Red tide threatens Florida’s coastal communities

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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 our citizens as well. Red tide blooms, fueled by nutrients, including anthropogenic sourced nutrients, put our tourism-based economy and quality of life at risk.

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

The threat of red tide to our southwest Florida environment is apparent:

  • In 2017, 67 endangered manatees were killed directly by red tide. 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 be extremely lethargic.
  • Red tide also causes fish kills due to lowered dissolved oxygen in the water and also by impacting the fish directly.
  • Birds are affected mainly by their consumption of these toxic fish. Afterward, they show signs of paralysis and nervous system shut-down.
  • Similarly, people may experience severe breathing problems 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 and hold deposits of toxins.

Red tide watches are regularly conducted by state and county agencies.

Red tides are a nearly annual occurrence along Florida’s Gulf coast.

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.

Nutrient pollution — excess nitrogen and phosphorous — comes from 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.

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, in addition to naturally occurring nutrient sources. Excess nutrients also spark blooms of other kinds of harmful algae, which red tide can feed upon.

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 tank, 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. Visit to learn more about what you can do to support clean water.

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Protecting Southwest Florida's unique natural environment and quality of and forever.

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