Despite cleanup efforts, most of the Bay’s major tributaries had seen no reduction in nutrient loads through the late 1990s, according to a new report from the U.S. Geological Survey.

The reason, according to the report, is that many of the recent years have been wetter than normal. That caused more nutrients to be swept off the land surface, join higher than normal river flows, and ultimately increase the total amount of nutrients — or nutrient “load” — entering the Bay.

At the same time, had river flows been near normal, many waterways could have seen nutrient load reductions, according to the report.

While the model predicted the amount of nitrogen entering the Bay fell 15 percent by 2000, and phosphorus dropped 25 percent, those predicted reductions were not detected in the rivers — at least through 1998. In that regard, the results in the new report, “Factors Affecting Nutrient Tends in Major Rivers Entering the Chesapeake Bay,” serves as a sort of reality check.

It shows that from 1985 through 1998, there was no trend in the nitrogen load in seven of the nine rivers it monitored, while nitrogen loads increased in one and decreased in another. Similarly, with phosphorus, no trends were seen in six of nine rivers monitored.

When USGS scientists adjusted the nutrient levels to take out the influence of stream flow variations, they did find more encouraging results. The “flow-adjusted” figures show downward nitrogen concentration trends at six of the nine monitoring stations, and downward phosphorus trends in seven rivers — results generally similar with Bay Program modeling.

But the report said, it is the lack of real-world nutrient load reductions that are important to aquatic organisms in the Bay.

“What the living resources in the Bay see is the load, which is, of course, affected by stream flow, and not these flow-adjusted concentrations that we use to evaluate the success of the management practices,” said Lori Sprague, the lead author of the report. “So we really want to see a decrease in the load.”

Indeed, the amount of underwater grass beds in the Chesapeake, which are greatly affected by water quality, remained almost stagnant during the 1990s. Also, monitoring has shown no Baywide improvement in dissolved oxygen concentrations, which are heavily influenced by both nutrients and stream flow.

The report was an attempt to integrate results from the Bay Program’s Watershed Model with trends in nutrient loads and concentration provided from river monitoring programs.

The model predicts reductions in nutrient loads based on the implementation of various nutrient control practices within the watershed. But it makes predictions based on “average” flow conditions. The trends in loads and concentrations from monitoring data provide a measure of how the rivers are responding in the real world.

The report suggested that the Bay Program needs to take fluctuations in stream flow into account when setting new water quality standards and associated nutrient reduction goals. In addition, it said the Bay Program should work to develop runoff control practices that work during high flows — many of those commonly used are not effective during heavy storms.

The report examined, river-by-river, the available water quality monitoring, computer model results, fertilizer sales, groundwater and a range of other information to explain nutrient trends. Part of the reason for the study was to gauge whether modeling efforts used to guide policy and measure progress match up with actual water quality monitoring.

On broad issues it found general agreement. And the report reinforced what was generally known.

It confirms that agriculture is the dominant source of both nitrogen and phosphorus in almost all of the river basins examined. But the amount of nutrients coming from agriculture appears to be decreasing. Improved farm practices and the reduced use of manure and fertilizer is contributing to the decline, according to the report, but so is the steady decrease in the amount of agricultural land.

Likewise, the report states that the amount of nutrients from urban areas is increasing, mainly because more land in the watershed is being developed. Increased development, and an increasing population, are offsetting some of the nutrient reductions made on agricultural lands.

The report found a number of areas where better information is needed.

For example, controlling runoff from agricultural lands and urban areas is done through a variety of activities collectively known as Best Management Practices, such as stormwater ponds, vegetated buffers and so on. But the report said better information about BMP effectiveness — especially in urban areas — is needed to guide future nutrient control activities.

Similarly, the report found it difficult to obtain consistent, reliable information about fertilizer sales and use, both in agricultural and urban areas across the watershed. That made it difficult to relate nutrient trends seen in rivers to trends in fertilizer sales.

Nutrient loads are also affected by natural factors, such as soil types, groundwater and in-stream biological processes. Those factors can vary significantly from place to place, dramatically affecting the amount of nutrients actually reaching the Bay from a particular area.

The report said those factors must be taken into account when setting nutrient reduction goals for various rivers in the future.

Scott Phillips, who oversees Bay-related research for the USGS, said understanding those complex factors is critical. Because of the influences of natural soil and groundwater stream processes, nutrient control efforts are destined to be more effective in some places than others and can explain why the same level of nutrient-control activity in two different areas can yield different results. This information can help to better target nutrient reduction efforts.

Likewise, understanding how natural variations in stream flow affect the amount of nutrients entering the Bay is important in explaining results — or the lack thereof.

Just as the rash of high flows explains the lack of real-world improvements during the past decade, the lower-than-normal flows in 1999 and 2000 could show improving trends when those results are analyzed. But, again, the results have to be tempered by understanding flow.

“When we update the trends this year, I could very well envision us seeing a decrease, not just in concentration, but also in actual load,” Phillips said. “But we have to be very careful and not start thinking the nutrient reduction actions are the only cause for the load reduction. It could just be more influenced by having two years of lower stream flow into the Bay.”

Phillips he would like to redo the river-by-river analysis in about three years.

The report, “Factors Affecting Nutrient Trends in Major Rivers of the Chesapeake Bay Watershed,” is available on the USGS web site: www.chesapeakebay.usgs.gov