Stony corals that are reef-building corals (also known as Hermtypic corals [see http://en.wikipedia.org/wiki/Hermatypic_coral]) require algae from the genus Symbiodinium [see http://en.wikipedia.org/wiki/Symbiodinium] to be in a healthy condition. The loss of these intracellular Symbiodinium from the host coral is known as bleaching, a condition which leads to the deterioration of a reef. The loss arises when the conditions necessary to sustain the Symbiodinium cannot be maintained. Anything that impacts the coral's ability to supply the Symbiodinium with nutrients for photosynthesis (e.g., carbon dioxide, ammonium) will result in the expulsion of the Symbiodinium. As noted in http://en.wikipedia.org/wiki/Coral_bleaching#Great_Barrier_Reef: "This process is a 'downward spiral', whereby the coral's failure to prevent the division of zooxanthellae leads to ever-greater amounts of the photosynthesis-derived carbon to be diverted into the algae rather than the coral. This makes the energy balance required for the coral to continue sustaining its algae more fragile, and hence the coral loses the ability to maintain its parasitic control on its zooxanthellae."
The loss of such microalgae leaves the coral vulnerable to invasive algae and death. One of the maxims of bleaching has been that reefs could recover only after young new corals from neighboring, healthy reefs settle into the bleached reef. Now this maxim appears to be of questionable validity.
Researchers in Australia studied Scott Reef, which suffered a bleaching in 1998 and is more than 150 miles from other reefs, seemingly a kiss of death. However, the researchers found that the corals have grown back. The coral cover increased from 9% to 44% within 12 years of the coral bleaching event despite a 94% reduction in larval supply for 6 years after the bleaching. The initial increase in coral cover was the result of high rates of growth and survival of remnant colonies, followed by a rapid increase in juvenile "recruitment" as colonies matured.
The researchers concluded that, contrary to the maxim, the benefit of isolation insulated the reef from chronic anthropogenic (human induced) pressures, which greatly enhanced its ability to recover.
The report can be found at: http://www.sciencemag.org/content/340/6128/69.