Scientists and the government officials directing the Chesapeake Bay cleanup offered varying assessments Friday of the progress in controlling pollution in the estuary during a congressional committee hearing, but they agreed that by anyone's measure, much more needs to be done.

"We have made modest gains in reducing the number of pollutants flowing into the bay, especially in the face of a 20 percent increase in population," Rebecca Hanmer, director of the Environmental Protection Agency's Chesapeake Bay Program Office, told the House Committee on Government Reform.

In contrast, the president of the University of Maryland's Center for Environmental Science, Donald F. Boesch, told the committee that there has been virtually no change in actual pollution flows, largely because of an above-average amount of fresh water flowing to the bay. By his calculation, the average annual amount of nitrogen and phosphorus pouring into the bay over the past decade is about the same as it was when the Chesapeake Bay Program got underway in the mid-1980s.

"It is clearly misleading to state that nutrient loading [pollution] has actually been reduced by a certain amount, based on watershed model estimates," his written testimony states.

The committee met at this bay-front center amid questions about exactly how much progress has been made in reducing the flows of two key pollutants, nitrogen and phosphorus, into North America's largest estuary.

The Chesapeake Bay Program, an alliance of the federal government and Maryland, Virginia, Pennsylvania and the District, has been under pressure to show progress since 1987, when members signed a landmark agreement to reduce the flow of nitrogen and phosphorus into the bay. Those two chemicals, called nutrients by scientists, set off algae blooms that cut off sunlight to creatures below and ultimately deplete the water of oxygen.

For several years, in its "State of the Chesapeake Bay" reports and other publications, the bay program has used computer model estimates as a primary means of reporting its achievements. But they have twice been found to have significantly overestimated pollution reductions, most recently by about 10 or 15 percent.

The model has been revised as the science has evolved, and it currently shows that nitrogen pollution has dropped 18 percent and that phosphorus pollution has dropped 28 percent since 1985. But some scientists say that the model continues to overestimate progress and that the reported estimates of pollution reduction should have been issued on the basis of actual water monitoring, which suggests less progress.

Environmentalists who are critical of the bay program also note that the amount of oxygen-depleted water in the bay, a key indicator of nitrogen and phosphorus pollution, has shown little or no improvement.

"In fact, in most of the bay and its tributaries, the data show no improvement or declining trends," said Theresa Pierno, a vice president of the Chesapeake Bay Foundation.

"While the watershed model estimates a downward trend in nutrient concentrations, the actual amount of decrease differs greatly between model and monitoring estimates," Boesch noted.

The committee hearing only hinted at the myriad complexities that face anyone seeking to determine the state of pollution in the Chesapeake Bay. The expanse of land that drains into it is vast -- about 64,000 square miles -- and detecting trends in pollution flows is complicated by wide variations in annual rainfall and the lag time between when nitrogen hits the ground and when it gets washed into the bay.

The Government Accountability Office has just begun to review the issue, however, at the urging of three senators, Barbara A. Mikulski and Paul S. Sarbanes, both Maryland Democrats, and John W. Warner, a Virginia Republican.

Both sides in Friday's Chesapeake Bay debate -- those in the bay program who have reported significant gains against pollution and those who see far less progress -- used the hearing to press Congress for help in the cleanup.

According to estimates from the Chesapeake Bay Commission, the costs of cleaning up the bay by 2010 amount to $18.7 billion. Thus far, the commission projects federal and state spending for the restoration at about $6 billion.

"If the dream is we want a Chesapeake Bay that is truly clean, then you have to put the cash in," Ann Pesiri Swanson, director of the commission, told the committee.

"It won't be easy, cheap or fast," Hanmer said.

The chairman of the committee, Rep. Thomas M. Davis III (R-Va.), said afterward that he will try to find more money for the program, particularly for efforts at reducing pollution from wastewater treatment plants.

"There is a wide consensus on what we need to do," Davis said. "I don't think we can do this on the cheap. [But] we all buy crabs and oysters. We all want a clean bay."

Boesch, in summing up the state of the bay and the needs ahead, said the situation is something like being down in the fourth quarter of a football game by the score of 42 to 14. The time has passed for small gains, he said.

"We're still trying to run the ball up the middle," he said. "We need to go long."

The sun rises over fishing nets at Cedar Point Hollow on the Chesapeake Bay near Solomons.