Relating Algal Bloom Frequencies to Phosphorus Concentrations in Lake Okeechobee

ABSTRACT

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 stations, 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 μg L⁻¹ chlorophyll a. For bloom criteria in the range of 40 to 60 μg L⁻¹ chlorophyll a, blooms were not detected when phosphorus was less than 30 μg L⁻¹, 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 die north littoral, and 48% of the time in die south littoral. When phosphorus concentrations were between 30 and 60 μg L⁻¹ in die 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 μg L⁻¹. This condition was observed 70% of die time in the open lake, 29% of the time in the north littoral, and 15% of the time in die south littoral. When phosphorus concentrations exceeded 60 μg L⁻¹, risk of a 40 μg L⁻¹ bloom was 19% in the open lake, 28% in the north littoral, and 60% in the south littoral. Regional variations in die relationship between phosphorus and bloom frequency may be due to variations in nitrogen availability. Highest total nitrogen concentrations were found in the south littoral zone, where bloom frequencies at a given phosphorus concentration were also highest. Reductions in lake water phosphorus concentrations can be expected to cause reductions in bloom frequencies in die littoral zones, but have little impact on die open lake. This result has positive implications for lake users, because die more responsive littoral region is also the location of die most frequent algal blooms.

Key Words:

• algal blooms
• chlorophyll a
• phosphorus
• algae
• eutrophication management