In the past week, the SCCF Marine Lab recorded varying concentrations of the algae Margalefidinium polykrikoides within Tarpon Bay and in areas nearshore to Sanibel, Captiva, North Captiva, and Cayo Costa. 

Blooms of this species are often referred to as “rust tides” as high concentrations can color the water with a rust-like hue. The photo above shows an aerial perspective of Tarpon Bay on Sept. 12, where the algae caused water discoloration (photo courtesy of Ralph Arwood & Calusa Waterkeeper). 

M. polykrikoides is a dinoflagellate often found in nutrient-rich, estuarine water worldwide that can lead to fish and shellfish kills at high concentrations. This species is rare in local waters but not unprecedented; concentrations near Sanibel were last recorded in 2023. 

It is not known to harm humans and does not produce airborne brevetoxins that cause respiratory irritation, as red tide (Karenia brevis) does. 

Basic precautions are still recommended, as with any algae bloom, such as avoiding contact with discolored water or dead fish and staying aware of local health advisories. 

No known fish kills or wildlife impacts have been reported, and SCCF continues to monitor the situation. Please refer to this week’s Caloosahatchee Conditions Report for more details and stay tuned for further updates.

Learn more about M. polykrikoides at Woods Hole Oceanographic Institution’s Northeast HAB website>>>