Temporal Water Chemistry Trends (1967–1997) for a Sample (127) of Florida Waterbodies

Total phosphorus, total nitrogen, total chlorophyll and Secchi visibility data from 127 Florida lakes across three time periods, Early (1967–1972), Middle (1979–1981), and Present (1996–1997) were compared using paired t tests and repeated measures analyses of variances. Although the population of Florida has increased 116% over the last 27 years, expected increases in lake concentrations of nutrients were not found. For this sample of lakes, total phosphorus concentrations decreased over a 30-year time period and total nitrogen concentrations showed no significant change. Secchi visibility, one of the simplest parameters to measure and easily understood by lake-users, also showed no significant change over time. The sample of lakes showed a significant but small increase in total chlorophyll concentrations. The increase was unexpected because total phosphorus concentrations decreased and total nitrogen concentrations stayed the same, suggesting that other factors were impacting total chlorophyll concentrations in this group of lakes. It is speculated that changes in color, fluctuating water levels, and increases in the amount of aquatic plant control over time may help explain some of the increases in total chlorophyll. Independent estimates of normal month-to-month and year-to-year temporal variance for Florida lakes were calculated using long-term Florida LAKEWATCH data (71 lakes, with greater than 4 years of monthly data) and compared to the variance among the Early, Middle, and Present time periods. Most of the differences detected among the time periods using statistical analyses were within calculated normal monthly temporal variance. Thus, increased nutrient concentrations or decreased water clarity, that is often speculated to occur with population growth and watershed development, have not been documented in this sample of Florida lakes.