Red tide threatens Southwest Florida’s coastal communities

December 4, 2022

Southwest Florida is experiencing continued growth and development and the population has increased a staggering 10-fold during the past 50 years. Similarly, instances of harmful algae blooms have also increased in size and frequency, poisoning not just manatees and other native wildlife, but threatening the health of residents and visitors, as well. Red tide blooms, fueled by nutrients, including nutrients from human activities, put our tourism-based economy and quality of life at risk.

Image: NOAA

What is Red Tide and how is it Impacting Southwest Florida?

Red tide (Karenia brevis) is a type of algae that occurs in the Gulf of Mexico, and occasionally the Atlantic. Though each organism is very small, they can number in the billions to form a “bloom.” They also produce toxins that are released into the air and water.

The threat of red tide to our Southwest Florida environment and our quality of life is apparent:

  • In 2018, red tide killed 288 threatened manatees1.  After ingesting the toxin, which coats their food, manatees become paralyzed and eventually drown.
  • Sea turtles are negatively affected by red tide as well. Sea turtles exposed to red tide will swim in circles, lack coordination, exhibit head bobbing, have jerky or twitchy movements, and appear extremely lethargic2.  From late 2017 to early 2019, red tide was responsible for 600 sea turtle deaths. 3
  • Red tide causes fish kills due to lowered dissolved oxygen in the water and through absorption of the harmful toxins directly. 4
  • Birds are affected mainly by their consumption of these toxic fish. Afterward, they show signs of paralysis and nervous system shut-down.5
  • Similarly, people may experience severe breathing problems 6 when near red tide, or even get seriously ill to the point of requiring hospitalization from eating affected seafood, such as oysters and clams, which filter water and hold deposits of toxins. 
  • Red tide also affects our way of life and our economy in southwest Florida. In the summer of 2018, the City of Sanibel and Fort Myers Beach lost a total of $46 million due to red tide and other harmful algal blooms’ impact on tourism. 7 

What Causes Red Tide?

Red tides are a nearly annual occurrence along Florida’s Gulf coast. 8  K. brevis blooms originate offshore, proliferating from both organic and inorganic nutrients. Once the bloom is carried inshore, however, anthropogenically introduced nutrients from stormwater and fertilizer runoff may contribute to the severity and duration of red tide events. 9

How Does Nutrient Pollution Influence Red Tide?

Nutrient pollution – excess nitrogen and phosphorous – comes from a number of sources, including fertilizer runoff, inadequately treated sewage and leaking septic tanks, as well as animal waste. Many of Florida’s waterways do not meet state water quality standards due to high levels of nutrient pollution. For more information review our Estuaries Report Card.

What Does the Science Tell Us?

  • A 2014 study published in the journal Harmful Algae identified twelve sources of nutrients in southwest Florida that feed red tide, including estuary water carrying excess nutrients from anthropogenic sources. 10  Excess nutrients also spark blooms of other kinds of harmful algae, which red tide can feed upon.11 
  • More recently, a 2020 study suggests that the duration and magnitude of red tide blooms could be reduced if better nutrient source controls were implemented within the contributing watersheds, such as the Caloosahatchee.12  
  •  A study published in The Science of the Total Environment in 2021 suggests that moderating nutrient-rich water discharges from Lake Okeechobee into the Caloosahatchee during the presence of red tide in coastal waters can reduce the intensification of the bloom. 13

How Can I Help?

The Conservancy of Southwest Florida is dedicated to protecting our waterways and the aquatic life that rely on clean water. Protective fertilizer ordinances and regulations regarding septic tanks, wastewater, and agricultural best management practices will help keep nutrient-laden runoff from entering our waterways in the first place.

Likewise, new developments need to have adequate measures to treat their stormwater runoff to protect adjacent waters. Stay informed about the current status of red tide in your community. Visit to learn more about what you can do to support clean water.

[1] Florida Fish and Wildlife Conservation Commission

[2] Florida Fish and Wildlife Conservation Commission

[3] Mote Marine Laboratory & Aquarium. 2019 Annual Report.

[4] Karen L. Kimm-Brinson and John S. Ramsdel. “The Red Tide Toxin, Brevetoxin, Induces Embryo Toxicity and Developmental Abnormalities” Environmental Health Perspectives , Vol. 109, No. 4 (Apr., 2001), pp. 377-381

[5] Danielle Stanek, et al. “Karenia Brevis Red Tides And Brevetoxin-Contaminated Fish: A High Risk Factor For Florida’s Scavenging Shorebirds?” Botanica Marina 55.1 (2012): 31-37. Academic Search Premier. Web. 3 Apr. 2013.

[6] Florida Department of Health. HABs: Harmful Algae Blooms.

[7] The Islands of Sanibel Captiva Chamber of Commerce. 2018 Water Quality Economic Impact Survey Results. PowerPoint. [cited 2021 Jan. 13]

[8] Dixon KL, Murphy PJ, Becker NM, and Charniga CM. The potential role of benthic nutrient flux in support of Karenia blooms in west Florida (USA) estuaries and the nearshore gulf of Mexico. Harmful Algae (2014) 38: 30–39.

[9] Brand, Larry E., Compton, Angela. Long-term increase in Karenia brevis abundance along the Southwest Florida Coast. Harmful Algae 6 (2007) 232–252 [cited 2017 May 30].

[10 ]Heil, CA, Dixon LK, Hall E, Garrett M, Lenes JM, O’Neil JM, Walsh BM, Bronk DA, Killberg-Thoreson L, Hitchcock GL, Meyer KA, Mulholland MR, Procise L, Kirkpatrick GJ, Walsh JJ, and Weisberg RW. Blooms of Karenia brevis (Davis) G. Hansen & Ø. Moestrup on the west Florida shelf: nutrient sources and potential management strategies based on a multi-year regional study. Harmful Algae (2014). 38: 127–140.

[11] Patricia M. Gilbert, JoAnn M. Burkholder, et al. “Grazing by Karenia brevis on Synechococcus enhances its growth rate and may help to sustain blooms.” Vol. 55: 17–30, 2009 doi: 10.3354/ame01279.

[12] Medina M, Huffaker R, Jawitz JW, and Muñoz-Capena R. Seasonal dynamics of terrestrially sourced nitrogen influenced Karenia brevis blooms off Florida’s southern Gulf Coast. Harmful Algea (2020) 98 [cited 2021 Jan 12].

[13] Medina M, Kaplan D, Milbrandt EC, Tomasko D, Huffaker R, Angelini C. Nitrogen-enriched discharges from a highly managed watershed intensify red tide (Karenia brevis) blooms in southwest Florida. Sci Total Environ. 2022 Jun 25;827:154149. doi: 10.1016/j.scitotenv.2022.154149. Epub 2022 Feb 25. PMID: 35227724