IMPACT OF DROUGHT AND WET SEASON CONDITIONS UPON NUTRIENT CONCENTRATIONS IN AN AGRICULTURALLY-INFLUENCED STREAM

Nutrient concentrations in stream water are inherently dynamic, varying at all temporal scales. The summers of 2002 and 2003 provided a unique opportunity to interpret the relationship between nutrient (phosphates and nitrates) concentrations and annual precipitation changes in an agriculturally-influenced stream, Spruce Creek—coincidentally, one of the premier trout streams in central Pennsylvania. The summer of 2002 was characterized by drought conditions, and the stream showed an overall progressive decrease in discharge. The summer of 2003 was an extremely wet summer, and the stream had high discharge with distinct surges on an almost weekly basis. The lowest stream discharge during the summer of 2003 was significantly greater than the highest discharge during the summer of 2002.

During the drought summer of 2002, phosphate was not effectively delivered to the stream. Peaks in discharge in late spring correlated with increases in phosphorus concentration, and as such, represent transport of phosphorus through surface runoff. During the summer however, peaks in discharge produced no visible change in phosphorus levels. Stream discharge did not reach requisite threshold values (minimum discharge at which phosphates reach measurable values) to effectively transport phosphate to the stream.

During the wet summer of 2003, discharge was always above threshold values for phosphate delivery. Initial increases in discharge following heavy rains coincide with increased phosphate concentrations. However, maximum discharge, in many cases, resulted in dilution of phosphate concentrations.

Nitrates are affected differently by drought and wet summers. During 2002, nitrate concentrations increased as discharge decreased throughout the summer. The relationship between nitrate concentration and discharge was more complex during the summer of 2003. There is a weak relationship between peaks in discharge and in nitrates; however, the magnitude of discharge events is disproportionately high compared to increases in nitrate concentrations. Additionally, the greatest increase in discharge coincides with a decrease in nitrate concentrations. These observations suggest that after sufficiently high discharges, nitrate concentrations are diluted by the large influx of water into the system.