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Climate Change - Impacts on Coral Reef Ecosystems
Climate Change Response Programme, Great Barrier Reef Marine Park Authority (GBRMPA)
Coral reef ecosystems are highly susceptible to the impact of climate change and recent events, such as mass bleaching, have lead to an increased focus on coral reefs in climate change discussions. Reef managers around the world are looking at resilience-based management, with an emphasis on using research to underpin practical management and response actions.
The concern about the future health of coral reefs partially emerges from their importance to our lives. Around the world coral reefs are a source of inspiration, motivating art, music and poetry. The natural values of coral reefs also attract a huge and growing tourism industry, forming the basis of many regional economies. Coral reefs also provide important ecosystem services, which underpin economic activity in fisheries. If unprepared, the ramifications of large-scale devastation to coral reefs could have serious outcomes for the communities and industries that depend on them.
Coral reef ecosystems will face a range of climate-driven changes in the future, caused by increasing atmospheric carbon dioxide and rising temperatures. Sea temperatures have already begun to rise, and will continue to do so in the future. Sea temperature rise occurs at a slightly slower rate than the atmosphere; the huge volume of the oceans confers some buffering capacity slowing the rate of change. The oceans will also continue to absorb some of the carbon dioxide being pumped into our atmosphere, leading to changes in ocean acidity and chemistry. Coastal and very shallow reef habitats face additional challenges including sea level rise and increasing intensity of disturbance events such as storms, rainfall and runoff.
Few ecosystems are likely to benefit from climate change, however coral reefs are particularly vulnerable. The list of potential impacts starts with coral bleaching. Mass coral bleaching events, resulting when sea temperatures become unusually hot, have already caused serious damage to over 16 per cent of the world’s coral reefs. It takes a rise in average sea temperature of only 1.5–2°C for six to eight weeks for corals to bleach. Such rises in temperature can occur at regional or even global scales, such as those seen during the 1998 mass coral bleaching event.
High sea temperatures reduce the ability of the coral’s symbiotic zooxanthellae to process light, leading them to produce damaging types of oxygen. The zooxanthellae are expelled from the coral, and, as they usually give the coral their colour, the coral’s tissue becomes transparent. The white carbonate skeleton can be seen through the transparent tissue resulting in a bleached appearance. A bleached coral is still alive but, without the zooxanthellae that usually provide most of the coral’s energy, the coral’s growth rate can be impaired. Bleached corals can recover and regain their zooxanthellae but the stress from bleaching can have long-term consequences for growth rates and reproductive output. However, bleaching corals often die if high temperatures persist. Coral reefs suffering severe mortality following bleaching can take years or decades to recover, even longer if the underlying resilience of the reef has been compromised.
Disease, sedimentation, pollutants and changes to salinity can also cause coral bleaching, however these tend to affect corals at a local scale. Bleaching caused by rising sea temperature is of particular concern because of the scale at which temperature-induced bleaching can affect coral reefs. Projections of future temperatures suggest that mass coral bleaching events could become an annual phenomenon in the course of this century. As bleaching events increase in frequency, the resilience, or the ability of reefs to cope with change, will become critically important to their long-term prospects.
While we have been grappling to understand the implications of increased temperatures for coral reef systems, other vulnerabilities are also coming to light. The implications of ocean acidification for the physical and ecological foundations of coral reefs could be extreme. Increasing CO2 concentrations have already resulted in a reduction of oceanic pH but further reductions are expected over the next 100 years. As pH declines in the oceans, the ocean carbonate system also changes to lower aragonite (a form of calcium carbonate) supersaturation. This has important implications for all calcifying reef-building organisms, such as corals and crustose coralline algae, as they rely on carbonate supersaturation to form their skeletons. As the ability of calcifying organisms to produce carbonate is slowed, the dissolution of them will also increase. This is predicted to alter the calcification balance so that rates of reef building will fall below the rates of erosion. This will make low-lying coastlines more vulnerable to oceanic wave energy.
Coral reefs form the foundation of a diverse and rich community of plants and animals. In addition to declines in coral cover, benthic communities will shift towards species more resistant to climate change impacts, including macroalgae and fewer species of corals. Many animals, including a range of reef-dwelling fish, are reliant on the structure of coral reefs for habitat. These, along with obligate coral feeders, will be the most rapidly affected by changes in coral cover. The faunal communities of coral reef ecosystems will also face direct impacts of climate change. Many species associated with reefs, such as microbes, fish, marine turtles and seabirds, have temperature-sensitivities that indicate the potential for future impacts under climate change scenarios.
While the need is clear, identifying, practical and effective management responses to climate change impacts has proven challenging. Coral reef managers are unable to directly influence the source of the stress — rising atmospheric CO2 concentrations and temperature. This makes managing coral reefs for climate change a particularly challenging task. There are current moves towards resilience-based management; efforts are made to optimise the ability of reefs to cope with the impacts of climate change. This new and developing field for coral reef management holds promise for the future of coral reef ecosystems in the face of climate change. Such management approaches, however, must also be coupled with strategic efforts to reduce greenhouse gas emissions.
Further information: GBRMPA, (07) 4750 0700
