MANCHONES REEF, Mexico, Nov 18 (Reuters) - Mexican scuba divers are struggling in surging seas to repair one of the world's biggest coral reefs after it was badly damaged by Hurricane Wilma last month.

Buffeted by strong currents, it takes three divers to hold broken chunks of coral in place and tie them down with plastic straps that are tricky to fasten even above the surface.

After an hour of silent underwater work in Mexico's turquoise Caribbean waters, several pieces of the fragile coral gardens are back in place.

"We got some good work done but it's moving a lot down there; it makes it very difficult," said diver Monica Escarcega, panting as she surfaced from the Manchones coral reef off Isla Mujeres and clambered onto a waiting boat.

"There's still a lot of live coral down there which is great, but we have a lot more work to do, and this weather's not helping," said Luis Guerra, water pouring off his wetsuit as the dive boat lurched over a huge swell.

Weather in the area worsened on Friday as Tropical Storm Gamma brewed off the coast of Honduras on its way toward Mexico's Yucatan Peninsula.

As well as battering luxury resorts and clawing away entire beaches in Cancun, Wilma damaged up to half of the spectacular coral reef chain that runs along Mexico's Caribbean coast, biologists say.

Mexico's reefs are part of the Mesoamerican Barrier Reef which runs for hundreds of miles (km) to Belize, Guatemala and Honduras, and is second only in size to Australia's Great Barrier Reef. It first won fame in a 1961 documentary by French filmmaker Jacques Cousteau shot off Mexico's Cozumel island.

The coral devastation not only hurts the marine ecosystem, but it is one more blow to Cancun's ravaged tourism sector, which normally sees boatloads of vacationers and diving enthusiasts heading out daily on snorkeling and scuba trips.

UNDERWATER GLUE

Three weeks after Wilma, the only dive boats out of Cancun carry 40 professional divers hired by the government to clear debris from the reef and repair the coral, hoping to speed up its agonizingly slow recovery time.

Coral grows just a few millimeters (fraction of an inch) a year, so left to its own devices the reef would take decades to recover -- especially as clumsy snorkelers and passing ships often chip off bits of coral. Pollution is another threat.

"If we leave it to nature, the pressure of tourism and water contamination don't give the coral much chance," said Juan Carlos Huitron, who is in charge of the repair mission.

"We can't change the rate at which coral grows, but we can try and make sure more of it survives."

The scuba teams use various means to fix broken coral back in place, including metal rods inserted in holes drilled into rocks on the seabed and glue made from cement mixed with sand.

The cement can only be applied on calm days, otherwise most of it seeps away and dissolves before it can set.

"The reef is the ocean's most powerful ecosystem. A great quantity of flora and fauna depend on its health," said Alfredo Arellano, regional head of Conanp, the government body in charge of Mexico's protected natural areas.

As well as damaged reefs and shrunken beaches, biologists are worried about the damage Wilma did to local forests.

They estimate the area around Cancun lost some 1.98 million acres (800,000 hectares) of tropical forest in this season's storms and say the dead trees and brush are a serious fire hazard.

"There is a lot of dead wood and leaves about. It's a latent danger," said Mauricio Limon, who headed a visit last week by Mexico's environmental protection agency to assess local storm damage.

"Nature's capacity to recover from disasters is marvelous and the forests will grow back. But it's certain we will see forest fires next year."

• Reuters