Such hard surfaces, which flush nutrients and other contaminants into streams, increased 41 percent during the past decade compared with an 8 percent increase in population, the study said.

The findings challenge the bay's recovery, said Scott Phillips, the service's Chesapeake Bay coordinator who compiled the report, which is the work of several scientists who have studied the bay for years.

"I've heard the mantra that we've studied the bay to death," Phillips said, "but that's not true. The more we understand these things, hopefully the more we will be able to develop solutions to rehabilitate the bay."

Among new findings is that much of the sediment that smothers ecologically valuable underwater grass beds and other marine habitats originates far inland. It's coming from the Virginia, Maryland and, especially, the Pennsylvania piedmont.

State and federal farm programs geared toward controlling erosion should be concentrated in that region to make the most efficient use of cleanup dollars, Phillips said.

The Chesapeake Bay Commission, made up of state legislators from Virginia, Maryland and Pennsylvania, urged Congress this month to support farm-conservation measures for the Chesapeake Bay at the "highest possible levels." Programs funded by the federal farm bill help farmers control erosion, which releases sediments into the bay, and help reduce nutrients, such as nitrogen, which impair the bay's health.

Agriculture represents the single largest source (40 percent) of nitrogen in the watershed, which causes harmful algae blooms and a menacing dead zone in deep portions of the bay every summer, the bay commission said.

In other findings, rising global temperatures are expected to produce heavier rainstorms in the mid-Atlantic. That will lead to more runoff from paved urban areas and make streams vulnerable to scouring, which washes away important organic material.

Related stresses continue to raise questions about the continued rebound of striped bass. Fishing bans and restrictions enacted in the 1980s have rebuilt the striped bass population to record levels, but bacterial lesions are being found on many of the fish.

"The findings imply the resistance of striped bass populations to disease appears to have been lowered due to low dissolved oxygen, contaminant concentrations and improper diet," the report said.
Contact Lawrence Latané III at (804) 333-3461 or llatane@timesdispatch.com.