As well as global warming, there is a second problem facing our seas which is potentially catastrophic to all marine organisms that, like corals, produce limestone shells or skeletons - ocean acidification. Since the industrial revolution began, vast amounts of carbon dioxide (CO2) have been pumped into the atmosphere, increasing the concentration of atmospheric CO2 from 280 ppm to 390 ppm in 2010 (click here for a video illustration). Approximately one third of the CO2 released by human activity has dissolved in seawater, affecting the carbonate chemistry of the oceans and decreasing ocean pH.
The changes in carbonate chemistry of the oceans will affect cold-water corals in many ways. Firstly, increasing concentrations of dissolved CO2 in the oceans decrease the carbonate saturation of the water, which means there are less carbonate ions available for corals to make their calcium carbonate skeletons. Field and lab studies of tropical corals found that increasing CO2 and assocated pH decline reduce coral skeleton growth by between 3 and 54%, and a similar pattern may be seen in cold-water corals. Indeed, one study found a 0.3 unit decline in pH reduced growth by 56% in Lophelia pertusa. As it has been predicted that by the end of the century pH will have dropped by 0.3 to 0.5 units and temperature increased by a further 1.1 to 6.4 °C, these findings are worrying.
As well as this, ocean acidification could even affect the distribution of cold-water corals by changing the depth of the aragonite saturation horizon. Below this horizon, corals cannot form their calcium carbonate skeletons, as the availability of aragonite (the form of calcium carbonate that scleractinian corals use to form their skeletons) is too low. Furthermore, these skeletons and the skeletons of dead corals that form deep-sea reefs would actually start to dissolve and get worn away by other organisms. By the end of the century, the water chemistry of some oceans and areas will be such that cold-water corals cannot exist there. This would have profound implications not only for the distribution of cold-water corals, but also for the thousands of species which rely on these reefs as habitats. Indeed, any processes affecting the oceanic food web could have far-reaching consequences for these out-of-sight ecosystems.
Future research goals include assessing the influence of increasing CO2 and temperature on cold-water corals from the larval stage right through to reproductively viable adults, and to the ecosystem beyond. Many such studies are in their early stages, with the aim of understanding how these species may respond to environmental change. You can learn more about the experiments being conducted, and conduct your own at the ocean acidification virtual lab