Coastal waters of Nassau and Duval counties test positive for the organism that causes red tide

On Aug. 6, the Fort George Inlet tested positive for the presence of red tide, which led the Duval County Health Department to issue a health alert due to the potential for a toxic algal bloom and associated health risks.

The Florida Fish and Wildlife Conservation Commission published a map (see below) and a report relating to recent red tide detected in our area. 

It is unusual on the Atlantic coast, especially this far north. The organism that causes red tide is a single-celled alga called Karenia brevis, typically found along Florida’s Gulf Coast. Most red tide originates in the Gulf during upwelling.

The algae that cause red tides can survive in dark, sunless places deep in the ocean by transforming into a dormant, heavy resting stage, called cysts. They sink and rest on the dark sea floor. When coastal winds trigger an upwelling, the algae rise to the surface and start a rapid growth stage when exposed to sunlight, warm waters and nutrients. 

Background levels of this species are normal. But if conditions are right, a toxic bloom can occur as the algae reproduces quickly. 

Karenia brevis, a microscopic marine alga, is a dinoflagellate. A dinoflagellate uses two tail-like parts called flagella to spin and move through the water. Unfortunately, it can produce powerful neurotoxins called brevetoxins when the population becomes large. These neurotoxins are dangerous to fish, causing fish kills; they can also harm marine mammals and lead to respiratory distress, burning eyes, and other human health problems. 

The reason for the name of red tide is that large concentrations of the algae can discoler the water, becoming red, green, or brown. 

Just because the algae cells are in our area does not mean they will bloom. An algae bloom needs the right conditions, which are optimal light and many bright, sunny, clear days in a row. This is important because the algae are photosynthetic, and the sun provides the energy they need to reproduce. Warm water also speeds up the reproduction rate, so most red tides occur in late summer. Another condition needed to create an algal bloom is nutrient availability. High amounts of nitrogen and phosphorus from farm fertilizer, lawn food, or leaking septic systems act like a massive meal for the algae, triggering explosive growth. 

But red tides also occurred before humans were adding so many nutrients into the ocean. Red tides are natural and happen in oceans without human influence. The first scientifically documented red tide bloom in Florida dates back to 1844, predating extensive human development, therefore not influenced by humans. 

I remember the historic red tide in 2007 on Florida’s east coast. There was an extensive bloom, spanning about 400 kilometers from the Georgia border down to Jupiter Inlet, lasting from September 2007 through early 2008. This bloom caused discomfort to beach-goers, respiratory irritation in people working on the beach, and closures of shellfish fisheries.

Starting on Sept. 25, 2007, several employees of the dredging company performing beach restoration on Amelia Island reported respiratory illnesses to the Nassau County Health Department. All the workers reported symptoms such as coughing, sneezing, sniffling, throat irritation and respiratory distress. The Florida Department of Health promptly conducted epidemiological and environmental investigations. They determined the illnesses were most likely associated with exposure to a red tide along the Atlantic coast at that time.

Red tide blooms are hard to predict. Low cell counts do not always turn into a big bloom. A small patch can fade away or grow fast in a few days. Locals may want to stay aware of the possibility and monitor the local news or the FWC reports during the late summer months and into the fall of 2026.

Another health warning typical in Florida when waters are warm is the potentially debilitating and sometimes deadly threat of flesh-eating bacteria. These bacteria are naturally occurring, being a normal part of the brackish ecosystem. Humans can be infected by eating raw oysters or by having an open wound in water where the bacteria occur. Florida has reported 15 Vibrio vulnificus cases and two deaths in the latest update for 2026. This infection is relatively rare.