Rising water temps could hurt corals

MIAMI, Florida – 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.


“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.

Eakin’s models show most of the damage from thermal stress will occur around the island of Hispaniola, the western Virgin Islands, and the northern coast of South America.

“The fact that we have a likelihood of severe bleaching in the Caribbean means Florida better keep its eyes open, too,” Eakin said.

Corey Walter coordinates a group of 100 trained volunteers that monitor coral bleaching throughout the Florida Keys marine sanctuary. The observers are professional divers, fishermen, lobster hunters and researchers.

Walter said coral bleaching around the keys has affected up to 10 percent of the corals, but Elkhorn have so far fared better than other species. In previous years, the percentage of affected corals reached 30 percent though most recovered.


“It’s nothing dramatic, it’s natural for this time of year,” Walter said about the extent of coral bleaching.

Located between Turkey Point Nuclear Plant and the South Dade Landfill, Biscayne National Park is one of the few areas where Elkhorn are recovering from 2005 lows. But bleaching and little genotypic diversity among the population there might stunt the recovery.

Like all corals, Elkhorn are not rocks or plants, but rather animals. They breed by releasing sperm and sacks of eggs into the ocean water, letting the ocean mix and match.

Low genotypic diversity — an important variable in evolutionary resistance to threats — means that Elkhorn colonies have very few distinct genetic strands. In the Caribbean, for every two to three colonies, one new, distinct genetic strand is produced. In Biscayne National Park, the Elkhorn need about 10 colonies to produce one new strand.

This low rate of genotypic diversity means Elkhorn are more vulnerable to disease and bleaching because if the colonies in the park do not develop genetic resistance to these threats, one massive episode could wipe out the whole lot.

“[Biscayne Bay] is an example that appears to be doing quite well, but may actually be at risk because there’s only one individual,” said Dr. Margaret Miller, a coral ecologist at NOAA.

The fate of Elkhorn coral is also indicative of larger coral reef ecosystems. If they rebound, then others could do the same, said Dr. Andrew Baker, a professor of marine biology at the University of Miami.

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.