M. Brenner et al., PALEOLIMNOLOGICAL EVALUATION OF HISTORICAL TROPHIC STATE CONDITIONS IN HYPEREUTROPHIC LAKE-THONOTOSASSA, FLORIDA, USA, Hydrobiologia, 331(1-3), 1996, pp. 143-152
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.