You are currently viewing The Rising (Red) Tide
Fish Kill, National Geographic (Ben Depp). This year’s red tide off the coast of southwestern Florida has caused a devastating loss of marine life and nationwide alarm. What’s the science behind the apocalyptic images?

The Rising (Red) Tide

Gulf Counties Affected by HABs since November 2017

This article was written by guest writer and UGA alumna, Katie Russell. 

An aerial image of the Floridian coast shows the discoloration caused by millions of Karenia brevis cells. Photo credit: Image: National Geographic (Ben Depp)

Fish, beached and bloated, the silver going out of their scales; a manatee floating lifeless in lurid green waters; pale carcasses of eel, crab, grouper, and dolphin hauled from the searing beaches day by day; reeking seas gone murky red. These images seem like something out of a disaster film, causing a visceral sense of alarm. The pictures, however, are flashing on the local news, not in a darkened theater and that tinge of fear comes not from the drama of a gripping story, but from reality, provoking a question that has been asked thousands of times this summer – what is happening in the waters of southwestern Florida?  

Over one hundred miles of coastline, stretching from north of Sarasota to south of Naples, are currently being monitored for the manifestations of an especially aggravated outbreak of red tide and the ensuing ecological fallout. As of August 16th, twelve bottlenose dolphins have been found dead in Sarasota County since August 7, a death toll referred to by Gretchen Lovewell of Mote Marine Laboratory as “extremely unusual,” given that Mote typically recovers 15 deceased dolphins per year. The laboratory has also logged almost 90 dead sea turtles since August 7, with many more ill or injured. Lovewell reports that they “do not see an end in sight” to the tide.  We see the results, but what are the causes?  

This map, generated on August 10 by the Florida Fish and Wildlife Conservation Commission, illustrates K. brevis concentrations at all sites monitored by the agency. The current bloom began last October, an unusually long life span for an event that typically lasts three to five months – though climate change and other factors could be altering the baseline for ‘normal’ events in the warming Gulf.

Red tide, a type of harmful algal bloom (HAB), results from explosive growth of certain species of algae, which discolor the water by sheer numbers as they multiply – hence the name – though HABs may be dark brown, green, or even colorless. Algae occur naturally in all marine ecosystems and, as phytoplankton, form the photosynthetic base of food webs. Most species are beneficial or harmless, save for the hundred or so species known to cause illness and sometimes death in marine life and humans.

Blooms in the Gulf of Mexico are caused by Karenia brevis – a phytoplankton which produces compounds, called brevetoxins, which are harmful if inhaled or ingested. These brevetoxins act on the nervous, respiratory, and gastrointestinal systems in humans and other animals, causing illness and, in rare cases, death. The toxins that irritate the respiratory systems are cast into the air by the force of wave action that breaks apart the algae cells, which is why beachgoers are warned to stay away from the surf during red tides. Since brevetoxins may also be consumed through contaminated shellfish and the tissues of infected fishes, government agencies test the catch for the toxin and restrict shellfish harvest as necessary during HABs. Brevetoxins have also been linked to neurological and respiratory damage in fish, birds, marine mammals, and other ocean life. The remains of a whale shark, which washed ashore on Sanibel Island in late July, are currently being tested for brevetoxins. If such a link could be established, it would be the first recorded case of a whale shark dying due to the effects of a red tide. 

K. brevis is found year-round in the waters of the Gulf, but normally in such small numbers that they don’t pose a threat. Deadly effects only manifest when they begin out-competing other algal species. Red tide is not a new phenomenon – blooms are a semi-regular occurrence in coastal areas and records of their appearance in Florida date back to the Spanish colonists. K. brevis outbreaks often appear in the late summer or early fall when a “perfect storm” of water temperature, salinity, and nutrient levels promotes their growth. The current bloom, first recorded offshore last October, has far outlasted the “normal” life span of a HAB, surpassing the benchmark in severity and intensity. Why? Experts list a variety of interacting factors:

  • Climate – The waters in the Gulf have been warmer than usual this year, an effect that is likely to worsen in coming years due to climate change. Nutrient-rich runoff and warm water were also introduced to the Gulf in great quantity by last year’s hurricanes. Storms like Maria or other severe weather activity could prove the tipping point between a “normal” red tide, to which south Florida’s marine systems are accustomed and can recover from, and an extreme event, which could destabilize fish populations, alter plankton communities, and ultimately change the character of the local ecosystem itself.
  • Discharges – Water discharges loaded with nutrients, like nitrogen and phosphorus, can contribute to algal blooms by providing energy for their growth when they are flushed into water sources that empty into the ocean. These discharges gain their nutrients from runoff, mostly from fertilized lawns and agricultural operations. According to a statement by the Florida Department of the Environment, the current bloom is being fed, in part, by nutrient-laced discharges from Lake Okeechobee that enter the Gulf through the Caloosahatchee River. The lake itself suffers from periodic blooms of blue-green algae, which have been linked to human disease and can kill freshwater life by depleting oxygen in the water.
  • Lake Okeechobee Aerial. This aerial view of Lake Okeechobee, taken July 21, shows an oxygen-consuming algal bloom on the surface of the waters, its growth fueled by high nutrient loads. Photo by Taylor Ebken
    Development – Lake Okeechobee does not naturally drain into the Gulf of Mexico. Extensive engineering projects in the last century altered south Florida’s hydrology to render the naturally marshy land suitable for settlement and agriculture, reducing the ecosystem’s resilience to disturbance. With the ecosystem’s natural filtration systems gone, the water that leaves Okeechobee has no opportunity to be rid of nutrients prior to entering the ocean. Development continues to displace marshes, estuaries, and other habitats that could filter polluted discharges.

These factors are predicted to worsen with climate change, creating conditions increasingly favorable for red tides. Scientific understanding of HABs, as with most complex ecological processes, is incomplete, but growing. Efforts are underway to improve red tide forecasting, develop methods of mitigation, understand the full set of associated health risks, and analyze data to determine the contribution of different factors to sustained, damaging HABs. 

Today’s models cannot tell us how the coast will look next year, whether the waves will break dull red or clear blue upon the beaches of Siesta Key and Boca Grande, or if the residents of Sarasota county will find more marine life floating listlessly, dead or half-paralyzed due to brevetoxins, in their canals and on their beaches. Public outrage and awareness of the current red tide has caught the attention of lawmakers and ignited public consciousness and consciences. Residents mourn the magnitude of the dead and push for action, even as they fear the long-term debilitation of the natural places from which many draw their livelihoods and sense of identity.

Beyond the capitol and the canals, individuals are making decisions that impact Florida coastal ecosystems in tangible ways. Here are some ways you can reduce your own impact on the red tide, wherever you live:

  • If you have a yard, choose to sow a hardy native landscape, pollinator garden, or fruits and vegetables instead of maintaining a monocultural lawn with fertilizers and pesticides. Florida Yards and the Georgia Gardener provide excellent resources for planning an ecologically-friendly yard, as do many state extension services.
  • When vacationing in coastal areas, make sustainable choices. Reduce your contribution to runoff by opting for a walk on the beach or a kayak tour instead of a round of golf or visit to a water park. You can also add a service-learning component to your vacation by finding a coastal clean-up group to volunteer with. Many beaches have programs in conjunction with the International Coast Cleanup Day on September 15, including one on the Georgia coast hosted by UGA Marine Extension and Georgia Sea Grant. 
  • Live more sustainably. Purchase food grown according to sustainable principles, such as no-till farming or organic practices, that limit fertilizer and pesticide input and reduce runoff at the source. 
  • Learn about coastal ecosystems and red tides using resources like our blog, and share what you’ve learned with friends and neighbors.
  • Advocate for a healthier ecosystem with environmental groups. The Calusa Waterkeeper is a strong voice for both freshwater and marine conservation in southwestern Florida. 

We cannot know when the tide shall cease and the waters clear, when the air will no longer burn our eyes and sinuses with fumes from brevetoxins or when the reek of death will be gone, but we can choose to acknowledge the ramifications of our actions today and make choices for a better tomorrow. If our coastal and marine ecosystems as we know them are to survive, we can do no less.


Still curious? Check these links for current updates on HABs in Florida:

Red Tide Status Report – Florida Fish and Wildlife Conservation Commission

Harmful Algal Bloom Forecasts – NOAA