NUTRIENT CONCENTRATIONS AND TEMPORAL VARIATIONS: A STUDY OF PHOSPHATES AND NITRATES IN RAYSTOWN LAKE, PA

Raystown Lake is a large (3360hm2) reservoir located in Huntingdon County, Pennsylvania. Despite being the largest in the state, it has received only modest attention with concern to its water quality since a post impoundment study written shortly after the reservoir’s completion in the mid-seventies. The intention of this study was to collect extensive nutrient data, specifically nitrate and phosphate (both total and reactive) values, from Raystown Lake to quantify seasonal variations in nutrient levels, and record nutrient concentrations both vertically and horizontally throughout the lake. Samples were collected in alternate weeks along horizontal and vertical profiles of the lake as well as from four major inputs. Colormetric methods were used to determine total phosphate, reactive phosphate, and nitrate values.

The horizontal profile yielded a pattern showing a gradual decline of nitrates through mid summer until fall turnover. A departure from the anticipated pattern was a dramatic decrease in nitrates in early September. This decrease was also observed in the nitrates for the vertical profile, which showed a linear drop in nitrate concentrations at all depths from the start of the sampling in June. The nitrates from the vertical profile exhibited a pattern that gives reasonable resolution to the epilimnion, metalimnion, and hypolimnion layers within the lake while the total phosphates lacked any definable trends. Measurements from the two largest inputs to the lake, Trough Creek and the Raystown Branch of the Juniata River at an upstream sampling site (Saxton), show anticipated behaviors. Trough Creek, draining primarily heavily forested land, had lower initial and final total phosphate values while the Raystown Branch, reflecting predominately agricultural land, showed both high initial and final phosphate values. Analyses from these two sites showed a consistent and unnerving pattern of total phosphate at lower concentrations than reactive phosphate. This calls into question the accuracy of current, commonly available techniques for phosphate measurements in lakes.