Dying Coral Could Be Environment Warning

by Roger Segelken
Cornell University

Increasing global temperatures and worsening pollution, the ecologists say, could place so much stress on ecosystems that organisms of all kinds will face new challenges.

"When we see corals that have persisted for hundreds of years suddenly die from opportunistic infections, we have to wonder what has changed in their environment," says C. Drew Harvell, associate professor of ecology at Cornell.

Harvell organized a session,"Diseases of the Ocean: A New Environmental Challenge," at the annual meeting of the American Association for the Advancement of Science (AAAS) last week to bring together leading scientists to evaluate the environmental threats from disease in the ocean.

Kiho Kim, a postdoctoral research associate, said that monitoring of sea fan corals in the Florida Keys, where up to 40 percent of sea fans are infected by a fungal disease and many have already died, suggests that lower water quality and higher ocean temperatures stress corals and increase their susceptibility to disease. He said the Florida findings support a growing consensus among scientists worldwide that as ocean ecosystems become degraded they will offer more favorable places for disease outbreaks and the emergence of new pathogens.

"We didn't begin our study of sea fans to monitor death and destruction," Harvell said. "Originally, we were interested in the natural disease-resistance properties of corals, such as the anti-bacterial and anti-fungal chemicals they produce, because some of those compounds may be useful in human medicine. That disease resistance normally keeps a coral alive for hundreds of years, despite living in an ocean full of potential pathogens."

"Somehow, a soil pathogen that was best known for infecting aged and immune-compromised humans has crossed the land-sea barrier," Harvell said. "Now, one of our jobs is to discover what has compromised the resistance of the corals at some sites. Although a significant number of sea fans have died at a few sites, at many locales they recover from infections, pointing to the success of their natural resistance."

While coral disease is reported throughout the Caribbean, the reef ecosystems of the Florida Keys may be particularly vulnerable because they are close to what ecologists call "natural stressors," such as fluctuating water temperatures and substantial freshwater runoff, Harvell said. The situation has worsened in recent years, she observed, with multiple "anthropogenic stressors," such as intensive human use of the land and off-shore waters.

"Then you have rising water temperatures of the oceans," Harvell added. "Whether you believe that global warming is a function of human activity and whether last year's El Nino was a symptom of global warming, the fact is that sea temperatures globally in 1998 were high. And 1998 was the worst year ever recorded globally for coral bleaching."

Corals bleach (or lose their symbiotic algae) when stressed by high temperatures, Harvell explained, adding: "I think we have to question the relationship between temperature stresses and diseases of the oceans."

Lately in the Florida Keys, coral death has been occurring so suddenly and rapidly that Harvell and Kim must monitor their research sites three times a year.

"We don't know if new diseases are emerging, if the hosts are becoming more susceptible or both," Harvell said. "We need to identify these new diseases and we should do it now while we have the chance. Disease ecology is poorly understood in the ocean because diseases are like lightning strikes -- they hit unexpedly, burn through a population, and then they are often gone."