Even when the beaches are free of dead fish and bad smells, it is evident: Coughing, sniffling and itchy, teary eyes confirm that red tide is out there, killing sea life by the ton and tainting the air along Florida's southwest coast for a ninth straight month.
What started in January with a fisherman's report of an algal bloom off St. Petersburg has spread south to Naples and, in recent weeks, up to Pensacola. It is one of Florida's worst red tides in decades, and as scientists puzzle over its tenacity, tourists are fretting, businesses are suffering and residents, some wearing surgical masks to the beach, are weaving their own anxious theories.
"The earth is an entity just like us, and it's saying, 'You're abusing me bad,' " said Juda Bynum, feeling the familiar catch in her throat as she watched the sun set over Bradenton Beach one night this week.
Scientists say that Karenia brevis, the Gulf Coast's strain of red tide, is a naturally occurring alga that has appeared here since at least the 1800's. It produces a toxin that causes short-term respiratory problems in humans when it vaporizes and, more seriously, attacks the central nervous systems of fish, birds and sea mammals. Its victims this year include at least 163 sea turtles, 63 manatees, 25 dolphins and thousands of fish, many of which have carpeted beaches.
Pinellas County, home to St. Petersburg, has removed more than 950 tons of dead creatures from its beaches since June. Some towns have dispatched prisoners to tackle the mess, while others have hired day laborers.
The red tide has shut down Panhandle oyster beds, another blow to the industry after Hurricane Dennis walloped it in July. Eating shellfish that carry red tide toxins can cause numbness, nausea and diarrhea, but scientists say other fish from affected areas remain safe to eat as long as they appear healthy when caught.
Over the summer, the bloom became so intense off St. Petersburg that it created a 2,000-square-mile "dead zone," large parts of it devoid of oxygen and marine life. Hurricane Katrina stirred up the water enough to dissipate the zone, but unlike past hurricanes, it failed to vanquish the stubborn red tide, which extends about 40 miles offshore.
"I've heard of people coming here to stay a week, and they're gone after a day, wanting their money back," said Kumar Mahadevan, president of Mote Marine Laboratory in Sarasota, which is studying the bloom along with state scientists. "The worst could be yet to come because if it stays during the busy season, tourism will really take a beating."
The bloom moves around, shifted by currents, winds and other conditions. It is hard to notice unless highly concentrated, when it looks not red but "very dense and dark green," said Cynthia Heil, senior research scientist at the Florida Fish and Wildlife Research Institute. People sputter and sneeze the most when rough surf churns the algae and wind pushes it toward shore.
People with asthma and other chronic lung diseases lose breathing capacity after red tide exposure, and emergency rooms treat more pneumonia and bronchitis during blooms.
With so many people affected this year, more are asking whether pollution, overdevelopment or other human factors are to blame. Nitrogen and phosphorus in lawn fertilizer, carried by runoff into coastal waters, are especially suspect.
"It's very likely that coastal pollution and the effluence are enhancing the red tides once they get into shore," said Richard H. Pierce, who oversees red tide research at Mote. "The problem I have as a scientist is the data we have so far does not really support that. But my take is it's because we really don't have enough of the right kind of information."
The state spends $1 million to $2 million a year on red tide research, much more than it did a decade ago, scientists said, but only about a 10th as much as they say is needed to solve red tide's riddles.
Dr. Mahadevan said it was crucial to detect outbreaks earlier because red tide can be eradicated only when small. Ozone and phosphatic clay can kill it, he said, but they can also hurt other marine life if used broadly.
Satellite photographs are the best detection tools for now, though blooms have to be large to show up. Mote has five automated detectors stationed at sea, and Dr. Mahadevan said at least 500 detectors would be needed to monitor the state's entire Gulf Coast.
Karenia brevis is singular to the Gulf of Mexico, different from the red tide that has plagued New England this year and from other strains around the world. It occasionally wanders to Florida's east coast and has traveled as far as the Carolinas, but it most often crops up between St. Petersburg and Naples. There are only theories as to why.
Around Anna Maria Island, rental and vacancy signs are widespread, and business owners say the quiet season has been slower than usual. Some declined to discuss the impact, suggesting it was bad for business to acknowledge the problem.
"It's killing us," said a shop owner in Bradenton Beach who refused to give his name, but said red tide had sharply cut his sales and left him playing computer solitaire all day. "We used to have a lot more people than this, but I don't blame them; it's disgusting."
Surfers are reporting eye infections, said Savanna Spivey, who works at Surfing World in Cortez and said business was stagnant. Bait shops and charter boat captains like Allen Walburn, owner of A & B Charters in Naples, complain that there are no fish to catch.
"I mean, literally, no fish," said Mr. Walburn, who added that his company's revenues had declined 5 percent to 10 percent this year. "The water just looks putrid. It's just polluted, stagnant water. It doesn't even look like water. It looks like chemical waste."
David Rodgers, owner of Pirates Den, a small resort on Anna Maria Island, said he had suffered cancellation after cancellation.
"We'll be here four years next month," Mr. Rodgers said, "and we're seriously talking about selling next year, partly because of red tide."
Tourism officials say the impact has not, in fact, been devastating. According to the St. Petersburg/Clearwater Area Convention and Visitors Bureau, the region has had only a 2.5 percent drop in visitors from last summer's level. Still, said Wit Tuttell, a spokesman for the bureau, "if we got into a situation where it's like this every year, that would be a problem."
This week's water tests, posted on Friday by the Florida Fish and Wildlife Research Institute, held promise: Red tide concentrations significantly decreased in most test spots, possibly because of westerly winds.
For some, though, the image of this serene slice of Florida has changed.
"Once I started to experience it, the paradise feeling left," said Norman Smith, who moved to Bradenton Beach nine years ago from Toronto and was coughing in his beach chair on Thursday. "People here always talk about another day in paradise, but it really takes the spirit way."