Total Phosphorus Trends in the Greater Everglades Headwaters: Upper Kissimmee Chain-of-Lakes
Nellie Morales
South Florida Water Management District, West Palm Beach, Florida USA
Changes in total phosphorus concentration were studied in the Upper Kissimmee Chain-of-Lakes in order to address the mandates of the Lake Okeechobee Protection Act. Twenty years of monthly total phosphorus data stored in the South Florida Water Management District database were analyzed for East Lake Tohopekaliga, Lake Tohopekaliga, Cypress Lake, Lake Hatchineha, and Lake Kissimmee.
Lake wide annual means reveal a monotonic decrease in total phosphorus concentration in these lakes, except in East Lake Tohopekaliga where total phosphorus concentration fluctuated around 0.020 mg/l. Lake wide annual mean total phosphorus concentrations decreased from 0.500 – 0.040 mg/l, 0.160-0.040 mg/l, 0.100-0.030 mg/l, and 0.065-0.040 mg/l in Lake Tohopekaliga, Cypress Lake, Lake Hatchineha, and Lake Kissimmee, respectively. This decrease appears to be related to the removal of sewage treatment plant effluents from tributaries to the lakes and from decreases in farmland around the lakes.
In addition, lake wide annual means revealed an approximate four-year cycle in total phosphorus concentration superimposed on the monotonic decrease. This cycling is most evident in East Lake Tohopekaliga, which was not affected by sewage discharges and, therefore, did not show the decreasing trend in total phosphorus concentration observed in the other KUB lakes. Total phosphorus peaks in these cycles were observed in 1984, 1987, 1990-92, and 1998 during the sampling period. The cycles appear to be intrinsic to the ecology of the lakes, probably related to some climatic variability expressed at the local scale, and not man-made. Cyclic climatic variability with a frequency of about 4-years could be linked to El Niño/La Niña events. During El Niño years, lake wide annual mean total phosphorus concentrations tend to be higher, whereas, during extreme La Niña years they tend to be lower. The cycling tendencies in lake wide annual mean total phosphorus concentrations seem to persist in spite of different activities in the lakes such as sewage diversions, drawdowns, muck removals, and chemical treatments of aquatic vegetation.
Lake wide monthly means, combined for the period of record, reveal a seasonal trend in total phosphorus concentration in these lakes. Higher monthly mean total phosphorus concentrations occur during the summer and lower concentrations occur during the winter. The onset of the increasing trend occurs in March. This increase could be associated to in-lake treatment of aquatic vegetation or indirectly to increases in temperature. The end of the increasing trend could be associated with increases in precipitation during the summer, perhaps due to dilution. This seasonal trend in lake wide monthly mean total phosphorus concentration in the Upper Kissimmee Chain-of-lakes indicates that increases in total phosphorus concentrations during the wet season is not created by precipitation runoff alone, as is commonly believed.
The results of the current study indicate that natural ecological cycles influence total phosphorus patterns in the Greater Everglades headwater lakes and that these cycles need to be taken into consideration when developing restoration evaluation criteria and ecosystem health indicators for the Everglades.
Nellie E. Morales, South Florida Water Management District, 3301 Gun Club Road, West Palm Beach, Florida. 33406, Phone: 561-682-2180, Fax: 561-682-5136,