Mass coral bleaching has devastated coral colonies around the world for almost three decades. Now scientists have found that bleaching can make corals more susceptible to disease and, in turn, coral disease can exacerbate the negative effects of bleaching. A paper in the October issue of the journal Ecology shows that when they occur together, this combination of afflictions causes greater harm to corals than either does on its own.
"Traditionally, scientists have attributed coral declines after mass bleaching events to the bleaching only," says Marilyn Brandt, a post-doctoral researcher at the University of Miami and the lead author on the paper. "This study shows that the interplay between diseases and bleaching can play a much larger role than we realized."
Corals rely on algae that live inside each coral polyp to provide nutrients and supplemental oxygen. Bleaching occurs when these colorful algae die out or leave the polyps, often in response to overly warm conditions. Without their brightly colored algae, the coral's skeleton becomes visible through its transparent tissue, making it appear white. Although the tissue remains intact and can recover over time, this stressful condition can cause corals to stop growing and reproducing.
Warmer water temperatures can also lead to increased incidence of coral diseases, which, unlike most bleaching, can cause irreparable loss of coral tissues. In many cases, bleaching and disease occur concurrently on coral reefs. Brandt and her colleagues wondered if the occurrences of bleaching and disease were linked beyond simply occurring under the same conditions.
"Coral bleaching and coral diseases are both related to prolonged thermal stress," says Brandt. "But we wanted to look closer to find out whether they were interacting and what was actually causing the decline we see."
In the summer and fall of 2005, the same oceanic temperature shifts that contributed to
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Ecological Society of America