Acidity levels in some parts of the Chesapeake Bay are increasing faster than in the ocean, scientists have found, and that could spell trouble for rebuilding oyster populations in those areas.

Looking at 23 years of water quality readings, researchers found acidity levels increasing in the saltier waters of the bay, while decreasing in the fresher portions. The study, conducted at the University of Maryland Center for Environmental Science, appears in the online journal Estuaries and Coasts.

Acidity levels have been rising in the oceans, which scientists have attributed to increasing carbon dioxide levels in the atmosphere. But the study's authors suggest increasing the acidity levels are a byproduct of the decomposition of algae blooms in the upper Bay - blooms that are fed by an influx of nutrients from sewage treatment plants, farm runoff and other sources.

Laboratory studies show that acidity levels akin to what are being measured in some parts of the bay can lead to thinner shells for oysters, making them more vulnerable to crabs and other predators.

"With oyster populations at historically low levels, increasingly acidic waters are yet another stressor limiting the recover of the bay's oyster populations," marine biologist Roger Newell said in a news release about the study.

George Waldbusser of Oregon State University, the study's lead author, said lower acidity in some bay rivers that once supported large oyster populations could help them come back. But increasing acidity levels in other areas may exacerbate their recovery from disease, habitat loss and historic overfishing.

For more on the study, go here.

(Oysters at Horn Point Laboratory, 2006 Baltimore Sun photo by Kim Hairston)