PALEOLIMNOLOGICAL EVALUATION OF HISTORICAL TROPHIC STATE CONDITIONS IN HYPEREUTROPHIC LAKE-THONOTOSASSA, FLORIDA, USA

We used paleolimnological methods to evaluate historical water quality in Lake Thonotosassa, Hillsborough County, Florida, USA. Sediment map ping shows that organic deposits are unevenly distributed in the lake. Two short (<130 cm) sediment cores from the depositional zone were an alyzed for radioisotopes (Pb-210, Ra-226, and Cs-137), bulk density, o rganic matter concentration, nutrients (C,N,P), and diatoms. Pb-210 re sults indicate that the profiles represent >100 years of sediment accu mulation. There is an abrupt change in sediment composition at about t he turn of the century (similar to 80 cm depth), above which bulk dens ity decreases and concentrations of organic matter, total C, total N, total P, and Ra-226 activity increase. Diatom-based reconstructions of historical water-column trophic conditions indicate progressive nutri ent enrichment in the lake during the past similar to 100 years. Strat igraphic changes in diatom assemblages suggest that anthropogenic nutr ient loading converted Lake Thonotosassa from a naturally eutrophic sy stem to a hypereutrophic waterbody after similar to 1900. Given the ed aphic setting of Lake Thonotosassa, efforts to mitigate recent anthrop ogenic impacts will, at best, yield the eutrophic conditions that char acterized the lake prior to human disturbance. This study illustrates the importance of paleolimnological data for targeting realistic water quality conditions when lake restoration is contemplated.