An Introduction to Ocean Acidification How could Ocean Acidification affect Coral Reefs?

In shallow tropical waters, massive coral reef structures, composed primarily of calcium carbonate (CaCO₃), offer important habitats to numerous marine organisms and provide coastal protection from storms. Building and maintaining these reef structures requires sustained CaCO₃ production through a number of different processes. Reef-building corals construct intricate three-dimensional frameworks by calcification (biological precipitation of CaCO₃). The reef structure is subsequently bound and strengthened by encrusting coralline algae and marine cements [Manzello et al., 2008].

Tropical coral reefs represent the accumulation of calcium carbonate over many years.

The production rate of CaCO₃ must exceed that of the removal processes (dissolution, storm export, and bioerosion) in order for the reef to grow (termed 'reef accretion'). Studies of CaCO₃ budgets on coral reefs suggest that these building and erosive processes are nearly balanced at most modern reefs, and net reef accretion is small. Most experiments now demonstrate a reduced rate of calcification as a consequence of ocean acidification. Laboratory studies have examined the effects on many types of corals and coralline algae, revealing a range of responses from a 3% to 60% decline in calcification rate for a doubling of atmospheric CO₂ [Kleypas et al., 2006]. Additionally, a recent study of Brain Corals in Bermuda found that calcification rates have declined by 25% over the past 50 years and ocean acidification is a likely contributing factor [Cohen et al., 2008]. Thus, a primary threat of ocean acidification is the potential to compromise the ability for reefs to maintain a positive net accretion, thereby resulting in the loss of critical habitat and coastal protection. Effects beyond reduced calcification rates have also been observed, such as reducing the abundance of crustose coralline algae [Kuffner et al., 2008; Jokiel et al., 2008 ] and a reduced fertilization success in sea urchin's [Havenhand et al., 2008] which would cause significant changes to the community structure of coral reefs. Other potential effects include an increased susceptibility to coral bleaching [Pecheux, 1993], a reduced capacity to tolerate ultraviolet radiation [Reef et al., 2008], and increased bioerosion rates [Tribollet et al., 2008].

	Most coral reefs exhibit very low annual accretion, with net carbonate production nearly balanced against carbonate export, bioerosion, and dissolution. By reducing the growth rate, ocean acidification could shift the balance in favor of net carbonate loss.