die-off.
“It appears that this misfortune is not a new experience in the eastern Gulf of Mexico,” writes US Fish Commission scientist Ernest Ingersoll. “One of the oldest inhabitants of the coast of Florida, Mr. Benjamin Curry, of Manatee, told me, and confirmed by others, that as early as 1844 there was a widespread extermination of all kinds of salt-water animals, apparently from , exactly similar to those which produced the desolation lately noticed.
Ingersoll and others with whom he corresponded did not outline the cause of these deaths, other than to note that people blamed “poisonous waters,” but these reports were consistent with later reports of red tide.
Read more: Algal blooms may have threatened these ancient cities
Red tide in modern times
Modern science has revealed that while red tide appears to appear out of nowhere, there are always cells of algae K. brevis in the deeper waters of the Persian Gulf. There may be other reservoirs in the sediment, although scientists are still unclear about the life cycle.
“Red tide doesn’t just occur at the surface, it happens throughout the water column,” says Hubbard.
Red algae blooms
Red tide blooms can occur closer to shore when offshore currents push the algae into shallower waters. “When they move inshore, they have access to different light levels and different nutrient sources,” says Hubbard, adding that this can help the algae bloom.
Although large algal blooms can be unpredictable, they also have some degree of seasonality. Blooms typically begin in late summer or fall, then disappear by late winter or spring in the Gulf and surrounding waters. But when large blooms occur, they can last year-round—even in the time frame when a new blooming season usually begins.
Massive algal blooms
These larger algal blooms are less predictable and can last for years. The worst recorded case occurred in the late 1940s to early 1950s.
More recently, the Gulf Coast experienced a bad bloom between 2017 and 2019, with a very large area of red tide and lots of dead fish.
It’s hard to tell how big the bloom is because it can’t be detected as easily in deeper water. But Hubbard says the types of fish that come out can give us clues — when bottom-dwelling species surface, 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 fish kills can be massive. During some blooms, individual counties in Florida have reported more than 1,000 tons of dead fish.
Red tide report
Reports of red tide and algal blooms are difficult to evaluate for a number of reasons. It seems to do well in a wide variety of conditions and does not appear consistently in the same areas. As a result, it is difficult to track trends and determine whether the algae are deteriorating due to climate change or other large-scale regional changes.
Hubbard and her colleagues overcame these difficulties by looking at sediment samples from the past 25 years to see if blooms could leave behind biomarkers. Researchers in China have found traces of related species of red tides found worldwide, Karenia mikimotoiand markers of the resting stage of this species from 50 years ago.
Red tide forecast
Agricultural runoff has led to blue-green algae blooms in freshwater basins across North America, but it’s also difficult to track red tide once it enters estuaries, so scientists still don’t know how these chemical pollutants contribute to large flowering.
From K. brevis seems to thrive in a variety of temperatures and conditions, Hubbard says it’s also difficult to determine whether climate can exacerbate or reduce red tide blooms. one study found that warmer waters led to increased growth rates, while another study found that a warmer climate could lead to increased toxins released by the algae.
How it will fare in different areas is unclear, but research predicts that red tide will thrive under different future conditions and currents, Hubbard says.
The fact that K. brevis is different from red tide species found in Asia and not found elsewhere in the world, probably means it evolved in the Persian Gulf ecosystem. This means that even though algae kills large amounts of marine life, it may not be bad for the ecosystem.
No research has been conducted on the potential ecological benefits of these blooms, but Hubbard speculates that they may be analogous to wildfires, which are important for forest regeneration. A red tide bloom can kill many species, but it can provide new growth opportunities for other marine life.
Read more: Biologists marvel at the pristine deep coral reef