Just before Hurricane Michael made landfall last month, a ferocious red tide that had scoured Florida’s Gulf Coast for a year, depositing countless dead sea turtles, dolphin and other marine life on beaches before spreading to the Atlantic coast, had finally started to wane.

In most places, with the wet season winding down and the U.S. Army Corps of Engineers easing up on releasing polluted water from Lake Okeechobee, the toxic algae that had become a key election year campaign issue had dropped to relatively low levels. Fish kills were down and so were the coughing fits among beach-goers.

But in the weeks following the storm, red tide that is already considered the worst in a decade has roared back.

On Monday, state wildlife officials logged high to medium levels along beaches from Clearwater to…