Trends are overall increases or decreases in a measurement such as concentration during a specified period. Trends in nutrient concentrations can be affected by nutrient control strategies, which manage point and nonpoint sources in an effort to achieve downward trends. In many cases, concentration trends are also affected by natural processes, especially streamflow. If a point source, such as discharge from a wastewater treatment plant, is the dominant nutrient source to a stream, increased streamflow would cause a downward trend in nutrient concentrations from dilution. In contrast, if a nonpoint source, such as runoff from agricultural land, is the dominant nutrient source, increased streamflow would cause an upward trend in nutrient concentrations from increased storm runoff. Often, a decrease in nutrient concentrations resulting from a BMP can be offset by an increase in concentrations resulting from higher streamflow.
The removal of streamflow as a variable affecting in-stream concentrations in a trend analysis allows trends caused by other factors to be identified, and impacts of nutrient control strategies can be assessed. These flow-adjusted concentration trends are independent of variation in streamflow. When considering the health of aquatic organisms in Chesapeake Bay and its tributaries, however, trends due to both streamflow and nutrient control strategies must be examined, as both influence the overall state of the ecosystem. These unadjusted concentration trends account for all factors that affect trends in nutrient concentrations.
Flow-adjusted and unadjusted trends in concentration of selected constituents at the River Input Monitoring stations in Maryland and Virginia are shown below. These trends are presented as 95-percent confidence intervals - the interval in which there is a 95-percent probability that the true trend falls.
Cross-hatching indicates the trend was significant at the 95-percent confidence level. 1. Sampling period was Jan. 1985-Dec. 1999. 2. Sampling period was July 1988-Dec. 1999. 3. Sampling period was July 1989-Dec. 1999. 4. Sampling period was Oct. 1989-Dec. 1999. 5. Total suspended sediment was measured at the Susquehanna, the Potomac, the Patuxent, and the Choptank Rivers, and total suspended solids were measured at the other rivers. 6. Unadjusted concentration trends reported here are the same as flow-weighted concentration trends as reported in Langland and others, 1999.
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