Levels of the two nutrients that cause poor water quality and algae blooms in Chesapeake Bay have risen, reversing a decade of decline or stability, according to a new analysis by the Environmental Protection Agency.
However, an EPA official said that because the changes seem to result from natural causes rather than pollution caused by human activities, they may not inflict major harm.
The analysis found a 10 percent increase in nitrogen compared with levels in 1984. Levels of phosphorus, which had declined 16 percent from 1984 through 1992, are back where they were a decade ago, said William Matuszeski, director of EPA's Chesapeake Bay program office in Annapolis.
"We have had some upticks in the nitrogen and phosphorus numbers," Mr. Matuszeski acknowledged. But he said the increases have not worsened water quality because the nutrients were bound to dirt and other organic matter, making them less available as food for algae.
The rise in nutrients measured in the bay stems from unusually heavy spring rains in 1993 and 1994 washing more mud, leaves and other organic matter into the water, Mr. Matuszeski said. Heavy rain and snow melt in spring 1993 generated freshwater flows over a 40-day period that exceeded the flooding caused in 1972 by Tropical Storm Agnes, a storm blamed by many for damaging the bay's underwater grasses and fisheries.
Scientists had predicted as long as two years ago that heavy spring runoff would spark extensive algae blooms and declining water quality, Mr. Matuszeski said, "but it's not as bad as we thought it would be."
Dissolved oxygen in the bay has shown no improvement over the past decade, Mr. Matuszeski said, but it hasn't gotten any worse either. And water-quality sampling actually has detected an 18 percent decline in algae overall since 1984.
However, William C. Baker, president of the Chesapeake Bay Foundation, said the recent nutrient increases show "we are a long way from declaring success" in restoring the bay.
Nutrients harm the bay by spurring massive growths of algae, or microscopic plants, that block out sunlight needed by underwater bay grasses. The algae die and sink to the bottom, where their decay consumes oxygen that fish need to breathe.
Maryland, Pennsylvania, Virginia, the District of Columbia and the federal government pledged in 1987 to restore the bay's water quality by reducing nutrient pollution 40 percent by 2000.
Nutrients enter the water from sewage, runoff of fertilizer from farms and lawns, and air pollution, but some also come from natural sources, such as dirt and dead leaves.
"There is good phosphorus and bad phosphorus, and good nitrogen and bad nitrogen," Mr. Matuszeski explained. "The stuff that seems to be pushing the numbers up appears to be mostly just dirt, which comes down [rivers into the bay] with a lot of spring rainfall."
Levels of dissolved inorganic phosphorus, the type that prompts rapid algae growth, have actually declined about 20 percent in the past decade, the EPA analysis found, and there has been essentially no change in the amount of biologically active nitrogen measured in the bay's water.
Mr. Matuszeski said he is unsure whether the upsurge in "good" nutrients poses long-term problems.
But Mr. Baker, president of the Annapolis-based bay foundation, said, "This sort of trend should give us all concern that we need to do more to save the bay.
"There are natural factors that affect the bay, whether you're talking fisheries or water quality," he said.
"In the decades past, the bay could overcome those natural variations, but we have done something to create a situation where those natural variations overwhelm the system."