A toxic red tide, the biggest in nearly a decade, is threatening tourism and endangered manatees as it moves down the Florida coast.

The culprit is Karenia brevis, microscopic algae that explode in numbers when the conditions are right, usually in late summer or early fall.

“These kinds of blooms damage wildlife, people, tourism, everything,” Don Anderson, a senior scientist at the Woods Hole Oceanographic Institution, told NBC News. “It can kill fish by the millions.”

The current red tide bloom is around 20 miles off the southwestern coast of Florida, too far away to bother beachgoers, at least for now…

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