Coral bleaching occurs when corals degrade or expel their dinoflagellate symbionts in response to environmental stressors such as elevated sea surface temperature and increased UV radiation. Although corals can reacquire symbionts and recover in weeks to months, recovered corals may grow slower and have reduced fecundity as compared to previously unbleached corals, giving bleaching-resistant corals an ecological advantage after bleaching events. In severe cases, bleaching may occur on the scale of hundreds to thousands of kilometers and radically alter coral cover and composition with coral mortality from bleaching events approaching 100% in extreme cases. Branching corals such as acroporid and pocilloporid corals are often more susceptible to bleaching and mortality than are massive corals, allowing the slower-growing massive corals to be more persistent on reefs after bouts of strong bleaching. Bleaching events not only decrease live coral cover but also provide large areas for seaweed colonization, and these seaweeds can prevent corals from reestablishing if herbivores are not present in sufficient numbers to suppress seaweed colonization and growth. Additionally, large-scale bleaching and mortality of branching corals can suppress fish populations that are dependent on live coral for shelter and food.
Analyses of coral bleaching on Caribbean reefs over the past two decades suggests that small increases in regional sea surface temperature (0.1 °C) result in large increases in the geographic extent and intensity of coral bleaching events. Given that climate change models suggest an increase in sea surface temperatures of 1-3 °C over the next 50-100 years, coral bleaching events may become an intense, annual stress on coral reefs throughout the Caribbean and even the world. Although corals may adapt and their bleaching thresholds may increase over time as sea surface temperatures rise, the threat of repeated, intense bleaching events over the next several decades is a significant concern. If even the conservative predictions of global climate models are realized, these climate changes could result in the fundamental reorganization of the ecology of coral reefs.
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