Author(s): William W Walker Jr, Karl E Havens
Empirical relationships were developed between algal bloom frequencies and total phosphorus concentrations for three distinct regions of Lake Okeechobee, and hypotheses were derived to explain observed spatial variation In those relationships. The analyses were based on water quality monitoring data collected monthly between 1986 and 1993, at 10 open-water lltatlons, 12 north littoral stations, and 14 south littoral stations. Using a cross-tabulation procedure, phosphorus/bloom-frequency relationships were developed for bloom criteria of 10, 20, 30, 40,50, and 60 ~g VI chlorophyll a. For bloom criteria In the range of 40 to 60~g Uchlorophyll a, blooms were not detected when phosphorus was le~ than SO ~g L·I. During the 1986-1993 monitoring period, this threshold phosphorus concentration was exceeded 3% of the time In the open lake, 25% of the time In the north littoral, and 48% of the time In the south littoral. When phosphorus concentrations were between Wand 60 I1g L"iln the littoral regions, frequency or risk of an algal bloom increased with phosphorus concentration. The maximum risk of an algal bloom generally occurred when phosphorus exceeded 60).lg L·I. This condition was observed 70% oCthe time In the open lake, 29% of the time in the north littoral, and 15% of the time In the south littoral. When phosphorus concenlr,ltlolls exceeded 60 ~g VI, risk ora 10 I1g VI bloom was 19% in the open lake, 28% in the north littoral, and 60% in the south littoral. Regional variations In the relationship between phosphorus and bloom frequency may be due to variations in nitrogen availability. Highest total nitrogen concentrations were found In the south littoral lone, where bloom frequenclesatagiven phosphorus concentration were al.o hlghe.t. Reduction. In lake water phosphorus concentrations can be expected to cause reductions In bloom frequencies In the littoral lones, but have little Impact on the open lake. This result has positive Irnpllcatlonsfor lake users, because the more responsive littoral region Is also the location of the most frequent algal blooms.