University of Georgia researchers have determined that unusually cold seawater, as with unusually warm seawater, can be deadly for the coral reefs found in places such as Virgin Islands and Biscayne national parks.

While many recent news concerning threats to coral reefs have involved overly warm ocean waters attributed to climate change, this new study is tied to cold snaps that plunged far south into Florida and stunned and killed sea turtles and manatees.

The lead author of the latest findings, Dustin Kemp, said he was motivated to study the effect of cold water on corals by a cold snap in January and February of 2010 that dropped water temperatures on reefs in the Florida Keys below 54 degrees Fahrenheit at times and held them below 64 degrees Fahrenheit for two weeks.

Mr. Kemp, a postdoctoral associate in the university's Odum School of Ecology, and his colleagues had planned to sample corals at Admiral Reef, an inshore reef off Key Largo, just three weeks after the cold snap. When they arrived, they discovered that the reef, once abundant in hard and soft corals, was essentially dead.

“It was the saddest thing I’ve ever seen,” Mr. Kemp said in a release from the university. “The large, reef-building corals were gone. Some were estimated to be 200 to 300 years old and had survived other catastrophic events, such as the 1998 El Niño bleaching event. The severe cold water appeared to kill the corals quite rapidly.”

Odum School Professor William Fitt, Kemp’s doctoral advisor and one of the paper’s co-authors, realized that the team had a unique opportunity.

“Nearly 100 years ago, Alfred Mayer described the temperature tolerance of different corals in the Dry Tortugas and found very similar results,” Mr. Kemp said. “We decided to take the next step and learn how and why the cold temperatures caused the corals to die.”

 
The researchers took samples of Siderastrea siderea—one of the few reef-building corals to survive—from Admiral Reef. They also took samples of three common Florida Keys corals, Montastraea faveolata, Siderastrea siderea and Porites astreoides from Little Grecian Reef, a nearby offshore reef that had not experienced the temperature anomaly to the extent of Admiral Reef.

Mr. Kemp explained that Little Grecian Reef is far enough offshore that the cold-water temperatures were likely buffered by the warm waters of the Gulf Stream, which resulted in offshore coral reefs being less severely affected by the cold air mass that was pushed by an unusual weather pattern over much of the U.S. during that two-week period.

Back in the lab, the researchers simulated the temperatures that had been recorded at Admiral Reef during the cold weather event, testing the different corals’ physiological responses at 12 C and 16 C (61 F), and then, after the corals’ exposure to the cold, returned the temperature to 20 C (68 F). They found that although responses varied depending on the coral species, in general the stress of extended cold temperatures had an effect similar to that of high temperatures.

 
Mr. Kemp explained that corals depend on Symbiodinium, a type of symbiotic algae that lives inside them, for nutrition. Through photosynthesis, the algae produce sugars, which are passed on to the corals.

“The cold temperatures inhibited photosynthesis in the algae, leading to a potential net loss of carbon transferred from the algae to the coral,” he said.

The studies also determined that each coral species had its own unique type of Symbiodinium, some of which were better able to tolerate and recover from cold temperatures than others.

All of the corals experienced a significant decrease in photosynthesis at 12 C. Siderastrea siderea and M. faveolata were able to handle the 16 C temperatures, but P. astreoides was not, and did not show signs of recovery once the temperature was returned to 20 C. Siderastrea siderea was the only coral able to recover.

 
“Corals and their symbiotic algae have a range of stress tolerance,” said Mr. Kemp. “Some can handle moderate stress, some are highly sensitive, and some are in between. But extreme cold is just one stressor among many.”

Other threats to coral health include increased seawater temperatures, diseases, ocean acidification, and pollution.

“Adding stress from wintertime cold episodes could not only quickly kill corals but also may have long-term effects,” he said. “For corals found in the Florida Keys, winter is typically a ‘non-stressful’ time and corals bulk up on tissue reserves that are important for surviving potentially ‘stressful’ summertime conditions (i.e. coral bleaching).”

Researchers at the National Oceanic and Atmospheric Administration attribute the record-breaking cold anomaly to a negative trend in the North Atlantic oscillation, an atmospheric pressure pattern that influences the weather in the northern hemisphere.

“They speculate that if the trend continues, these kinds of extreme cold events may become more frequent,” Mr. Kemp said.

The researcher stressed that the study’s findings should not be interpreted to downplay the major role of higher temperatures on corals’ decline. “The study shows that warming may not be the only climate-related problem for coral reefs in the future,” he said.

He also pointed out that it was not only the corals that were devastated by the cold snap.

“The corals provide the framework for the entire reef ecosystem,” he said. “The lobster, shrimp, clams, fish—all the creatures that depend on the reef—were affected too. The potential consequences for coral ecosystems are extremely alarming.”

Besides Kemp and Fitt, the paper’s coauthors were Clinton Oakley and Gregory Schmidt of the UGA Department of Plant Biology, Daniel Thornhill of the nonprofit Defenders of Wildlife and Bowdoin College, and Laura Newcomb of Bowdoin College. The research was supported by the National Science Foundation and Bowdoin College.