Declining coral growth on the Great Barrier Reef
AIMS has identified widespread and rapid decline in the growth rate of massive Porites coral colonies on the Great Barrier Reef, with rising sea surface temperatures and coral bleaching episodes causing a decline in coral calcification.
Coral calcification is the rate at which reef-building corals lay down their calcium carbonate skeleton. It is a measure of coral growth, which is important for healthy reef ecosystems.
AIMS’s recent study of the central Reef has demonstrated that mass bleaching events could suppress coral calcification rates for up to four years.
This research comes in the wake of our 2012 study, which revealed a loss of over 50 per cent of the coral cover since 1985 due to storm damage, crown-of-thorns starfish and coral bleaching.
For the recent study, our scientists analysed skeletal records of 328 colonies of massive Porites coral housed in the AIMS coral archive. These colonies originated from 69 reefs throughout the Reef.
Long-term records show that this severe and sudden decline in coral growth is unprecedented in at least the last 400 years, and that 1990 may have been a ‘tipping point’ for coral calcification rates on the Reef.
Follow-up research at AIMS shows a high tolerance in massive Porites to ocean acidification, but a non-linear response in growth rates to temperature. Average coral calcification rates are related to average sea surface temperature but they can decline when short-term seawater temperatures are above or below optimal levels.
Similar studies have now been conducted in other parts of the world and on other coral species. Most of these studies came to similar conclusions.
The widespread nature of the observed decline—affecting reefs from inshore to offshore and north to south along the Reef—argues against localised agents, such as water quality, being the main factor causing the decline.
Although water quality and land-based runoff are not the causes of declining calcification, research from AIMS and other organisations has shown that high levels of nutrients and sediments lead to high macroalgal cover, low coral biodiversity and low rates of coral recruitment on inshore reefs, slowing rates of coral recovery after disturbances, and increasing frequency of outbreaks of crown-of-thorns starfish.
Coral reefs, including the Great Barrier Reef, are facing unprecedented pressure worldwide due to climate change, changes in water quality from terrestrial runoff and over-exploitation of reef resources.
Substantial reductions in global CO2 emissions, together with effective water quality management to reduce macroalgal competition and facilitate coral recovery, are required to ensure the long-term biodiversity, and perhaps survival, of coral reefs.
Despite the Great Barrier Reef being one of the best-managed reef system in the world, coral cover has declined alarmingly since 1985. Fundamental environmental changes are under way, driven by increased concentrations of carbon dioxide in the atmosphere, to which no reef can be immune.
Efforts to reduce local and regional pressures must be maintained and enhanced to maximise the resilience of the Reef.
To stabilise long-term atmospheric carbon dioxide concentrations at a level that would prevent a global warming by more than 2 °C, the available science suggests that substantial emission reductions are needed. Even if carbon dioxide concentrations were stabilised at 450 ppm (parts per million), the growth and health of the Reef will continue to be affected.
As Australia's tropical marine research agency, AIMS faces the significant challenge of filling current knowledge gaps about how the Reef will respond to ongoing climate change. Without global commitment and implementation of greenhouse gas reductions strategies, the Reef’s marine climate of the will change. This knowledge will underpin global and Australian policy and management decision-making, helping to ensure the ongoing health of our reefs and providing a basis for developing mitigation options if they are needed in the future.