Shellfish Face An Uncertain Future In High Carbon Dioxide World

ScienceDaily (May 28, 2009) — Overfishing and disease have decimated shellfish populations in many of the world's temperate estuarine and coastal ecosystems. Smithsonian scientists, led by Whitman Miller, ecologist at the Smithsonian Environmental Research Center in Edgewater, Md., have discovered another serious threat to these valuable filter feeders—rising levels of atmospheric carbon dioxide that contribute to the acidification of open ocean, coastal and estuarine waters

For shellfish and other organisms that have calcium carbonate shells and structures, the problem begins when atmospheric CO₂ dissolves in seawater and creates carbonic acid that is then rapidly transformed into carbonate and bicarbonate ions in the water. Increased acidity tips the balance toward bicarbonate formation and away from carbonate. Less carbonate in the water means that shellfish have fewer building blocks to generate their shells. If the water is acidic enough, shells can even begin to dissolve.

"Estuarine and coastal ecosystems may be especially vulnerable to changes in water chemistry caused by elevated CO₂ because their relative shallowness, reduced salinity and lower alkalinity makes them inherently less buffered to changes in pH than in the open ocean," said Miller. For many calcifying organisms, CO₂-induced acidification poses a serious challenge because these organisms may experience reduced rates of growth and calcification that "when combined with other environmental stresses, could spell disaster."

Their findings are being published in PLoS One.

Larval oysters are thought to be particularly susceptible to acidification since larvae produce shells made of aragonite, a crystalline form of calcium carbonate that is prone to erosion at low pH. Adult oysters continue to build shell but generate calcite, a more durable form of calcium carbonate. In Miller's study, the larvae of Eastern oysters (Crassostrea virginica) and Suminoe oysters (Crassostrea ariakensis) were cultured in estuarine water that was held at four separate CO₂ concentrations, reflecting atmospheric conditions from the pre-industrial era, the present, and those predicted in the coming 50 and 100 years. To test the effects of acidification, Miller monitored their growth and measured the amounts of calcium carbonate deposited in larval shells over the course of one month.

Miller and his team found that Eastern oysters experienced a 16 percent decrease in shell area and a 42 percent reduction in calcium content when specimens in the pre-industrial CO₂ treatment were compared with those exposed to the levels predicted for the year 2100. Surprisingly, the closely related Suminoe oysters from Asia showed no change to either growth or calcification.

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