Unusually high summer temperatures are forcing Czechs to head out to swimming holes in record numbers this year.

Petr Sobol, who runs accommodation at Máchovo jezero, a lake in north Bohemia, says he saw more than 50,000 people take a dip there in June.

"Last week, we were on the edge of collapse because there were so many people coming that we had absolutely no room left," he says.

But the heat has also caused a flurry of unwelcome visitors — photosynthetic cyanobacteria known as sinice in Czech — that have blanketed the surface of swimming holes throughout the country, creating hazardous conditions in several regions.

This type of bacteria, single-celled organisms normally impossible to see, have reached such high levels that they have formed a visible blue, filmy surface on many ponds, forcing the Health Ministry to close nine locations as of July 13.

Ministry officials have declared seven other places unsuitable for children, pregnant women and people with weak immune systems including the Hostivař reservoir in Prague 10. Health officials are advising swimmers at 20 other locations, several within a day's drive from Prague, to shower as soon as they leave the water.

Anežka Sixtová, director of public health protection at the Health Ministry, warns that the problem may get worse before it gets better.

"Warm weather and long hours of sunshine contribute to the bacteria's spread," she says. "If this current weather lasts, we can expect swimming conditions to deteriorate in already effected areas."

Nationwide woes

Cyanobacteria flourish in slow-moving, nutrient-rich fresh water. Ponds, lakes and dams allow warm water to sit on the surface, where the bacteria accumulate.

Certain types can cause problems that range from the seriously hazardous, like poisoning, to the mildly annoying irritations known as swimmer's itch.

Overexposure to the toxins produced by the bacteria can lead to poisoning, says Jiří Komárek, a water expert with the Academy of Sciences in south Bohemia, although, he says, swimmers "would have to drink the whole pond to die."

Such bacteria are an annual problem in the Czech Republic, one that has been exacerbated this year by temperatures consistently reaching tropical levels.

An increase of pollution has also made things worse, Komárek says. The bacteria thrive on the phosphorus and nitrogen found in cleaning agents that have been dumped in certain areas.

Although many individual swimming spots are safe, no region has escaped the hazard.

Water experts report worsening conditions in Plzeň, west Bohemia. Swimming has been banned since late May in parts of Liberec, north Bohemia. In Moravia, the bacteria has even made its way into artificial swimming pools, and hygiene officials were forced to close city pools in Litovel and Šternberk.

Reluctant messengers

Regional hygiene officials have been quick to monitor the country's swimming holes, conducting tests at least every other week and issuing up-to-date warnings via their Web sites. But that doesn't mean that the message is getting to swimmers.

Some proprietors of unhealthy swimming sites, often private businesspeople, have been reluctant to advertise the problem.

Matěj Čermák, a spokesman at the Prague Hygiene Office, said July 12 that the swimming pond at Hostivař was unsuitable for children and pregnant women. But when Vojtěch Tajovský, the owner at Hostivař, was questioned that same day about unsafe conditions there, he said there was "nothing of the kind."

"I have the Hygiene Office report here with me," he said. "It speaks of a slightly higher concentration of bacteria, but otherwise everything is normal."

Tajovský said he did not put up any signs warning swimmers about the presence of the bacteria because he said he was not required to do so.

"That's not true," Čermák said in response. "The law strictly states that the owner or administrator of a swimming hole is obligated to inform visitors of this."

Čermák added that his office would make sure swimmers were warned of the danger, adding that he would consult with Hygiene Office lawyers if necessary.

Two days later, hygiene officials said the water at Hostivař had worsened, making it unsuitable for all people, but bathers are now being warned.

Battling the bacteria

Sobol at Máchovo jezero says the water at his lake is clean because of the measures taken last year to combat the bacteria.

"We solved that problem by using sodium chlorine, which keeps the bacteria in the lake bed and prevents them from spreading."

Sodium chlorine can kill the bacteria if it is put in the water before it can grow, but the measure is expensive; one application of sodium chlorine costs 3 million Kč ($134,000).

Máchovo jezero, which the Environment Ministry runs, could afford that treatment, but others are priced out of such cleaning measures.

Komárek says there is a greater price to pay for that treatment because sodium chlorine remains long after the bacteria are gone.

"Chemical materials should be banned because the bacteria could appear again in a few days," he says. "The water in bathing areas should be kept clean."

The only lasting solution to get rid of the bacteria is draining the lakebed, which would cost "hundreds of millions of crowns," Sobol says.

Sixtová says other swimming areas are taking measures to artificially circulate the water so that it is harder for the bacteria to flourish.

Recreation area operators have asked fishermen to rid their waters of carp, which help also spread the bacteria.

Sixtová says hygiene officials will monitor nearly 130 other swimming holes through September.

Other dangers

Bacteria hardly form the only threat to swimmers, though; heat, cold water, inattention and alcohol consumption can prove just as hazardous.

In north Moravia, a 28-year-old man drowned while swimming in a surface mine near Přerov July 16. His drowning was the third that weekend and one of 19 that has occurred throughout the country this summer.

A bystander found the body of a 9-year-old in a swimming hole three days earlier in Kladno, central Bohemia. That same day, a 68-year-old wheelchair-bound woman was found dead in the water in nearby Benešov.

A 25-year-old man became paralyzed in May after jumping headfirst into shallow water. So far this year, five people have suffered serious spine injuries from such dives. Alcohol is often cited as a factor.