With its half-a-million lakes, Quebec is nirvana to fishermen and boaters fleeing inner city stress for the peace and calm of summer cabins and mansions on the shores of cool lakes.

But this summmer, a pall has fallen over this idyllic paradise and over the surface of many lakes in the form of blue-green algae, or cyanobacteria.

Besides turning the lake surface a putrid shade of green, the pond scum or bloom, as cyanobacteria is also called, can be toxic, causing skin irritation on contact and liver or nervous system problems when swallowed.

The Quebec government has posted warnings on the Internet for 72 lakes and rivers people should not drink from -- three times the number from last year.

"For two weeks, they've been providing us with water for drinking and cooking. At the beginning of the summer it was bottled, but since last week it comes in tanker trucks," said Cowansville Mayor Arthur Fauteux, whose 12,500 citizens live some 100 kilometers (60 miles) southeast of Montreal.

Most of the polluted lakes in Quebec are resort areas where decades of growing human activity has battered ecosystems to the point of fragility.

"They've built houses and cut down trees to get better views of the lakes, they've replaced natural vegetation with lawns that need fertilizing. There are many factors that over the years have wrought a change in the quality of the lake waters," Department of Environment biologist Marc Simoneau told AFP.

Blue-green algae chiefly get their nourishment from phosphorous, which is rarely found in water but becomes abundant in the presence of fertilizer that washes off lawns and farms.

The bloom is a headache for the tourist industry and to owners of lake shore real estate who fear property values will fall with the rising pollution.

"You can be sure people think about that. That's one of the incentives that leads to a change in behavior and the reforestation of lake shores," said O'Donnell Bedard, mayor of Lac-St-Joseph, a town 30 minutes from Quebec.

The lake the town is named after became fodder for blue-green algae last year. The bloom forced local authorities to start thinking green: they banned fertilizers and offered 480 dollar grants to people to reforest the lake shore.

"Quebec is no more polluted than other regions around the world," David Bird, a cyanobacteria specialist with the University of Quebec, Montreal, told AFP.

"These precautions are the result of global awareness to the real danger of toxic cyanobacteria after all those people died in Caruaru, in Brazil."

In 1996, some 50 people at a blood dialysis center in Brazil died after getting injections of cyanobacteria contaminated water in their veins.

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