Can corals reefs recover from bleaching?

Coral reefs can recover from bleaching events in the short-term. 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 decades for the area to recover, particularly if large, old corals have been killed.

Underwater surveys are used to check whether corals survive bleaching. Photo by GBRMPA.

Coral reefs recover as the surviving corals grow, and new recruits (coral larvae) settle and grow. The rate at which a reef recovers from coral bleaching depends on many factors. Some reefs are more likely to receive large numbers of coral larvae carried with ocean currents. The reefs that receive large numbers of larvae can have many young corals growing on them within a few years. However, conditions need to be suitable for their survival and growth. 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 inhibit the recovery of the corals. Where conditions are ideal, coral communities can recover within about 10 years. Other reefs that do not receive a plentiful supply of coral larvae, or are polluted or degraded, 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.

The status of coral reefs in the long-term will depend substantially on the frequency and severity of future bleaching events. If maximum temperatures become higher and bleaching becomes more frequent in the 21st century, reefs may change greatly in character. Unless 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 only be found in cooler areas. A major shift in the nature of tropical reefs is possible.

An additional potential stress to coral reefs is changing ocean chemistry due to the absorption of carbon dioxide. Several experimental and modelling studies have demonstrated that ocean acidification can decrease the rates of coral calcification which would weaken the structure of coral reefs. Other reef organisms such as fish that depend on hard corals to provide habitat or food may also be seriously affected.

CRC Reef Research Centre Ltd was established and is supported under the Australian Government’s Cooperative Research Centres Program. Its mission is to plan, fund and manage world-leading science for the sustainable use of the Great Barrier Reef World Heritage Area. CRC Reef is a joint venture between Association of Marine Park Tourism Operators, Australian Institute of Marine Science, Great Barrier Reef Marine Park Authority, Great Barrier Reef Research Foundation, James Cook University, Queensland Department of Primary Industries and Fisheries, Queensland Seafood Industry Association, Sunfish Queensland Inc. The University of Queensland is an associate member.