ALGAL BLOOM PROBABILITY IN A LARGE SUBTROPICAL LAKE

Algal blooms, defined as chlorophyll a concentrations greater than 40 mu g l(-1), are common in Lake Okeechobee, Florida. Using logistic reg ression techniques, we have developed equations that relate limnologic al variables to algal bloom occurrence in four distinct open-water reg ions of this large shallow lake: central pelagic, northwest, southwest , and a transition region between the western and pelagic regions. Win d velocity and total phosphorus, which are closely related to resuspen ded material in the central region, are negatively related to algal bl oom occurrence there. In the transition region, algal bloom occurrence is positively related to total nitrogen and wind velocity. Algal bloo m occurrence is strongly and positively related to total nitrogen and total phosphorus concentrations in the western regions. The logistic r egression model predicts an algal bloom probability greater than 95 pe rcent 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 t he model predicts algal bloom probability of 100 percent when total ph osphorus 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 l(-1) in the northwest and southwest regions, respectively, total nitr ogen 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 i n-lake regional basis for Lake Okeechobee.