Water samples collected last week showed the, when a red tide bloom killed marine life in a wide part of the bay, said Tom Cardinale, head of the Hillsborough County Environmental Protection Commission's water quality laboratory.
"It's at a dangerous concentration that would normally cause major fish kills," Cardinale said.
Dead fish have been found in areas hardest hit by the algae, although not yet in the numbers expected, he said.
The rare intrusion of the salt-loving algae in Tampa Bay is caused, in part, by the extended drought reducing the flow of fresh water into the bay, said Karen Steidinger of the Florida Marine Research Institute in St. Petersburg.
Clam farmers in southwest Florida have been seeing the effects of the red tide since August. Shellfish, which absorb the algae as food, become toxic and are closed to harvest during a bloom.
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