“Oyster shells are like slow-dissolving TUMS in the belly of Chesapeake Bay”

Says Roger Mann of Virginia Institute of Marines Sciences

“Oyster shells are like slow-dissolving TUMS in the belly of Chesapeake Bay,” explained Mann, in a release. “As ocean water becomes more acidic, oyster shells begin to dissolve into the water, slowly releasing their calcium carbonate, an alkaline salt that buffers against acidity. An oyster reef is a reservoir of alkalinity waiting to happen."

The team’s calculations suggest that in 1870, before people began large-scale harvesting of oyster meat and shells from the Chesapeake, the amount of oyster shell exposed to Bay waters was more than 100 times greater than today, with an equally enhanced capacity to buffer acidity.

“Our data show that that oyster reefs likely played a key role in the pH budget of pre-harvest Chesapeake Bay,” Mann said. “The amount of carbonate in the shells of living oysters at that time was roughly equal to the total amount of carbonate dissolved in the modern Bay. If similar numbers of oysters were alive today, they could take up about half of the carbonate that rivers currently carry into Bay waters.”

Many people are familiar with the notion that the cloudy waters of the modern Bay would be clearer if over-harvesting and disease hadn’t drastically reduced the oyster population and its capacity to filter particles from the water. Mann said, “Our study suggests a similar loss of ecosystem function, but in terms of buffering acidity rather than improving water clarity. This has significant ecological ramifications, but hasn’t really been on anyone’s radar screen.”

Assuming that the Bay water is in equilibrium with atmospheric CO2 (the source of acidity), the Bay and because the alkalinity and pH of the Bay tends to decrease with decreasing salinity, the upper part of the Bay would be undersaturated with respect to CaCO3 (calcium carbonate would tend to dissolve), while the lower part would be net supersaturated (calcium carbonate would tend to spontaneously precipitate) is balanced on the knife edge of calcium carbonate saturation.

Of course the world is not that simple. Because the bay is almost certainly net heterotrophic, (meaning it uses more fixed carbon than it produces), the Bay tends to produce CO2 on average, and net undersaturation is more likely than predicted base on atmospheric CO2 concentrations. Moreover, phytoplankton blooms locally can use up the CO2, and produce more saturating conditions.

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0005661