NUTRIENT CONCENTRATIONS IN RAYSTOWN LAKE, PENNSYLVANIA

Raystown Lake is a large reservoir (514,000 acre-feet storage capacity) located in the Valley and Ridge region of south central Pennsylvania. The Army Corps of Engineers completed the present dam in the early 1970s. Primary purposes include flood-control, fishing, boating and hydropower. Nearly half of the watershed drains agricultural land. To date, extensive nutrient data for Raystown Lake is not published. This study was conducted to obtain a horizontal profiling of concentrations along the length of the lake and to investigate seasonal patterns. Identification of nitrate and phosphate sources was another objective. The water quality was monitored for temperature, transparency, orthophosphate, and nitrate concentrations every 2 to 4 weeks from July to November 2000. Epilimnion water samples were taken at 6 sites along the lake, chosen according to equal geographic distribution and relationship to input streams. Total phosphate was tested using persulfate acid digestion and nitrate was tested using a cadmium reduction method. Both were analyzed using a spectrophotometer. Transparency, measured with a seechi disk, ranged from 1.5 to 6.9 meters. Values increased with decreasing distance to the dam. Total phosphate concentrations averaged 0.18 mg L-1. Values varied significantly throughout the season (0.01 to 0.38 mg L-1). A slight increase near mid-lake was the only spatial pattern detected. A sharp decline in total phosphate occurred at the beginning of November before fall turnover. Nitrate values averaged 0.71 mg L-1, ranging from 0.47 to 1.00 mg L-1. Concentrations were highest near the main river input and decreased toward the dam. This relationship faded with time. A previous sampling in early summer found this pattern to be statistically significant, though ours did not. Nitrate concentrations remained relatively constant throughout the summer then dropped substantially in late September. Data suggests a possible local nitrate input near mid-lake. Observed nitrate patterns differed from the expected trend of nutrient enrichment toward the river. The sudden and drastic drop in nutrient values in mid-fall was unanticipated.