Scientists recently found that coral calcification on the Great Barrier Reef is declining, at a rate unprecedented in the last 400 years. Global warming and ocean acidification are again key suspects behind this decline.

AIMS scientists recently found that calcification of the Porites genus, a group of massive coral species, has declined by 14,2% since 1990, which is unprecedented in the last 400 years. Global warming and ocean acidification are again key suspects causing the decline. AIMS, the Australian Institute for Marine Science, is a respected research institute where many top marine scientists actively study our oceans. Recently, November 2008, AIMS scientists (De’ath et al.) published their latest discovery in the renowned scientific magazine Science.

Figure 1: 328 samples of Porites corals were taken from all over the Great Barrier Reef, from 69 different locations (image: Google Earth).

The marine biologists studied an extensive collection of 328 Porites corals sampled on the Great Barrier Reef at 69 different locations. They compared several growth parameters from the coral samples over a long time-interval, ranging from the year 1572 to 2005. They studied the amount of calcification stored in the samples, in terms of density and linear growth, similar to studying year-rings on a tree. They found that calcification declined by 14,2% from 1990 to 2005 (fig.2). In 1990 this decrease was only 0.3%, however, in 2005 linear growth dropped by 1.5% in a single year. When they considered a long period ranging from 1572 to 2001, they found an increase in calcification from 1700 to 1850, and a decrease since 1960, in accordance with their first results.

Calcification of Porites corals on the Great Barrier Reef declined by 14,2% from 1990 to 2005 – De’ath et al, 2008

Figure 2: Plot showing the variation of calcification (grams per square centimeter per year) in Porites corals over time. Calcification was regarded as a combination of extension (centimeters per year) and density (grams per cubic centimeter). These data are based on 1900–2005 data for all colonies. Calcification has declined with 14.2%, from 1.76 g/cm²/y to 1.51 g/cm²/y (modified from De’ath et al, Science, 2008).

According to the researchers, these observed declines are most likely due to global warming (in this case a rise in sea surface temperatures) and ocean acidification. Since the 19^th century, humans have been combusting fossil fuels, which has increased tremendously during the last century. This had lead to an increase in atmospheric CO₂ concentration of 36%, from 280 to 387 parts per million (ppm). About 20% of the released CO₂ has been absorbed by our oceans, which have in effect partly masked global warming. Unfortunately, CO₂ releases hydrogen particles when dissolving in seawater, thereby lowering pH levels. When seawater pH levels decrease, this changes a highly important chemical equilibrium (fig.3). As the pH-level sinks, the carbonate (CO₃^2-) ion concentration decreases. This lowers the ‘aragonite saturation state’, which in part depends on how much carbonate ions are present in seawater. Corals may have more trouble building their skeletons at lower carbonate concentrations, as aragonite - the principal component of coral skeleton - dissolves more easily under such chemical conditions. If pH levels will drop too low, that is ta level of about 7.4, coral skeleton can completely dissolve within a matter of months!

Figure 3: The CO₂ equilibrium. When pH levels drop, more carbonate ions (shown in red) are converted into bicarbonate ions (shown in green). This provides more 'room' for new carbonate ions to dissolve, making it harder for corals to build their skeleton (equation by Tim Wijgerde).

The scientists also report that although calcification increases linearly with temperature, it actually drops severely when temperatures rise above 30°C (86°F). This is because the symbiotic algae which are harbored by the corals are expelled, as they are starting to die off at these temperatures (for more information on pH, CO₂, global warming and coral symbiosis, see the coral science archive). 

Figure 4: Without their zooxanthellae, corals slowly starve to death and will stop growing. Shallow coral reefs are lit by intense sunlight, of which the symbiotic algae residing in the corals make good use. When temperatures get too high, the algae die and are expelled by the corals. Without their partners, corals cannot survive for long, and they have to reacquire their algae quickly (photograph: Hans Leijnse).

They rule out other factors which might explain the decrease of Porites corals, such as competition between coral colonies. Coral colony density however has not increased, but has even declined at several locations. Terrestrial runoff and salinity fluctuations have also been dismissed, because these factors mainly play a role on inshore reefs. The scientists also found a decline in calcification on off-shore reefs. Diseases are common amongst corals, and may decline their growth as well, however the sampled corals were all healthy specimens. The amount of light may also influence coral growth, as it is known that light stimulates this process. Corals may receive up to 95% of their daily required energy from their zooxanthellae, which produce carbohydrates by harnessing the sun’s energy; a process known as photosynthesis. However, it was also found that water turbidity and cloud cover did not change significantly on the Great Barrier Reef over the long sample period. Finally, changes in oceanic currents and pH levels caused by long-term oscillations have also been ruled out.

The fact that this decline in coral calcification has not been as severe as ever recorded during the past 400 years, again underlines the importance of a reduction of CO₂ emissions. Current oceanic pH is already 0.1 degree lower compared to 100 years ago, and the aragonite saturation state (a measure related to the amount of carbonate ions dissolved in seawater) has dropped by 16%. Recent studies have shown that a doubling in atmospheric CO₂ concentration reduces the growth of stony corals by 9 to 56%. Coral larvae have now also been found to decrease in settlement because of decreasing pH levels, thereby reducing successful coral reproduction.

“The fact that this decline in coral calcification has not been as severe as ever recorded during the past 400 years, again underlines the importance of a reduction of CO₂ emissions”

Coral reefs are home to thousands of (in)vertebrate animal species, ranking them amongst the highest species-diversity ecosystems on earth. Millions of people depend on the reefs as their source of food and income, and many countries have economies which rely in part on ecotourism fuelled by the presence of coral reefs. Furthermore, reefs protect the coastlines of 109 countries, which will become more important now tropical storms are increasing in frequency. With the destruction of coral reefs worldwide, our planet would lose these unique ecosystems which are of great ecological, economical, societal and cultural importance.

Figure 5: A tiny, hairy crab living on a sponge. Coral reefs are home to millions of these unique invertebrate animals, of which many have still not yet been identified. With the decline of coral reefs worldwide, all of these species are under equal threat (photograph: Hans Leijnse).

References:

1. Glenn De’ath, Janice M. Lough, Katharina E. Fabricius, Declining Coral Calcification on the Great Barrier Reef, 2008, pp 116-119(323)

2. Ohde S, Hossain MMM, Effect of CaCO3 (aragonite) saturation state of seawater on calcification of Porites coral, Geochem J, 2004, pp 613-621(38)

3. Fine M, Tchernov D, Scleractinian coral species survive and recover from decalcification, Science, 2006, pp 1811(315)

4. J. M. Lough, D. J. Barnes, J. Exp. Mar. Biol. Ecol. 245, 225 (2000).

5. F. Bessat, D. Buigues, Palaeogeogr. Palaeoclimatol. Palaeoecol. 175, 381 (2001).

6. J. Bruno, E. Selig, PloS ONE 2, e711 10.1371/journal. pone.0000711 (2007).

7. M. McCulloch et al., Nature 421, 727 (2003).

8. Falkowski, PG, Dubinsky, Z, Muscatine, L, Porter, JW, Light and bioenergetics of a symbiotic coral. Bioscience, 1984, pp 705–709(34)

9. Muscatine, L. Porter, JW, Reef corals: mutualistic symbioses adapted to nutrient-poor environments. Bioscience, 1977,  pp 454– 460(27)

10. Edmunds, PJ, Davies, SP, An energy budget for Porites porites (Scleractinia). Mar. Biol, 1986,  pp 339– 347(92)

11. J. M. Guinotte, V. J. Fabry, Ann. N. Y. Acad. Sci. 1134, 320 (2008).

12. J. C. Orr et al., Nature 437, 681 (2005).

13. R. Albright, B. Mason and C. Langdon,  Effect of aragonite saturation state on settlement and post-settlement growth of Porites astreoides larvae, Coral Reefs, pp 485-490(27)

14. Caldeira K, Wickett ME, Anthropogenic carbon and ocean pH, Nature, 2003, pp 365(425)