Researchers say they have identified the villain in the decay of the Chesapeake Bay and not surprisingly, it is us.

Scientists wrapping up a six-year federal study say nutrients--mostly phosphorus from man's urban sewage treatment plants and nitrogen washing off his rural farming land--have overenriched the bay, spawning algae blooms.

When a similar problem existed in the Potomac River almost two decades ago, the solution--upgrading Washington-area treatment plants to remove phosphorus from the effluent-- was expensive: billion. It was undertaken anyway and today the algae problem is largely under control. The river in Washington is again widely used for recreation and fishing and many river problems have been reversed.

Now come algae blooms in the Chesapeake, though not yet as noxious as the Potomac's, and concurrent steep declines in rooted aquatic plants, oxygen levels in the water and populations of desirable fish and shellfish. There is, according to Eugene Cronin of the Chesapeake Research Consortium, "a general sense that the bay is growing murkier and dirtier."

In the sewage treatment vocabulary, nutrients are those chemicals that support the growth of algae and other undesirable forms of marine life. They are unaffected by the sewage treatment process because by law the principal function of those facilities is to remove disease-carrying bacteria. Nutrient removal, an expensive add-on, particularly for nitrogen, is one of the goals of advanced waste water treatment and is not common nor required by law.

The cost of cleaning the bay, a body of water hundreds of times larger and far more complicated than the Potomac, also will be immense. States bordering the Chesapeake are on the brink of a major cleanup investment that could, if Maryland gets its way, rival the cost of upgrading the Potomac.

Spurred by final recommendations of the $28 million Environmental Protection Agency study of the Bay, officials in Maryland, Virginia and Pennsylvania have pledged action to reverse troubling trends in the nation's largest estuary.

"We have identified the problems" through the EPA study, said state natural resources chief Torrey Brown, "and the exciting thing is that they are soluble. There's nothing wrong that we can't fix."

Brown said by taking advantage of federal programs and augmenting them with their own funds, the states could easily apply $1 billion over the next decade to cleaning the bay, and should.

He said EPA's confirmation of recent declines in water quality has galvanized public opinion to support such spending. Brown said his boss, Gov. Harry Hughes, believes Chesapeake restoration "could be the mark he leaves" from his tenure in the State House.

"It's exciting," said William Eichbaum, who heads Maryland's health department, "because it's doable."

A bay cleanup is "something I would look back on with pride," said Betty Diener, Virginia secretary of commerce and resources.

But Diener and Pennsylvania officials question Brown's money figures. "That's about twice as high as anything I've heard," said William Cook, a special assistant in Pennsylvania's Environmental Resources Department. Added Diener, "I would be very surprised if at the end of 10 years we could count up to $1 billion."

In the scientific community there is fear that despite the pressing nature of EPA findings, the states, when faced with spending local tax money, will propose what Carl Osborne of the Chesapeake Bay Foundation calls "Band-aid solutions" to massive bay problems.

The issue of money and who is willing to spend it arises now that EPA has issued the final section of its study, the "management plan." In it, the federal agency calls for spending $335 million immediately to reduce phosphorus and nitrogen output from sewage treatment plants along the Patuxent River and in the upper bay and its tributaries.

The report also seeks long-range phosphorus and nitrogen reductions from other treatment plants that eventually could cost "in the billions," according to project manager Virginia Tippie. And it seeks outlays of $10 million a year each in the three states to reduce phosphorus and nitrogen coming off farm fields and from other sources.

Nutrient reduction, EPA's top priority, is exactly the course taken in cleaning up the Potomac at Washington.

In 1965, when President Lyndon B. Johnson called for a Potomac restoration, the river was plagued by summer algae blooms that blocked sunlight from aquatic plants, gobbled oxygen needed by fish and other animal life and looked and smelled foul.

EPA has found similar problems in the Bay. The report says Chesapeake overenrichment results from huge and growing volumes of phosphorus from urban treatment plants and nitrogen washing off rural land, particularly along Pennsylvania's Susquehanna River, which provides half the bay's fresh water.

Whether the solutions can be similar is another matter. The Potomac is but one arm of the Chesapeake; the bulk of its problems were localized, attributable to a few Washington-area sewage treatment plants. The mammoth Chesapeake has many more urban runoff sources and a separate issue of nutrient-rich runoff from rural basins that Potomac managers did not have to address.

And while nutrient enrichment is clearly established by EPA as the pressing general problem in the bay, research also pinpointed large areas of life-threatening toxic metal contamination near Baltimore and Norfolk that will require separate cleanup strategies.

Brown believes the solutions are not so complicated and expensive as to drive the government into torpor.

With the study in hand, he said, "we know where the nutrients, toxins and sediments are coming from. Three states are now committed to doing something about it."

Brown said the fact that the federal government is cutting its support of sewage treatment plant construction, with plans to pay perhaps half of future costs rather than three-fourths, simply means the states will shoulder more of the burden.

And he offered a litany of common-sense solutions to curbing other pollution, such as collection areas for storm sewer runoff so street wash won't flush straight into the bay; seepage holes built into parking lots to percolate rainwater; contour plowing, no-till farming, timely application of fertilizers and containment of manure in agricultural areas to reduce nitrogen runoff.

Brown believes the bay's problems are soluble if the political problems of organizing and financing a cleanup in five states and the District of Columbia (New York and West Virginia also are in the watershed) can be overcome.

To that end, workshop committees with representatives of all the states, the District and EPA are preparing a menu of bay initiatives for consideration before a "summit" of the big-three governors in December.

EPA is awaiting congressional approval of a $4.2 million budget to continue overseeing and collecting data on the bay cleanup.

Virginia has pledged $6 million to generate bay initiatives in 1984. Maryland's Hughes is committed to spending state funds to upgrade sewage treatment plants where federal money is unavailable.