| CANCUN, Mexico
CANCUN, Mexico Dainty blue fish dart around coral shaped like moose antlers near the Mexican resort of Cancun, but sickly brown spots are appearing where pollution threatens one of the world's largest reefs.
Parts of the reef, nestled in turquoise waters, have died and algae -- which feed on sewage residues flowing out of the fast-growing resort city -- has taken over.
Coral reefs like Chitales, near the northern tip of a Caribbean reef chain stretching from Mexico to Honduras, are dying around the world as people and cities put more stress on the environment.
Climate change alone could trigger a global coral die-off by 2100 because carbon emissions warm oceans and make them more acidic, according to a study published in December.
But local environmental problems like sewage, farm runoff and overfishing could kill off much of the world's reefs decades before global warming does, said Roberto Iglesias, a biologist from UNAM university's marine sciences station near Cancun.
"The net effect of pollution is as bad or maybe worse than the effects of global warming," said Iglesias, a co-author of the study in the journal Science on how climate change affects reefs.
Human waste like that from Cancun's hotels and night spots aggravates threats to coral worldwide like overzealous fishing which hurts stocks of fish that eat reef-damaging algae.
Coral reefs, underwater structures that look like rocky gardens, are covered with tiny animals called coral polyps.
The polyps build the reefs by slowly secreting calcium carbonate over thousands of years, creating structures that can dull the blow hurricanes deal to coastal cities and are vital nurseries for fish.
The polyps also give the reefs their dazzling shades of pink and purple that delight scuba divers and boost tourism from the Great Barrier Reef of Australia to the Florida Keys.
Economically, reefs generate billions of dollars a year worldwide in tourism and fishing, the Nature Conservancy environmental group says.
Across the Caribbean, the amount of reef surface covered by live coral has fallen about 80 percent in the last three decades, the Global Coral Reef Monitoring Network says.
In the Pacific between Hawaii and Indonesia, reefs have been losing about 1 percent of their coral coverage annually over the last 25 years.
It is hard to tell how much of that damage was caused by global warming and how much by local factors like pollution.
Some scuba diving instructors around Cancun are worried about the future of their trade. Jorge Olivieri, who has been taking tourists out diving in the area for the last 16 years, says some reefs are so damaged he would not take an experienced diver to see them.
"There are still fish and coral, but it isn't like it used to be," Olivieri said.
With the fight against global warming largely outside of the reach of local officials, fixing problems like poor sewage treatment and overfishing are among the few things that countries and cities can do to help their reefs.
"The local factors are the only things we can manage at this point and they are absolutely critical," said Drew Harvell, a biologist at Cornell University.
In the late 1960s, Cancun was a barely inhabited strip of sand just off Mexico's Yucatan peninsula. Separated from the mainland by narrow straits on either end, just a handful of families tended coconut groves there.
Then Mexican bureaucrats, hungry for foreign currency and armed with statistics on sunshine, hatched a plan to turn the area into a tourist area.
Today, millions of people each year pack into hotels running the length of the strip, including American "spring breakers" drawn to bawdy bars and wet T-shirt contests.
In Cancun's urban sprawl on the mainland, where hotel and bar workers live, infrastructure has failed to keep up with a ballooning population of around half a million.
The lagoon next to the hotel strip is murky and gives off a foul odor in parts. Only crocodiles swim there now.
"It's kind of gross," said U.S. college student Leah, 19.
Away from the lagoon, seawater samples from around Cancun show the levels of chemicals from human waste have increased steadily over the last decade, said Jorge Herrera, a marine biologist at the Cinvestav research center in the nearby city of Merida.
Rising phosphate levels are disrupting a delicate chemical balance needed for coral to thrive, scientists say. Phosphates help algae grow so that it crowds out coral colonies on reef surfaces, making it harder for them to recover from storms or disease.
Rodrigo Hernandez, Cancun's top environmental official, says the city treats the majority of its sewage, unlike most other Mexican cities. "It is really under control," he said.
But Cancun's waste treatment plants do not clean sewage enough to make it safe for coral, marine biologists say. The treatment plants kill bacteria that can be harmful to people but do not remove chemicals like phosphates.
The treated sewage is deposited underground but seeps through the porous soil into the lagoon and the ocean, scientists say. "Little by little, this causes the coral to die," said Herrera.
(Editing by Doina Chiacu)