Ocean Acidity

Once I thought of the ocean as infinite. The constancy of seawater was taken for granted.  But, now as we see evidence of increasing concentrations of mercury in seawater, if seems that the human mobilization and disposal of wastes worldwide is taxing the infinite dilution capacity of the seas.

And, there is good evidence that the ocean is also getting more acid. Why?  When carbon dioxide dissolves in water, it creates a weak solution of carbonic acid.  If a lot of carbon dioxide dissolves, as in a bottle of soda, the solution is very acid.  When only a small amount dissolves, the solution is weakly acid.  But, as carbon dioxide rises in Earth’s atmosphere, more of it enters the oceans and they are becoming more acid.  There is no climate model to criticize in this chain of logic.

In water, acidity is expressed in units of pH.  Neutral is 7.0 and greater acidity is seen with lower values.  Lemon juice has a pH of about 2.0.  In remote regions, the pH of rainfall is about 5.6, reflecting a small amount of acid-generating CO2 that has dissolved in it as rain falls through the atmosphere.

Values above pH = 7.0 indicate increasing levels of alkalinity—that is, less acidic. Older textbook indicate the pH of seawater was uniform at 8.3.  Now, in many regions the value is as low as 8.1 and dropping at a rate of about 0.02 units per decade.  Can a drop of pH of 0.2 units be a big difference?  Unfortunately, yes. Since pH is a logarithmic value, a water sample of pH 8.1 is 37% less alkaline than water at a pH of 8.3.

Why should we care if the pH of seawater is declining, indicating that the oceans are getting less alkaline, or alternatively more acidic?  Many ocean organisms use calcium carbonate for their outer skeletal material.  This is obvious in clams, oysters, and mussels, but harder to see in the myriad small organisms that are part of the phytoplankton in the seas. Calcium carbonate dissolves in acid, so it is more difficult for these organisms to live in more acid waters.  Below a pH of about 7.5, mussels can’t precipitate calcium carbonate at all (Gazeau et al. 2007).

Studies of the oceans’ history, written in marine sediments, show that periods when the CO2 content of the atmosphere was very high, the oceans were very acidic, and many marine organisms went extinct.  Unfortunately, in the clamor about global climate change, the quiet side of rising CO2 in the atmosphere, is that we are making the oceans more acidic every year, with the likelihood of extinction of many of the hard-shelled marine organisms that we all enjoy. Not all areas and not all organisms will be impacted equally. Coastal areas of the Pacific Northwest and the Gulf of Maine are most vulnerable; areas in Massachusetts and southeastern North Carolina are not just vulnerable, but especially dependent on the shellfish economy.

What to do?  Short of speeding the transition to energy sources that do not increase the concentration of carbon dioxide in Earth’s atmosphere, there are only a limited number of options.  Some organisms, such as sea urchins, shift their metabolism to become more tolerant of acid waters.  Farm-raised shellfish could be reared in pens with additions of crusted limestone, which would help maintain the pH of the water above thresholds of vulnerability. But, it will be almost impossible to protect the ocean’s phytoplankton that are the basis of the food chain for all marine organisms.

What should be most disconcerting is this evidence that the effluent of modern society—from 7 billion humans on the planet—is causing a change in the basic chemistry of our environment.  When is enough, enough?



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