pH, salinity, & nutrients

More Acidic Oceans: A Threat to Coral, Mollusks, and Seaweed

May 7, 2012, 6:40 pm
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Solubility of calcium carbonate in seawater. Shown is the stability of aragonite, a form of CaCo3, in the oceans at various latitudes at three different times: preindustrial; in the year 2000; and in the year 2100 under four different scenarios for greenh

Rising atmospheric carbon dioxide concentrations have increased dissolved inorganic carbon (abbreviated as DIC) consisting of carbon dioxide (CO2), bicarbonate ions (HCO3-), and carbonate ions (CO3 2-). Increased levels of DIC have, in turn, increased water acidity. If atmospheric CO2 concentrations climb from 350 parts per million (ppm) to 1000 ppm, as they well may do, the DIC in a glass of pure water sitting on your table will go from 14.1 micromoles per liter (µmol L-1) to 37.4 µmol L-1, and its pH will decrease from 5.65 to 5.42. Such a pH shift may seem nominal, but pH is based on a logarithmic scale, and so the corresponding proton concentrations in the glass will increase 70%, from 2.2 µmol L-1 to 3.8 µmol L-1.

Seawater, because of the salts dissolved in it, is significantly more alkaline than pure water. The pH of seawater now averages 8.1, which translates to a proton concentration of 0.008 µmol L-1. Given this low starting point, proton concentrations in seawater may increase 250% as atmospheric CO2 concentrations rise. Sea life will be subjected to pH-induced changes in conformation of biochemicals, storage of biochemical energy, and the stability and availability of inorganic compounds.

Of particular concern is that many forms of sea life have exoskeletons of calcium carbonate (CaCO3) that will dissolve into, rather than precipitate out of, seawater. The balance between these two processes depends on pH and CO2 as well as on temperature, pressure, and salinity. Aragonite is the form of CaCO3 found in the exoskeletons of macroalgae (seaweeds), corals, and mollusks. According to some predictions, oceans in the southern hemisphere will become undersaturated for aragonite by the end of the century. This means that aragonite will be more likely to dissolve into seawater; exoskeletons will begin to thin; and macroalgae, corals, and mollusks will be put at risk. [1]

[1] Orr, J. C., V. J. Fabry, O. Aumont, L. Bopp, S. C. Doney, R. A. Feely, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, R. M. Key, K. Lindsay, E. Maier-Reimer, R. Matear, P. Monfray, A. Mouchet, R. G. Najjar, G. K. Plattner, K. B. Rodgers, C. L. Sabine, J. L. Sarmiento, R. Schlitzer, R. D. Slater, I. J. Totterdell, M. F. Weirig, Y. Yamanaka, and A. Yool (2005) Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms. Nature 437:681-686.

This is an excerpt from the book Global Climate Change: Convergence of Disciplines by Dr. Arnold J. Bloom and taken from UCVerse of the University of California.

©2010 Sinauer Associates and UC Regents



Bloom, A. (2012). More Acidic Oceans: A Threat to Coral, Mollusks, and Seaweed. Retrieved from


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