Ocean acidification leads to a reduction in the levels of calcium carbonate, an essential component of the shells and skeletons produced by many marine organisms, including mollusks and corals. As carbonate saturation declines, the rate at which these organisms are able to construct and maintain such vital supportive and defensive structures is significantly decreased. Although the effects of ocean acidification on non-calcifying marine organisms are not well documented, recent studies suggest that decreasing pH levels may interfere with the ability of some reef fishes to navigate using olfactory cues. Ocean acidification also has been shown to reduce metabolic rate and oxygen transport capacity in larger animals.
If current acidification trends continue, seawater will corrode calcium carbonate structures. Scientists predict that erosion rates may outpace calcification rates by 2050 (Orr et al., 2009). Organisms unable to adapt to the reduction in available carbonate are likely to experience significant population crashes. Because of the complexity of marine food webs, the disappearance of any single carbonate-dependent organism, such as the pteropod, could have a devastating impact on a large number of species.
The effects of ocean acidification, coupled with other global changes, such as increasing water temperatures and widespread coastal and marine pollution, are likely to result in increasingly fragile marine ecosystems. This fragility, in turn, poses an economic, as well as an environmental threat: coral reefs, for example, are of significant value to a variety of industries, including the fishing and tourism trades, and offer coastal regions a measure of protection against hurricanes (which may themselves become increasingly frequent and severe as global warming contributes to higher air and ocean temperatures).
It is important to note that ocean acidification does not entail negative consequences for all marine organisms. On the contrary, current research indicates that some species will be positively affected by ocean acidification – the photosynthetic rates of certain seagrasses, for example, have been shown to increase in response to elevated CO2 levels. It is essential, however, that we understand the ecological and societal implications associated with the changes in species distribution that seem likely to result from the continuation of current carbon-emission trends.