Signs of Red Tide are being found along Florida's southwest coast from southern Pinellas County to the Florida Keys.
Patchy concentrations of the toxin, scientifically known as Karenia brevis, are possible within Tampa Bay for the next week.
Heavier concentrations have been found recently in Manatee and Sarasota counties.
"It's a significant bloom and actually has been going on since late summer or early fall,'' said Mike Gittinger, emergency response meteorologist with National Weather Service in Ruskin.
Red Tide is a naturally occurring algae that depletes the water of oxygen, killing fish and shellfish, and sometimes producing an aerosol that causes breathing difficulty, especially for people with respiratory illness.
Over the past few days, complaints of respiratory irritation were reported in Sarasota and Charlotte counties.
Dead fish also were reported in Charlotte and Lee counties.