Dr. John Bruno, assistant professor of marine sciences at UNC, and colleagues at other U.S. universities, believe they have identified one reason why. Results of field experiments they conducted off Mexico's Yucatan Peninsula suggested that chemical nutrients washed and dumped into the sea can increase the severity of coral diseases.
A report on the findings appears in the December issue of the journal Ecology Letters, which is expected to be posted online Nov. 26. Besides Bruno, authors are Drs. Laura E. Petes of Oregon State University, C. Drew Harvell of Cornell University and Annaliese Hettinger of California State University in Northridge.
"Caribbean coral reefs have declined dramatically over the past 20 years or so as disease epidemics have swept through them," Bruno said. "In less than a year, the two most common species that covered 60 to 70 percent of the bottom were just wiped out, becoming functionally extinct and changing possibly forever the structure of those marine communities. It was analogous to losing all the pine trees in the Carolinas down into Georgia."
Since no one had gone into the field to test the nutrient hypothesis about what was happening, the UNC scientist and his colleagues did just that. They looked specifically at the fungi Aspergillus, which kills elegant gorgonian sea fans through a disease known as aspergillosis and two species of the reef-building corals Montastraea, which yellow band disease can kill.
The researchers placed various concentrations of time-release fertilizer rich in nitrogen and phosphorus in porous bags made from pantyhose and suspended them at sites on reefs some four to six inches from living colonies of the tiny animals. That enabled them to manipulate and boost nutrient levels in the water.
"We found that even modest rises in nutrient pollution could increase mortality of the three important Caribbean corals by facilitating the spread of disease," Bruno said. "Our results suggest that further steps should be taken to reduce nutrient pollution from agricultural runoff, sewage pollution and deforestation."
By increasing nutrient concentrations between two- and five-fold, the marine biologists recorded almost a doubling of tissue loss among the Monastraea from yellow band disease, he said. A separate experiment showed nutrient enrichment significantly increased two measures of the severity of sea fan aspergillosis.
"What we did was relatively minor enrichment so we were not doing it to the extent you might find in the Chesapeake Bay or the coastal Carolinas near a pig farm or something," Bruno said. "We did what we thought would be comparable to what is happening in the Caribbean."
Sea fans are the colorful, fragile-looking, plant-like animals that divers and snorkelers see waving gracefully back and forth in 10 to 20 feet of water, he said. The aspergillosis that kills them is common in plants, birds and humans with weakened immune defenses such as patients with HIV, the virus that causes AIDS.
What causes yellow band disease of the reef-building corals is unproven, but Dr. Garret W. Smith of the University of South Carolina believes that it is a bacterial infection that spreads over the surface of coral colonies like a yellow doughnut, Bruno said. Sometimes it kills the entire colony, and other times it stops and disappears.
Reefs rarely if ever recover and in death often become covered with algae and other microorganisms. "We don't think nutrients played the primary role in causing this Caribbean-wide shift from coral to algae-dominated communities, but we do think their role could be important," the UNC scientist said. "There are much more insidious things going on that likely are more important such as rising global temperatures and over-fishing.
"The good news is that we might be able to do something about lowering the growing nutrient levels through regulations or other methods," Bruno said. "It's close to impossible to do anything about rising temperatures and other effects of what humans are doing to the environment."
Support for the research came from the National Oceanographic and Atmospheric Administration's National Undersea Research Program, the Shoals Marine Laboratory, the PADI Foundation, Cornell University and UNC.
The marine sciences department is part of UNC's College of Arts and Sciences.
By DAVID WILLIAMSON UNC News Services
Note: Bruno can be reached at 919-962-0263 or jbruno@unc.edu.
News Services Contact: David Williamson, 919-962-8596.
Journal
Ecology Letters