A recent scientific paper has raised questions over whether dissolved oxygen conditions in the Bay have been getting better, worse—or stayed pretty much the same—in recent years. The answer, it seems, depends on how one looks at the data, and how one defines low oxygen—or hypoxic—conditions.

Since the mid-1980s when a comprehensive Baywide monitoring network was established, the state-federal Bay Program says hypoxia has stayed about the same, or has slightly improved.

But the Bay Program considers any water that has less than 5 milligrams of oxygen per liter of water as hypoxic. (By contrast, surface waters can have as much as 14 milligrams of oxygen per liter of water.)

A paper in the August issue of the journal Estuaries, though, said that dissolved oxygen conditions in the Bay had had not improved and may have worsened during the past decade. That paper didn’t use 5 mg/l as its definition of hypoxia. Instead, it looked at the volume of water with less than 2 mg/l—a level that is extremely stressful, if not lethal, to most organisms.

Jim Hagy, a scientist with the EPA’s National Health and Environmental Effects Research Laboratory in Florida, said that because he never looked at the 5 mg/l level, he has “no disagreement” with the Bay Program’s interpretation of the data. Further, he said, an increasing trend was not evident in his data if one only examined the 1985–2001 figures, but was clear when those years were examined in the context of the longer 1950–2001 period he studied. “In this light,” he wrote in the paper, “there does not yet appear to be substantial evidence that hypoxia has begun to stabilize or decrease.”

Rich Batiuk, associate director for science with the EPA’s Bay Program Office, said more light may be shed on the trend issue next year as the Bay Program undertakes its own effort to assemble a dissolved oxygen retrospective using data going back to 1950.

The goal, he said, is to look at a broader range of dissolved oxygen concentrations throughout the Bay over the years and—using computer enhancement techniques—extrapolate the historical data over broader areas of the Bay. Then, he said, as nutrient loads to the Chesapeake are reduced, scientists and state and federal officials can see if dissolved oxygen levels in the Bay are similar to those seen in the past under similar conditions. “Mother Nature has done a lot of natural experiments during this 50-year period that we have not fully taken advantage of,” Batiuk said.