The researchers investigated 328 colonies of massive Porites corals from 69 reefs of the Great Barrier Reef (GBR) in Australia.

Factors that determine coral growth and calcification rates include competition for space; water quality; salinity; diseases; irradiance; currents; large-scale and long-term oceanographic oscillations; SST (sea surface temperature); temperature stress; and carbonate saturation state.

After eliminating competition; terrestrial runoff and salinity; diseases; benthic irradiance; and the inter-decadal Pacific Oscillation, the researchers suggest that sea surface temperature and carbonate saturation state “are the two most likely factors” to have affected calcification at a GBR-wide scale.

The supersaturation of tropical sea surface waters with the calcium carbonate mineral forms calcite and aragonite is considered a prerequisite for biotic calcification, with saturation state being a function of pH and temperature. Since industrialization, global average atmospheric CO₂ has increased by 36% (from 280 to 387 parts per thousand), the concentration of hydrogen ions in ocean surface waters has increased by 30% (a 0.1 change in pH), and the aragonite saturation state (Ω_arag) has decreased by 16%. Studies based on meso- or microcosm experiments show that reduced Ω_arag, due to the doubling of CO2 as compared with preindustrial levels, reduces the growth of reef-building corals by 9 to 56% (6), with most of these experiments suggesting a linear relationship between calcification and Ω_arag.

Ω_arag data from the GBR or adjacent waters are sparse, but estimates of a global decline in Ω_arag of 16% since the beginning of global industrialization are similar in magnitude to our finding of a 14.2% decline in calcification in massive Porites. However, the decline in calcification observed in this study began later than expected, based on the model of proportional absorption of atmospheric CO₂ by the oceans’ surface waters. Thus, our results may suggest that, after a period of a slight increase in extension and prolonged decline in density, a tipping point was reached in the late 20^th century. The nonlinear and delayed responses may reflect synergistic effects of several forms of environmental stress, such as more frequent stress from higher temperatures and declining Ω_arag. Laboratory experiments have provided the first evidence documenting strong synergistic effects on corals, but clearly more studies are needed to better understand this key issue.

...If Porites calcification is representative of that in other reef-building corals, then maintenance of the calcium carbonate structure that is the foundation of the GBR will be severely compromised. Verification of the causes of this decline should be made a high priority. Additionally, if temperature and carbonate saturation are responsible for the observed changes, then similar changes are likely to be detected in the growth records from other regions and from other calcifying organisms. These organisms are central to the formation and function of ecosystems and food webs, and precipitous changes in the biodiversity and productivity of the world’s oceans may be imminent.

—De’ath et al. (2008)

Resources

• Glenn De’ath, Janice M. Lough, Katharina E. Fabricius (2009) Declining Coral Calcification on the Great Barrier Reef. Science 2 January 2009: Vol. 323. no. 5910, pp. 116 - 119 doi: 10.1126/science.1165283

• Elizabeth Pennisi (2009) Calcification Rates Drop in Australian Reefs. Science 2 January 2009: Vol. 323. no. 5910, p. 27 doi: 10.1126/science.323.5910.27