Concern has grown in recent years about the huge amount of airborne nitrogen that drifts into the Bay watershed from upwind pollution sources.

Now, a new study may give Bay states a new gripe about their upwind neighbors — some of their mercury is building up in fish within the watershed.

A recent report produced for the Maryland Department of Natural Resources found that large fish in some reservoirs had mercury levels in their flesh that exceed federal guidelines.

Too few fish were sampled to conclude whether human consumption advisories are warranted. But the state is conducting an expanded study this year to determine whether mercury levels in fish are high enough to pose a threat to humans who eat them.

“We’ve really got to get a better handle on it,” said Paul Jiapizian, who is overseeing the study for the Maryland Department of the Environment. “It is an important fishery, but we also have to protect public health.”

The study for the DNR found that large striped bass, chain pickerel, walleye, white crappie and largemouth bass in some reservoirs had mercury levels which are near, or exceed, levels for human consumption advisories that are used in other states or recommended by federal agencies. Problems were not found in fish from the Bay or rivers.

The issue reaches beyond Maryland’s borders, said Cynthia Gilmour, a scientist with the Academy of Natural Sciences’ Estuarine Research Center, who did the DNR study. Because most mercury results from air pollution that is spread over the entire region, she said it would be a “big surprise” if levels of the heavy metal are not elevated in reservoirs in neighboring states.

High exposures to mercury can cause developmental problems in fetuses and delay walking and talking in children, as well as lower scores on nervous system function tests.

Mercury is also one of the Bay Program’s “toxics of concern” because of its potential to bioaccumulate, or build up, in the food chain, ultimately affecting top predators.

As the Bay Program moves toward adopting a new toxics strategy, the issue illustrates the challenge it faces in trying to eliminate toxics from the system, as much of the mercury originates from sources outside the watershed.

Mercury exists naturally in the soil and can leach into waterways over time, but most elevated mercury concentrations are thought to originate from air pollution.

In a 1997 report, the EPA estimated that about 158 tons of mercury were emitted nationwide, about 87 percent of which was the result of combustion — either through the incineration of wastes, or the burning of fossil fuels, which contain trace amounts of mercury. The rest came from natural sources.

Globally, as much as 5,500 tons of mercury may be emitted to the atmosphere, both from natural and human sources, according to the EPA.

In recent years, the agency has cracked down on emissions from incinerators and is weighing whether to control emissions from power plants, something that could have a huge price tag.

Still, that wouldn’t end the problem: Once it enters the atmosphere, mercury can stay there for up to a year, traveling around the globe. Of the 87 tons of mercury that the EPA estimates is ultimately deposited on the United States annually, nearly half — 35 tons — is transported from other parts of the world.

While mercury deposition is widespread, determining whether it is a problem in a particular place is more complex.

Generally, mercury has to be retained in a lake or impoundment long enough to work its way up the food chain and build up in fish. Mercury buildup in fish, therefore, isn’t an issue in the Bay or in free-flowing rivers and streams. But it has been a worry in recent decades for areas with plenty of lakes, such as the Northeast and the Midwest. Nationwide, more than 40 states have fish consumption advisories.

Complicating the issue is that inorganic mercury in deposition is not as harmful as its organic form, methylmercury. Methylmercury is typically created after mercury is deposited and transformed by bacteria. Different types of bacteria turn mercury into methylmercury more effectively than others.

“For any given amount of mercury deposition to a system, you could have an extremely wide range of methylmercury production rates,” Gilmour said. “There are some systems that just don’t make much methylmercury, and there are some systems that make a lot.”

Two reservoirs could receive the same amount of mercury deposition, and one could have a methylmercury bioaccumulation problem, while the other would not. Factors such as acidity, the amount of organic material in the water and the type of soils in an area affect methylmercury production.

One of the most effective types of bacteria in transforming mercury, Gilmour said, are sulfate-reducing organisms. Because they are fueled by the sulfuric acid in deposition — acid rain — it could potentially compound the problem regionally, because the mid-Atlantic receives some of the nation’s most acidic rainfall.

While mercury deposition is a widespread problem, local sources can be important, too. Rob Mason, of the University of Maryland’s Center for Environmental Science, has found deposition in Baltimore to be about three times higher than rural monitoring sites in the state.

Deposition in urban areas, where contaminants can be quickly flushed through stormwater systems, may result in elevated levels of mercury in urban streams, Mason said. One study found more mercury coming from the heavily developed Anacostia River than other watersheds.

“In a forested watershed, maybe 90 percent of the mercury is being retained,” Mason said. “But in someplace like the Anacostia, it is going straight through. So not only do you have a higher deposition to the area, but you also have a much lower retention in the watershed.”

None of that automatically translates into a human health threat, though. Mercury can bioaccumulate in humans, but to do so, they have to consume large numbers of big, old fish that have been accumulating mercury for a long time. Small fish usually have little mercury.

Some people face greater risks, though. Studies show that fetuses may be especially vulnerable to exposure; many states recommend that pregnant women restrict their consumption of fish from affected lakes.

The expanded monitoring of Maryland reservoir fish will determine what level of mercury is present in the species and size classes that people are likely to catch and eat. “We know there are definite health benefits from eating fish,” Jiapizian said.“The study will determine what potential health risks are posed to fish consumers who eat the legal limit for these fish and allow people to make informed choices about their fish consumption habits.”

Although the health effects warrant study, scientists say larger concerns could involve environmental effects. Wildlife around a reservoir, for example, are more likely to eat mercury-tainted fish on a regular basis than humans.

Impacts of mercury on fish, birds and mammals include death, reproductive failure, reduced growth and development and behavior abnormalities. Studies in Florida have found potentially harmful mercury concentrations in predators such as the endangered Florida panther, wood storks, loons, eagles, mink and otters.

The fish, themselves, could be affected. “Initially, people wrote off some of the chronic effects, but they are starting to go back and readdress the issue,” said Mason, who is doing the analytical work for the new Maryland study. “There are early reports that maybe fish reproduction may also be influenced by the increase in mercury concentrations.”

Those elevated concentrations are likely to be around for years, Mason said, even if the EPA takes further action to reduce emissions. Enough mercury is in the atmosphere — and already on the ground — to pose problems for years to come as it slowly leaches into streams, he said.

“Even if we shut off the anthropogenic tap tomorrow,” he said, “we would still have a legacy for the next 50 years of this mercury slowly bleeding out that has built up in the surface soils.”

Ultimately, the mercury will end up buried in the sediment of the ocean or lakes.

Copies of the report, “A Preliminary Survey of Size-Specific Mercury Concentrations in Gamefish from Maryland Fresh and Estuarine Waters,” are available on the Maryland DNR’s web site at: www. dnr.state.us/streams/pubs