Algal blooms, defined as chlorophyll a concentrations greater than 40 micrograms l-1, are common in Lake Okeechobee, Florida. Using logistic regression techniques, we have developed equations that relate limnological variables to algal bloom occurrence in four distinct open-water regions of this large shallow lake: central pelagic, northwest, southwest, and a transition region between the western and pelagic regions. Wind velocity and total phosphorus, which are closely related to resuspended material in the central region, are negatively related to algal bloom occurrence there. In the transition region, algal bloom occurrence is positively related to total nitrogen and wind velocity. Algal bloom occurrence is strongly and positively related to total nitrogen and total phosphorus concentrations in the western regions. The logistic regression model predicts an algal bloom probability greater than 95 percent in the northwest region when total phosphorus exceeds 0.10 mg l-1 and total nitrogen exceeds 2.5 mg l-1. In the southwest region the model predicts algal bloom probability of 100 percent when total phosphorus exceeds 0.10 mg l-1 and total nitrogen exceeds 2.8 mg l-1. Given 1994 mean total phosphorus concentrations of 0.05 and 0.04 mg 1-1 in the northwest and southwest regions, respectively, total nitrogen would have to remain below 1.32 and 1.43 mg l-1, respectively, to keep the algal bloom probability below 10 percent. Because the lake is heterogenous, such nutrient standards should be considered on an in-lake regional basis for Lake Okeechobee.
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