Will human-induced changes in seawater chemistry alter the distribution of deep-sea scleractinian corals?

tion state of the world’s oceans are changing as a result of the addition of fossil fuel CO2 to the atmosphere (Kleypas et al. 1999; Feely et al. 2004; Orr et al. 2005). The pH of surface oceans has dropped by 0.1 units since the industrial revolution and if fossil fuel combustion continues at present rates, the pH of the world’s oceans will probably drop another 0.3 to 0.4 units by 2100 (Mehrbach et al. 1973; Lueker et al. 2000; Caldeira and Wickett 2003). “This influx of anthropogenic CO2 is causing the world’s oceans to become more acidic, to the detriment of corals and other marine calcifiers, including plankton, which occupies the base of marine food webs. Corals and some species of plankton (coccolithophores and foraminiferans) use carbonate ions obtained from the surrounding water to build their skeletons and protective shells. As oceanic pH and carbonate ions decrease as a result of rising fossil fuel CO2 levels, the calcification mechanisms and abilities of many marine organisms will be negatively impacted. In recent decades, only half of anthropogenic CO2 has remained in the atmosphere; the other half has been taken up by the terrestrial biosphere (20%) and the oceans (30%) (Feely et al 2004; Sabine et al 2004). This uptake initiates a series of chemical reactions, increasing the hydrogen ion concentration (H), lowering pH, and reducing the number of carbonate (CO3 ) ions available in seawater. All of this will make it more difficult for marine calcifying organisms to form biogenic calcium carbonate (CaCO3). Although little is known about the effects of decreasing aragonite saturation state on deep-sea corals, lab experiments have conclusively shown that lowering carbonate ion concentration reduces calcification rates in tropical reef builders by 7–40% (Gattuso et al. 1999; Langdon et al. 2000, 2003; Marubini et al. 2003). In fact, all marine calcifying organisms tested to date have shown a similar negative response to decreasing carbonate saturation state. As the world’s oceans become less saturated over time, corals are expected to build weaker 141