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The patchy bloom stretches from the curve of the Panhandle to the central Tampa Bay region. It is about 130km long and 80km wide.
Red tide occurs when naturally occurring algae bloom out of control, producing toxins deadly to fish and other marine life. The odorless chemicals can trigger respiratory distress in people, such as coughing and wheezing.
"It could have large impacts if it were to move inshore," said Brandon Basino, a spokesman for the Florida Fish and Wildlife Conservation Commission (FWC). "It has been killing a lot of marine species, especially fish, as it waits offshore."
The agency has received reports of thousands of dead fish, including snapper, grouper, flounder, crabs, bull sharks, eel and octopus. This is the largest bloom seen since 2006.
The phenomenon has existed for centuries, but such a large bloom is being closely monitored in Florida because it could impact beach tourism and commercial fishing.
A smaller red tide bloom, closer to shore, contributed last year to a record number of deaths among Florida manatees, an endangered sea mammal.
"I have seen analogies that equate red tide with a forest fire," said Kellie Dixon, manager of the ocean technology program at the Mote Marine Laboratory in Sarasota, Florida. "There is an ecosystem reset."
Researchers at that laboratory recently helped deploy two underwater robots, nicknamed Waldo and Bass, to collect data on the slow-moving red tide, which could linger for months or be rapidly dispersed by a storm.
To map the bloom and try to predict its movement, state wildlife officials also organized this week a three-day boating expedition, sending researchers to test water samples across 5180 sq km.
The team found evidence of red tide at the bottom of the ocean, where it is expected to be swept by currents and carried to land, potentially affecting beaches north and south of Tampa.
"It looks like it's coming in," said Alina Corcoran, an FWC research scientist on the expedition, adding that the bloom would not arrive at once. "All of southwest Florida is not doomed. This is normal. It happens all the time."