A tiny ocean organism called rust tide has been spotted in the waters off Southwest Florida.

Margalefidinium (mar-gah-leh-fih-DIN-ee-um) looks like red tide and can cause massive fish kills, but it is not known to produce the toxins responsible for respiratory irritation.

Calusa Waterkeeper volunteer pilot Ralph Arwood spotted it in Lee County waters near Sanibel Island, Tarpon Bay, Captiva Pass, Pine Island Sound and Cayo Costa.

Rust tide last bloomed in Southwest Florida in 2023 and is rare here, but the fact that it appeared matters because it has caused fish kills and thrives in warm coastal waters. By contrast, recurring rust tide blooms have affected parts of Chesapeake Bay for decades.

Researchers believe it has also existed in Asia, Europe and the Middle East. In some places, blooms stretch for miles and return year after year.

The Sanibel-Captiva Conservation Foundation is monitoring the bloom.

Environmental reporting for WGCU is funded in part by VoLo Foundation, a nonprofit with a mission to accelerate change and global impact by supporting science-based climate solutions, enhancing education, and improving health.

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