Florida’s Growing Red Tide

Katarina Samurović


Coastal algae bloom – the “Red tide” – is a common occurrence on Florida’s Gulf coast. Red tides were first recorded in the 16th century when Spanish explorers heard stories about the “red water” that resulted in fish and bird mortality, told by Florida Indians. In 1844, the first scientifically documented red tide episode in the U.S. occurred in the Gulf along the West Florida Shelf. However, not all red tides are of the same intensity.

This year’s red tide is so severe that it has forced the state to declare a state of emergency. More than 260 tons of marine life, including thousands of fish, a hundred manatees, a dozen dolphins, and hundreds of sea turtles– all have washed up dead along 150 miles of the Gulf Coast. Many people complain about respiratory issues related to the bloom, and unsurprisingly – tourism has suffered as well. The severity of 2018 red tide is not an isolated incident, but it continues to be a pattern of increasing intensity and duration of the bloom which has been going on since the 1950s.

What is a Red Tide Exactly?

The Red tide phenomenon is caused by the huge and swift proliferation of microscopic algae in the sea water. The name comes from water discoloration that occurs when algae multiply to such high concentrations. Although there are many species of algae that cause harmful blooms, the best known and most aggressive algae species is Karenia brevis– a red plankton algae that thrives in high-salinity areas and easily outcompetes other species of algae. Karenia brevis produces toxins called brevetoxins, which are capable of killing fish, birds, turtles and even big sea mammals such as dolphins and manatees. When waves brake open algae cells, toxins become airborne and cause respiratory issues and eye irritation in humans. Shellfish can accumulate brevetoxins, and individuals who eat them can end up with Neurotoxic Shellfish Poisoning. That is why all shellfish farms have to close during the red tide events. Additionally, when the short-lived algae die and decompose, the process leads to oxygen depletion of the water. Low levels of oxygen cause additional mortality in animals.

Why Have Florida Red Tides Gotten More Severe?

As with most of the algae blooms all over the world, the main reason for the increase is the overuse of commercial fertilizersin the crop fields on land. Large-scale agriculture and chemical fertilizer use have boomed since the 1950s – the same time when algae bloom events got more severe. Farmers continue to add more and more synthetic fertilizers to the often-depleted soils in order to reach higher yields – more than half of the fertilizer ever applied to global agricultural fields has been put in there in the last 30 years.

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The Karenia brevis bloom expanded and intensified in late-July. IMAGE: OCLI sensor on Sentinel 3. Image courtesy of Rick Stumpf, NOAA.
The Karenia brevis bloom expanded and intensified in late-July. IMAGE: OCLI sensor on Sentinel 3. Image courtesy of Rick Stumpf, NOAA.

Unabsorbed fertilizer washes away from the fields – it is estimated that less than half of the nutrient formula is actually used up by the crops. The rest runs off into the environment, entering streams and eventually the sea. Since the easily available nitrogen and phosphorus are a quick feed for both crops we eat and the algae in our seas, algae take advantage of this enormous influx of food to increase their numbers greatly.

Warming sea waters brought on by climate change also have a positive influence on phytoplankton proliferation. “As water temperatures remain persistently warm for longer periods, the likelihood of harmful blooms becomes greater”, states the UN Environment nutrient pollution expert Christopher Cox.

Bloom of Karenia brevis using Sentinel-3 #OLCI Enhanced Color satellite imagery.  Source: ESA.
Bloom of Karenia brevis using Sentinel-3 #OLCI Enhanced Color satellite imagery. Source: ESA.

What Can Be Done About Red Tides In Florida?

The toxic bloom in Florida is still ongoing. However, the Tropical Storm Gordon brought the much-needed relief by helping push the algae away from Southwest Florida’s coastline – though scientists warn that it may be only temporary. Although algae blooms have been known for centuries, there are no scientific inventions to stop or control the blooms. A study from the UK published earlier this year suggests that treating toxic algae in inland waterways with hydrogen peroxide can reduce fish deaths fish deaths; since hydrogen peroxide breaks down into water and oxygen, it offers far more environmental benefits than the commercial algaecides.

In a wider context, there are worldwide initiatives for better nitrogen management, such as United Nation’s International Nitrogen Management System which goal is to “address excess nitrogen in the environment and its negative effects”, by providing recommendations and strategies on how to reduce emissions of reactive nitrogen, including more efficient fertilizer use on farms.

Although the 2018 Florida red tide is likely to cause even more issues in the coming days, something good may come out of it. As the Sarasota Herald-Tribune journalist Nicole Rodriguez concludes: “Particularly lethal or long-lasting blooms like this one renew calls for intensified government and scientific interventions to address the problem.”


About Florida Red Tides. Florida Fish and Wildlife Conservation Commission. http://myfwc.com/research/redtide/general/about/

Red Tide Timeline. Herald Tribune. http://crca.caloosahatchee.org/crca_docs/Heraldtribune_Timeline.pdf

Resnick, B. Why Florida’s red tide is killing fish, manatees, and turtles. Vox. September 3, 2018 https://www.vox.com/energy-and-environment/2018/8/30/17795892/red-tide-2018-florida-gulf-sarasota-sanibel-okeechobee

Rodriguez, N. A hurricane may be only way to get rid of red tide, expert says. Herald Tribune. August 10, 2018. http://www.heraldtribune.com/news/20180810/hurricane-may-be-only-way-to-get-rid-of-red-tide-expert-says

Toxic algal bloom continues to suffocate Florida’s Gulf Coast. UN Environment. September 6, 2018. https://www.unenvironment.org/news-and-stories/story/toxic-algal-bloom-continues-suffocate-floridas-gulf-coast

Toxic ‘red tide’ algae bloom is killing Florida wildlife and menacing tourism. The Guardian. August 14, 2018. https://www.theguardian.com/us-news/2018/aug/13/florida-gulf-coast-red-tide-toxic-algae-bloom-killing-florida-wildlife

Wright, P. Tropical Storm Gordon Alleviates Florida’s Red Tide But Likely Not for Long. The Weather Channel. September 5, 2018. https://weather.com/science/environment/news/2018-09-06-tropical-storm-gordon-red-tide-southwest-florida

See Also

Satellite Image of a Phytoplankton bloom in Lake Erie

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About the author
Katarina Samurović
Katarina Samurović is an environmental analyst and a freelance science writer. She has a special interest in biodiversity, ecoclimatology, biogeography, trees, and insects.