Historical ecology of a hypereutrophic Florida lake.

Lake Hollingsworth lies in phosphate-rich central Florida. Recent limnological data show the lake has been hypereutrophic for >25 years, but nothing was known of the lake's trophic history prior to the late 1960s. A paleolimnological study was undertaken to: 1) evaluate the distribution of organic sediments in the basin, 2) reconstruct historical water quality to determine whether the lake is naturally (edaphically) or anthropogenically nutrient-rich, and 3) assess the feasibility of reducing in-lake nutrient concentrations. Organic sediments are distributed rather uniformly above the sandy substrate. Two ²¹⁰ Pb-dated sediment cores taken ≈0.7 km apart display similar stratigraphies with respect to density, organic matter, total C, N, P, radioisotopes, biogenic SiO ₂ , diatoms and δ ¹³ C in organic matter. Limnetic P _tot reconstructions, inferred from diatoms, show eutrophic (≈30 μg L ⁻¹ ) conditions in the late 1800s. Total P increased after 1920 and very high P _tot concentrations (125 to >200 μg L ⁻¹ ) are inferred for the 1950s through 1970s. Cultural eutrophication was probably a consequence of agriculture, residential development, construction of Florida Southern College, road building, stormwater runoff and regional phosphate mining. Total P inferences in the 1980s declined to ≈65 μg L ⁻¹ , reflecting soil stabilization and reduced mining activity. The historical perspective provided by paleolimnological data demonstrates that Lake Hollingsworth was naturally eutrophic, but became hypereutrophic as a consequence of anthropogenic nutrient loading. It is probably not feasible to reduce in-lake nutrient concentrations significantly in this shallow, urbanized, hypereutrophic waterbody.