Revisiting the total maximum daily load total phosphorus goal in Lake Okeechobee

Abstract

In 2000, a total maximum daily load (TMDL) total phosphorus (TP) goal of 40 µg/L was established for Lake Okeechobee, Florida. The goal was selected to reduce the “imbalance to flora and fauna” caused by excessive phosphorus loading in the lake. We recalculate the TMDL TP goal using a water quality data series of TP and chlorophyll-a concentrations from 1986 to 2018 for the original 30 in-lake stations plus an additional 29 stations. Using the cross-tabulation approach used to generate the original TMDL TP goal, we determined a new goal of 42 µg/L, nominally different from the original. We also reevaluate the goal’s ability to maintain an implicit goal of 11.5% bloom frequency. We conclude substantial changes in the frequency and scope of the water quality sampling scheme prevent a determination on the effectiveness of the TMDL TP goal. The implementation, however, appears to have failed as the median TP concentration has increased by 93 µg/L to 133 µg/L and bloom frequency, after correcting for the declining sampling frequency, has increased from 1986 to 2018. An increased sampling frequency of TP and chlorophyll-a sampling is needed, or else tracking responses of chlorophyll-a to future TP reductions will be virtually impossible.