Under the turquoise waters in Biscayne National Park, a story of promise or demise is brewing.

About a mile from the rusted steel slabs of the Mandalay -- a shipwrecked Windjammer cruises sailboat parked on the ocean floor -- Elkhorn corals have latched onto the reef and started to reproduce.

Once a prevalent reef builder in South Florida, the dulce de leche-colored Elkhorn corals were reduced to about 3 percent of their historic population in 2005 by tropical storms, disease, irresponsible boating practices and climate change.

Now, Elkhorn are ``coming back like gang-busters,'' according to Richard Curry, chief scientist at Biscayne National Park.

But if weather forecasters are correct, this recovery may soon melt away under the summer heat -- possibly in tragic proportions.

Like most shallow reef coral species, Elkhorn are susceptible to coral bleaching; a paling effect corals endure when under severe stress, usually but not exclusively as a result of increasing water temperatures.

When bleaching reaches massive levels , it looks as if a blizzard of snow has fallen on the coral reef.

``It's actually very beautiful, but its also depressing,'' said Curry.

This summer, water temperatures have been so warm that coral paling, the first step in the bleaching process, was found as far north as Biscayne Bay in July, even before the usual August scorchers.

Ocean waters in South Florida and in the Florida Keys reach their highest temperatures in August and September, and there is always some bleaching related to those seasonal increases.

The severity of the rise in temperature and the extent of time ocean waters stay at those temperatures determine whether corals can recover from massive bleaching episodes or whether they shrivel up and die.

In 2005, weeks of high water temperatures simmered coral reefs throughout the Caribbean. In the Florida Keys, some scientist estimate 50 percent of the Elkhorn population was lost; about 90 percent in the U.S. Virgin Islands.

South Florida's corals were spared a similar fate by hurricanes Katrina and Rita.

A BULLET DODGED

``We dodged the bullet here in Florida in 2005 because the storms cooled the water in the peak of the bleaching season,'' said Diego Lirman, a professor of marine biology at the University of Miami.

Hotter than average air and water temperatures this summer eerily resemble conditions in 2005.

Scientists at the National Oceanic and Atmospheric Administration predict coral bleaching will equal or surpass 2005 levels -- possibly causing massive coral death across the Caribbean.

Dr. Mark Eakin, NOAA's coral reef watch coordinator, tracks ocean water temperatures using satellites with infrared sensors. Using data collected from the satellites, Eakin compares current ocean temperatures to average historic temperatures.

When water warms one degree centigrade above average for a week it is called a degree week. Coral bleaching typically begins following four consecutive degree weeks.

In the Keys, six degree weeks have come and gone, and the two historically hottest months are next. That means there is a potential for up to 14 degree weeks by the end of September.

In 2005, coral reefs were getting 13, 14 or 15 degree weeks and the result was widespread mortality.

``We see conditions that look even potentially more extreme than 2005,'' Eakin said.

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