Measuring pH in the Bay is a challenge. Levels can vary widely over time and at different locations, and officials say that the available data don't indicate that acidity is rising in Maryland. The top graph shows the ranges of monthly pH at one station in the Bay, Cedar Point, from 1985 to 2010. The bottom graph shows variation of pH over 14 days in July 2010 at Fort Howard. Source: Maryland Department of Natural Resources, Eyes on the Bay.

LEVELS OF ACIDITY MAY BE RISING in parts of the Chesapeake Bay, and as a result, oyster larvae may be dying and oyster shells may be thinning. According to state and federal regulators, there isn't yet enough proof to say the Bay is suffering from acid indigestion or to prescribe an antacid. One environmental advocacy group, however, has been working the regulatory umpires to persuade them otherwise.

A byproduct of carbon dioxide inputs from the land and the air, acid levels can fluctuate wildly in an estuary like the Bay, presenting a challenge for anyone seeking to measure or control them. Levels of pH, the numerical scale for acidity, can vary hugely over months and even hours, much more than in the open ocean. In the Bay, measurements in a single year at a single location can vary from 6.5 (more acidic) to 8.5 (less acidic) on the pH scale (see box, Chemistry 102: The pH Scale). That's a difference of more than one hundred times. Ebbs and flows of fresh water from rivers contribute to this unevenness; so do tides that stir the water.

The federal government acknowledged the natural variability of pH in estuaries when it wrote an allowable range for acidity in salt water under the Clean Water Act back in the 1980s: the range is between 6.5 and 8.5 in pH. Maryland adopted the same range.

So far, the Maryland Department of the Environment, which has lead responsibility for enforcing the act in Maryland, has not seen evidence that the Bay's acidity has transgressed those boundaries, says the agency's Matt Stover. The data don't indicate that the water is growing more acidic. "At this point, it doesn't seem like the localized data are conclusive enough to show that there's an immediate effect on shellfish" from acidity levels, he says.

That conclusion doesn't satisfy Miyoko Sakashita, ocean director at the Center for Biological Diversity, an environmental advocacy group. The organization, based in Tucson, Arizona, has been pushing Maryland, Virginia, and other states to declare their coastal waters to be in violation of the law, so far without success. Sakashita argues that Maryland has grounds to declare portions of the Bay as "impaired" because new research, based on laboratory tests, indicates that current pH levels threaten to damage oysters (see separate article, Shell Game). The oysters are "the elephant in the corner," Sakashita says.

The U.S. Environmental Protection Agency, which oversees how the states enforce the Clean Water Act, weighed in on the issue in 2010, encouraging the states to pay attention to acidification in their coastal waters. (The EPA made the statement to settle a lawsuit filed against it by Sakashita's organization.) States should declare waters as impaired by acidity "where data and assessment methods are available," said the agency, while recognizing "that information is absent or limited for [acidification] parameters and impacts at this point in time in many states." The EPA encouraged states to focus on protecting vulnerable ecosystems, including those where shellfish live.

If acidified water indeed threatens the Bay's ecology, communities in the Chesapeake watershed are already planning measures that could ease the risk, by controlling the excess nutrients that contribute to acidified water. According to Sakashita, it's not clear that Maryland would need to do more to control acidity beyond the steps it has already promised to control nutrient threats to the Bay's water quality. Those steps include a reduction of about 25 percent each in nitrogen and phosphorus by 2025. Excess levels of these nutrients feed algal blooms that kill fish and contribute to rising acidity. The EPA has directed Maryland and other states in the Chesapeake watershed to improve stormwater systems and expand vegetative buffer areas that can filter out those nutrients before they reach the Bay.

While those efforts might help control acidity in the Bay, Sakashita and many scientists believe the controls on nutrients must be complemented by new national and global measures to curtail carbon dioxide emissions into the atmosphere. Those gases can dissolve in waters of both the Bay and the open ocean. The EPA won the legal authority to regulate CO₂ emissions under the federal Clean Air Act and began doing so in 2011. Legal challenges to that authority, however, are still underway.