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 feedersrising levels of atmospheric carbon dioxide that contribute to the acidification of open ocean, coastal and estuarine waters. Their findings are being published in the open-access, peer-reviewed journal PLoS ONE, May 27.
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."
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 Sum
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