Declining coral growth on the Great Barrier Reef
Work by scientists at the Australian Institute of Marine Science (AIMS) has identified a wide-spread and rapid decline in the growth rate of massive Porites coral colonies on the Great Barrier Reef. A systematic analysis of potential causes suggests that a combination of rising sea surface temperature and ocean acidification, due to increased carbon dioxide (CO2) in the atmosphere, is likely to be the cause of this dramatically slowed growth. With coral growth being an important determinant of the health of reef ecosystems, there is great concern that this trend will have significant consequences for the Great Barrier Reef, and perhaps for reefs worldwide.
A scientific paper entitled "Declining coral calcification on the Great Barrier Reef" by Glenn De'ath, Janice Lough and Katharina Fabricius, published in the prestigious international scientific journal Science on 2 January 2009, gained widespread attention and has been the subject of public debate. Presented below is an outline of the study and its findings together with a brief discussion of the implications.
Outline and findings:
- The study is based on the analysis of skeletal records of 328 colonies of massive Poritescoral coloniesfrom 69 reefs throughout the Great Barrier Reef (GBR).
- Coral calcificationis shown to have declined by 14.2% since 1990. 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 GBR.
- By a systematic process of excluding all potential alternative explanations, and by comparisons with similar results of published laboratory experiments, the authors suggest that increasing sea surface temperature and ocean acidification are the two most likely factors to have affected Porites calcification at a GBR-wide scale.
Discussion of the results:
- The study considered a range of factors known to determine coral growth and calcification including competition for space, water quality, salinity, diseases, irradiance, currents, large-scale and long-term oceanographic oscillations, temperature stress, sea surface temperature and ocean acidification.All but the last two, both attributable to rising atmospheric CO2, were excluded as being likely causes of the observed decline.
- The observations made during the study were based on the analysis of data recovered from coral cores gathered in the field – they were not based on computer-generated models.
- Coral calcification rates are known to increase linearly with long-term rises in sea water temperature (Lough, 2008a) but respond non-linearly when short-term sea water temperatures are above or below optimal levels (Lough and Barnes, 2000, De'ath et al, 2009).
- While the sharp decline occurred toward the end of a long-term period of increasing sea water temperatures - average temperatures having increased 0.120°C per decade since 1950 (Lough, 2008a) – it coincides with a period characterised by repeated thermal stress events (1990-2005).
- The study took into account variations in calcification responses of corals:
- along the north-south gradient in long-term average sea water temperatures;
- to the warming over the 20th century; and,
- in years that were unusually cold or warm (high and low temperature anomalies).
- Previous studies have shown that the combination of ocean acidification and rising sea water temperatures reduces the carbonate saturation state of seawater, slowing hard coral growth rates.
- The widespread nature of the observed decline – affecting reefs from inshore to offshore and north to south along the GBR – 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 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 (Brodie et al., 2005, Fabricius et al., 2005, De'ath and Fabricius 2008).
- 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 such as seafood (Lough 2008b).
- Substantial reductions in global CO2 emissions, together with effective water quality management to reduce macroalgal competition and facilitate coral recovery (Wooldridge et al., 2005) are required to ensure the long-term biodiversity and, perhaps survival, of coral reefs.
- The GBR is one of the healthiest and is the best managed reef system in the world, as recently confirmed inStatus of Coral Reefs of the World 2008 (Wilkinson, 2008). The Great Barrier Reef Marine Park Authority, informed by the very best and latest marine science, manages the reef superbly. However, there are fundamental processes underway, driven by increased concentrations of CO2 in the atmosphere, to which no reef can be immune.
- While AIMS congratulates the Australian Government on the recently announced Carbon Pollution Reduction Scheme (CPRS), ongoing efforts by State and Commonwealth Authorities to reduce local and regional pressures must be maintained to maximise the resilience of the GBR.
- It must be noted that the CPRS target of a unilateral 5% cut in CO2 emissions (with the possibility of a 15% cut by 2020 contingent upon international agreements) is well short of what science says is needed to achieve the stated objective of stabilising long-term atmospheric CO2 concentration at 450 ppm. Indeed, the science suggests that reductions of less than 25% will mean the goal of 450 ppm is unachievable and, even at 450 ppm, there will be continuing impacts on the growth and health of the Great Barrier Reef (Hoegh-Guldberg et al 2007).
- As Australia's tropical marine research agency, a significant challenge for AIMS into the future is the task of filling current knowledge gaps about how coral reefs, including the GBR, are responding to climate change. This knowledge will underpin Australia's 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.
Ocean acidification is a consequence of increasing atmospheric carbon dioxide, the main greenhouse gas. About 30 per cent of the extra carbon dioxide humans are putting into the atmosphere is being absorbed by the oceans, resulting in a change in the alkaline/acid balance (their "pH") the oceans. Oceanic pH has already dropped by 0.1 units on the pH scale (become more acidic) and could decrease by 0.4 by the end of this century. This would be well outside the realms of anything marine organisms have experienced over hundreds of thousands of years.
Massive Porites coloniesare commonly used to construct records of past environmental conditions as (like tree growth rings) they contain annual density bands, are widely distributed, are long-lived (individual colonies can continue to grow for several centuries) and changes in environmental conditions are recorded in their skeletons.
Coral calcification , a measure of coral growth, is a term used to describe the rate at which reef-building corals lay down their calcium carbonate skeleton and is a product of colony extension (dimensional growth) and density.
Hard corals extract the raw materials for their calcium carbonate skeletons from the surrounding water column. The availability of these raw materials is determined by the "carbonate saturation state" of that water.