Despite early progress reducing Chesapeake Bay pollution, levels of a key pollutant, phosphorus, have not come down in many rivers in the past decade — and are actually rising in several, officials say.

Phosphorus is one of two pollutants blamed for causing algae blooms and "dead zones" in the bay, where fish and shellfish can't get enough oxygen in the water. Plants and animals need phosphorus and nitrogen to live, but the bay is choking on an overdose.

The lack of progress in reducing phosphorus is a particular problem on the Delmarva Peninsula, officials say, where there's evidence it is washing off the many farm fields fertilized with chicken manure. But phosphorus levels also are on the rise in some urban and suburban watersheds, which scientists say may stem from the erosion of stream banks caused by storm runoff from buildings and pavement.

"We're seeing some trends not headed in the right direction," said Rich Batiuk, associate director of the Environmental Protection Agency's bay program office in Annapolis. "We've got to step back and refocus on phosphorus."

Phosphorus levels in the rivers that feed into the bay are still mostly below where they were three decades ago, when the bay restoration effort formally began, officials say. But in the past decade, phosphorus levels have remained unchanged in nearly two-thirds of the rivers and streams routinely monitored by the U.S. Geological Survey, while pollution worsened in 16 percent. Just 21 percent of those waters monitored recorded declines in phosphorus in recent years.

Efforts to reduce phosphorus get only passing mention in a recent draft of a new Chesapeake restoration agreement obtained by The Baltimore Sun. It is to be signed Monday in Annapolis by Gov. Martin O'Malley and representatives of five other bay region states, as well as the EPA. A spokeswoman for the bay program said the final agreement won't be made public until it's signed. But officials involved with the cleanup say they're very aware they need to deal with the declining progress in reducing that key pollutant.

"Phosphorus pollution absolutely has to be on our radar screen," said Ann P. Swanson, executive director of the Chesapeake Bay Commission, which represents state lawmakers from Maryland, Pennsylvania and Virginia. "The progress that we made in the last 25 years is being eclipsed by increases in the last 10, so we're losing ground."

Pollutants in the bay pour into rivers and streams from a variety of sources, including sewage treatment plants, runoff of animal waste and fertilizer from farms and lawns, and from power plants and vehicle exhaust.

The bay cleanup has focused largely on reducing nitrogen, which readily dissolves in water and tends to travel farther downstream, causing more problems. Officials long believed that phosphorus could be dealt with by upgrading sewage treatment plants and controlling erosion, as phosphorus tends to stick to soil particles. But the river and stream monitoring indicates there are holes in the cleanup strategy, some say.

"We are still seeing improvements from wastewater treatment plant upgrades," said Scott Phillips, the geological survey's bay coordinator.

There also appear to be water-quality improvements in agricultural areas where farmers have fenced their livestock away from streams, he said. But in other areas, the increased use of manure as a fertilizer is degrading phosphorous levels, he said. Stormwater runoff is suspected of boosting phosphorus in more developed areas, Phillips added.

"We have to do everything we can to work with the agricultural community and work with our local governments to find solutions to better control phosphorus," Swanson said.

More than half of Maryland's farms have phosphorus running off into waterways, and animal manure accounts for 59 percent of the phosphorus spread on fields to raise crops, Swanson told commission members in a briefing last month.

Poultry litter, a mix of chicken waste and wood shavings, is widely used as fertilizer on the Eastern Shore. Containing both nitrogen and phosphorus, farmers favor it as a cheaper alternative to chemical fertilizer.

The problem is two-fold, experts say. Farmers generally don't till their fields in an effort to prevent soil erosion, so the manure is spread on the surface, where it's more likely to wash off in a rain.

Second, because corn, soybeans and other crops need more nitrogen, the phosphorus from the fertilizer builds up in the ground. After repeated annual applications, phosphorus can become so concentrated that it no longer binds to the soil and dissolves in rainfall runoff or shallow ground water, eventually reaching streams and rivers.

Kenneth W. Staver, a scientist at the University of Maryland's Wye Research and Education Center, said he and others have been saying for nearly two decades that phosphorus as well as nitrogen runoff must be controlled. Only in the last nine years or so haveregulators began requiring farmers to start curtailing how much phosphorus they put on their fields.

A new rule adopted last year should help, Staver said. Under it, Maryland farmers are no longer allowed to leave manure — or sewage sludge — on the surface when fertilizing their fields. They must, in most cases, work it into the soil within 48 hours of applying it.

Dealing with phosphorus buildup in soils is more problematic, and controversial.

Phillips said some rivers and streams may not be showing improvements yet because it can take years for the phosphorus built up in soils to come down. That could be the case in the Eastern Shore's Choptank River, where nutrient pollution levels are higher than they were 30 years ago.