Can coral reefs recover from bleaching?

Bleached staghorn coral
Photo: GBRMPA

Short-term recovery of coral reefs from bleaching events has been demonstrated. When a reef is only slightly stressed, a few scattered corals will die, and the effect will be barely noticeable. When a reef has been exposed to prolonged and extreme heating, most corals will die, and it can take many years for the area to recover, particularly if large, old corals have been killed.

Corals recover as a result of growth of surviving corals, and from the settlement and growth of new coral recruits. The rate at which a reef recovers from coral bleaching depends on many factors. Recent research has shown that some reefs are more likely than others to receive large numbers of coral larvae carried to them with ocean currents. The reefs that receive large numbers of larvae can have many young corals growing on them within a few years. As a result, coral communities can recover within about 10 years. Other reefs that do not receive a plentiful supply of coral larvae will take considerably longer to recover, perhaps decades.

The coral communities on a recovering reef may be different from those that were present before the reef was bleached. A few species appear quickly after a reef is damaged and grow rapidly. Other species that are slower to appear and slower growing, may not be present on the reef at their previous levels for some decades. The community on a reef may shift from coral species that are prone to bleaching to those with higher resistance or a more rapid recovery rate. If a recovering reef is subject to high levels of dissolved or organic nutrients or has few grazing animals, the reef can become dominated by algae which inhibits recovery of the corals.

The long-term recovery of reefs from bleaching will depend on the frequency of bleaching events. If maximum temperatures become higher and bleaching becomes more frequent in the 21st century, reefs may change greatly in character. Unless there corals have a greater capacity for adaptation than scientists currently believe is likely, today’s diverse community of corals may be replaced by a smaller number of tolerant species - some vulnerable species may be found only in cooler areas. A major shift in the nature of tropical reefs is possible. Other reef organisms that are dependent on hard corals to provide habitat or food may also be seriously affected.