For the first time, nutrient monitoring in major rivers has detected a declining trend in the amount of nitrogen reaching the Chesapeake from its largest tributary, the Susquehanna River.

The annual river analysis, conducted by the U.S. Geological Survey, also detected a declining trend in the James for the second year in a row, while the Patuxent River continued its downward slope.

At the same time, loads appear to be increasing in some heavily agricultural areas, including the Choptank basin on the Eastern Shore and the Shenandoah River. Other rivers did not show a noticeable trend in nitrogen, according to the USGS analysis.

The drought likely played a factor in the nitrogen decline because reduced rainfall translates into fewer nutrients running off farms, lawns and streets. But monitoring has also shown reduced concentrations of nitrogen in the water, regardless of the amount of rainfall, particularly in parts of the Susquehanna and Potomac basins, said Scott Phillips, who oversees USGS’ Bay-related programs.

“We are actually starting to see a downward slope in concentration data,” Phillips said. “So I don’t think that loading trend was all just due to the drought. That would lead us to think that some of the management practices are starting to actually reduce the concentration numbers values in some areas.”

Still, the recently completed analysis of 2001 data failed to show any significant trends for most of the 34 monitoring sites scattered throughout the Bay’s nontidal tributaries, where nutrient levels have been monitored since the 1980s.

Of the 34 monitoring sites, nine showed declining nitrogen trends in the watershed, five of which were in the Susquehanna basin, according to Mike Langland, who has lead the USGS in conducting the trend analysis. Four showed increasing trends, and the remaining sites showed no trend.

For phosphorus, the monitoring showed decreases at four sites, increases at two, and no trend at the remaining sites. The decreases were in the Patuxent, the Maryland headwaters of the Potomac, the West Branch of the Susquehanna, and the Conodoguinet Creek, a Susquehanna tributary. The increases, as was the case for nitrogen, were in the Choptank River in Maryland, and the Shenandoah River in Virginia.

Sediment monitoring showed no change at any of the 34 stations.

To determine a trend, scientists analyze monthly nutrient loads gathered from monitoring programs from 1985-2001 by using a “trend test” designed to detect significant changes in loads. A significant trend is one in which scientists have a 95 percent level of confidence that the change is greater than what would be expected only from normal fluctuations observed since 1985.

Although computer models have suggested bigger nutrient reductions basinwide, actual monitoring continues to show less progress. Officials cite a number of reasons. Many of the nutrients enter the Chesapeake through slow-moving groundwater, which means it takes years for recent nutrient reduction actions to show up in the monitoring.

In addition, officials generally acknowledge that Bay Program estimates for many nutrient reduction activities are overly optimistic by assuming that nutrient control practices are properly installed and maintained.

Also, the USGS monitoring is conducted on nontidal rivers that feed into the Bay. Some big nutrient reductions — especially those by many wastewater treatment plants — have taken place in tidal portions of the rivers farther downstream.

“Most of the stations where we measure are above some of the major point source discharges,” Phillips said. “So we don’t pick up any reductions that they would have from the point sources that are directly tied to the tidal areas.”