PRACHUAP KHIRI KHAN, Thailand - For the first time in a decade a Thai lake has given up its dazzling secret -- the pink blooms of tens of thousands of lotuses.

The sea of colour in Khao Sam Roi Yot national park, three hours south of Bangkok, is a show-stopping feat of nature that has locals hoping for a tourist bonanza. But it has also sparked warnings over the fragility of the environment with park rangers pleading with people not to visit until the lake is healthy enough to deal with an influx of tourists.

The flowers, considered sacred in Thai culture, are good news for locals who have been ferrying camera-toting visitors through the lotuses which lie in knots on the lake's vast waters. But "currently they should not come," said head ranger Rungroj Aswakultarin.

"The national park is in the process of restoring the lotuses. When it comes to the right time the park will open them up to the public," he said.

Rungroj's caution is understandable. For the last decade the park -- a stunning jumble of lakes, tributaries and towering limestone karsts three hours south of Bangkok -- saw no widespread lotus bloom. No-one has been able to pinpoint exactly why. But officials blame a mixture of drought and pollution from fertilisers and nearby seafood farms.

In recent years rangers have tried to clean up the park and this year were rewarded with a sea of pink flowers. Thailand struggles to balance its lucrative tourism industry with preserving the environment. Few countries have experienced the kind of stratospheric growth in tourism.

In the last decade the number of foreign arrivals has more than doubled from 13.8 million in 2006 to a record 32.5 million last year, growth partly powered by Chinese arrivals. Those expecting to find the kind of pristine, untouched beaches in tourist brochures are often left disappointed among hordes of visitors.

Thailand's tourism minister has called for the industry to chase fewer holidaymakers but ones with deep pockets. This week some 61 national parks, including 25 marine parks, closed to visitors for the monsoon season, an annual move to try and help them recover.

But some environmentalists have argued the most vulnerable ecosystems need to close for longer, or face much stronger restrictions on visitor numbers.-AFP