One could think of it as a master plan for the Chesapeake Bay.

Officials from the Bay states are beginning to divide the Chesapeake into specific areas that are critical for different types of fish, crabs, grasses and other Bay dwellers.

Like local government zoning maps that identify where different land uses are appropriate, the new Bay maps detail areas suitable for use by different types of species.

There are shallow water zones where underwater grasses should be able to thrive; deep water zones used by crabs, oysters, clams, flounder and catfish; spawning zones for shad, striped bass and other anadromous fish; and so on.

This aquatic master plan, so to speak, is aimed at setting “designated uses” that are required under the Clean Water Act. It is a critical part of the Bay Program’s effort to define what a “clean” Chesapeake would look like. And, the outcome of the exercise — which will set more strict goals for some areas and less stringent requirements for others — will ultimately drive nutrient and sediment reductions for the next decade.

The Clean Water Act requires states to set standards — or water quality goals — for surface waters within their borders.

Standards consist of two parts: a designated use, and the water quality criteria needed to maintain that use. The water quality criteria can be expressed either numerically or narratively. A numeric goal could be the maximum concentration of certain pollutants that can exist in the water while still supporting a designated use; a narrative goal could be a simple statement that no pollutant may exist at levels that are acutely toxic to animals.

Under the Clean Water Act, the minimum designated use is that the water be “fishable and swimmable.” Existing designated uses in the Bay don’t go much beyond that, although freshwater designated uses routinely acknowledge some special areas, such as trout streams.

The new effort would dramatically change the situation in the Bay by sprinkling far more specific designated uses in a mosaic that would stretch horizontally and vertically throughout the Bay and its tidal tributaries.

Once set, officials would assign water quality criteria to each use. The Bay Program is simultaneously developing new criteria for chlorophyll (a measure of algae), dissolved oxygen, water clarity — and possibly other parameters — that are intended to be tailored to different designated uses. [See “Criteria being developed to determine what is a ‘clean Bay,’” January-February 2000]

Ultimately, water quality managers will match up a particular designated use with the numeric criteria that will achieve that goal. For example, a shallow water habitat designated use intended to support underwater Bay grasses may have water clarity criteria that requires light to be able to penetrate to a specific depth so grasses can grow. An open water habitat designated use may have specific criteria for chlorophyll and dissolved oxygen.

Once each of the designated uses is mapped and matched with new criteria, the Bay Program will crank up its computer models to determine the amount of nutrient and sediment reductions needed to achieve the new goals throughout the Bay.

Officials hope to accomplish that by the end of next year. Then, the Bay states will formally adopt the new designated uses and criteria as their water quality standards.

As that happens, the 40 percent nutrient reduction goal that has driven nutrient reduction efforts since 1987 will become moot. The new goals for nutrients — and, for the first time, sediment — are expected to require far greater reductions.

This new effort is being driven, in large part, because the Bay is on the EPA’s list of “impaired” waters. Unless it is cleaned up before 2011 — something the Bay jurisdictions promise to do in the new Chesapeake 2000 Agreement — the EPA will require the development of an enforceable cleanup plan known as a Total Maximum Daily Load for the watershed. Officials fear TMDLs would be more costly and difficult to implement.

But before they can achieve a clean Bay — and remove it from the impaired waters list — officials have to define exactly what “clean” is. Existing water quality standards don’t do the job. They are often so vague that they do not guarantee that some sensitive habitats — such as grass beds, which have declined dramatically in recent decades — will be protected.

At the same time, existing standards may overprotect other areas. For example, Virginia calls for 5 parts per million of oxygen throughout the water. That’s fine for fish on the surface, but scientists generally agree that even Capt. John Smith wouldn’t have found that much oxygen in the deepest parts of the Bay during the summer, had he looked nearly 400 years ago.

The reason: Natural conditions usually prevent that. Strong spring flows from rivers create a barrier, known as the pycnocline, between the top, fresher layer of water and the saltier bottom water. That keeps the layers from mixing, causing oxygen in the bottom to gradually be used up.

Excess nutrients, which spur algae growth, make matters worse — when algae die, they decompose in a process that uses lots of oxygen. But scientists who have studied sediment core samples from the Bay say that even before colonists arrived, the bottom was often oxygen-starved.

The new standards, for the first time, will recognize that there is no “one-size-fits-all” criteria, or even use, for the Bay. Different species, such as newly spawned juvenile fish, use different areas and have different water quality needs than adult fish. Or, for that matter, a crab or an oyster or a blade of underwater grass.

“We know the deep trench of the Bay is not a spawning ground for striped bass,” said Rich Batiuk, associate director for science with the EPA’s Bay Program Office. “We want to define — where we haven’t for the last 15 years — the water quality conditions we want to restore that make the most sense to the variety of living resource needs that we have.”

Setting a designated use for each Chesapeake-dwelling creature would be a mind-boggling task. So, the Bay Program approach is to use research from the past few decades to lump species with similar habitat needs together.

Migratory finfish, for example, tend to use tidal fresh to low salinity habitats in the upper reaches of the Bay and its tributaries to spawn. Other fish, such as menhaden, Bay anchovy and the adult striped bass that feed on them, use the top several feet of water throughout the Bay. Meanwhile, softshell clams, hard clams, spot, croaker, flounder, oysters and other species use deeper waters.

By lumping similar habitats together, officials hope to limit the number of designated uses to about five, although that could change. Still, the process has the potential to become staggeringly complex. Even if there are just five designated uses, each could be sprinkled here and there throughout the Bay and its tributaries, potentially creating scores of individual designated use zones.

“That is going to be one of the big challenges,” said Alan Pollock, director of the Office of Water Quality Programs in Virginia’s Department of Environmental Quality, who heads a special Bay Program workgroup to identify potential standards. “How much complexity do we go with here? You could go hog wild and draw lots of lines all over to be protective of different things.”

Not only do different species use different areas, they often use them at different times of the year. Spawning takes place mainly in the spring. The deepest areas of the Bay may not be real important in the summer, but they are refuges during the winter.

Officials acknowledge there will have to be some limits to their attempts at realism. For example, the amount of freshwater and low-salinity spawning habitat available for migratory fish varies with the amount of springtime river flows entering the Bay. The designated uses can’t capture all of nature’s complexities. “We’re going to have to draw straight lines where, in fact, Mother Nature has curves,” Batiuk said.

One sensitive part of the exercise is the implicit recognition that — as part of the standard-setting effort — some areas, mainly the deep parts of the Bay, will get designated uses with less stringent water quality criteria than exist today.

Bay Program officials say that reflects the real world. Just as people have learned that stricter standards are needed to protect grass beds, they have also learned that old dissolved oxygen criteria, set for the water surface, could never be attained in the deepest areas of the Chesapeake. Unless such areas get designated uses associated with less stringent criteria, they will never achieve their water quality standards and the Bay will never be declared clean and removed from the EPA’s impaired waters list

“We can’t make a whole lot of progress in the deep part of the Bay, because of factors that just can’t be ignored,” said Rich Eskin, of the Maryland Department of the Environment. “But in exchange, where we can do something in these shallow areas, and where we recognize there is critical habitat to support, we will support that. We’re just trying to set up a series of uses that are supported by the science.”

Any changes that result in the downgrading of designated uses would require the Bay Program and the states to complete a “use attainability analysis” to show that existing standards cannot be met because of irreversible human impacts, natural conditions or other reasons. Then, the EPA must approve the change.

“You have to have good science and good data,” said Evelyn MacKnight, of EPA Region III. “Luckily, in the case of Chesapeake Bay, we have lots of water quality modeling and monitoring.”

Mike Hirshfield, vice president for resource protection with the Chesapeake Bay Foundation, said the new designated use effort is, “conceptually, something we wouldn’t have a problem with.”

But, he expressed concern that the whole exercise could become so complex that few people will be able to understand or follow the process, resulting in decisions made “out of the sight of the general public.”

While recognizing that current water quality standards may not be realistic for deep parts of the Bay, Hirshfield said he was worried that officials would settle for lesser goals than are actually attainable in other areas.

“We want to make sure that in the search for greater accuracy or realism, people don’t play fast and loose with definitions that allow polluters to get off the hook,” he said. “We are expecting the Bay to be delisted because it is cleaned up, not because it is defined as cleaned up.”

The stakes in the whole standard-setting process are high. Unlike the old 40 percent nutrient reduction, which was only a goal, the new water quality standards will be legally enforceable and will define a “clean” Bay. Everyone — and everything — from crabs in grass beds, to anglers on the shore to the farmers and wastewater treatment plant operators asked to control pollution, may have to live with the zoning decisions made in the new Bay master plan for decades to come.

“In a sense, this time, this is really it,” Hirshfield said. “This is for real.”