SEDIMENTATION AND TROPHIC HISTORY OF EDINBORO LAKE, ERIE COUNTY, PENNSYLVANIA

Sediment cores obtained from Edinboro Lake provide a record of sedimentation rates and water quality over the past 9,000 years, which can be useful in developing a sound management strategy for the watershed. Historic documents suggest that Edinboro Lake has been rapidly in-filling with sediments. Bathymetric maps from1960 and 1973 show a decrease in maximum depth from 13.4 to 9.1 meters. In addition, data collected since 1960 consistently show the lake to be eutrophic, a condition which most authors consider to be a recent development. To reconstruct the sedimentation history of Edinboro Lake, a 5.52 m sediment core was extracted from the deepest part of the lake. Radiocarbon dates of 3,315, 6,430, and 9,575 years BP (calibrated) were obtained at depths of 1.90, 4.00 and 5.52 meters, respectively yielding an average sedimentation rate of only 0.56 mm/yr. These data clearly indicate that the 1960 bathymetric map is not accurate and that Edinboro Lake has not been 13.4 meters deep for at least 7,000 years. The occurrence of the authigenic phosphorous mineral vivianite below 220 cm in the core indicates a change in the trophic state of the lake at approximately 4,000 years B.P.. Prior to this time there was sufficient oxygen in the hypolimnion to allow phosphorous to be retained in the sediments as vivianite. In more recent sediments complete anoxia in the summer due to eutrophication allowed for the release of P from sediments into the water column. The results of this preliminary investigation show the importance of obtaining reliable historical data before developing a management plan for any watershed. A restoration plan for Edinboro Lake based on the widely held assumptions that the lake is rapidly in-filling with sediments with only recent eutrophication would likely attempt to return the lake to a condition that has not existed for 4,000 years.