Overview of the issue
Ocean acidification (OA) is the process of declining pH in seawater that results from the ocean’s absorption of anthropogenic carbon dioxide (CO2) from the atmosphere. The average pH of the surface ocean has already declined by 0.1 of a pH unit to 8.1 since the beginning of the industrial revolution and is predicted to decline by another 0.2 to 0.4 units by 2100. The carbonate ion concentration has also declined by more than 10% during this period. The carbonate concentration in seawater is the prime determiner of whether biogenic calcium carbonate either precipitates or dissolves; hence, concern is growing for those organisms with shells of calcium carbonate, particularly those made of aragonite that dissolves more readily than calcite. This growing concern is already well justified by the experimental studies undertaken so far of OA effects on calcifiers for realistic carbon emission scenarios provided by the Intergovernmental Panel on Climate Change (IPCC). The majority of corals tested reduce calcification, by up to 56% (Kleypas et al. 2006), an effect that can be enhanced by increased temperature (Reynaud et al. 2003). Thus, there is great concern for the future of coral reefs and the ecosystems that their physical presence represents. Calcification rates in commercially valuable mollusks decrease linearly with increasing CO2 in short-term exposures. Important species of calcareous phytoplankton called coccolithophores show reduced calcification at lower pH, but there is a species-dependent response to OA, and biogeochemical cycles may also be affected (from summary by Gazeau et al. 2007; Martin et al. 2008).
These emerging results are alarming to say the least and may herald a near future where we wave good-bye to significant calcified ecosystems and highly valued commercial shellfish species. As ecotoxicologists, should we really continue to investigate the nuances of using Mytilus spp. as bioaccumulating biomonitors if, perhaps, Mytilus spp. won’t be able to adequately grow a shell in the not-too-distant future?
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Jeffree, R. A., 2009. Ocean acidification: a prognosis and treatment for this eclipsing issue in marine ecotoxicology. Integrated Environmental Assessment and Management 5(1): 173–181. Article.
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