Relationships Between Total Phosphorus Concentrations, Sampling Frequency, and Wind Velocity in a Shallow, Polymictic Lake

The effect of sampling frequency and wind velocity on estimates of total phosphorus (TP) concentrations were evaluated in Lake Okeechobee, Florida, a shallow, polymictic lake. It was hypothesized that estimates of TP would be affected by sampling frequency because the lake's P-rich bottom sediments are frequently resuspended during wind events. However, differences in estimates of annual mean TP were small (< 5 %) when daily, weekly, bi-monthly, and monthly sampling frequencies were compared. In contrast, estimates of monthly mean TP often varied by 20 to 30 percent among sampling frequencies and differences in monthly maximum and minimum TP values commonly varied by 50 to 250 μg·L−1. Bottom sediments were most likely to be resuspended during the windy season (November-April) when calculated threshold wind velocities were exceeded more than 40 percent of the time. On a monthly basis, wind velocity explained as much as 70 percent of the daily variability in TP during the windy season. In comparison, threshold velocities were exceeded less frequently during the calm season (May – October) and wind velocity only accounted for 5 to 25 percent of the daily variability in TP. The effects of wind-driven mixing events on the chemical and physical attributes of shallow lakes and the influence of sampling frequency on estimates of these attributes should be considered when designing a monitoring program to evaluate short and long-term trends in water quality.