Phosphorus-algal bloom relationships in large lakes of South Florida: Implications for establishing nutrient criteria

A simple cross-tabulation procedure was used to examine the relationship between total phosphorus (TP) concentration and algal bloom frequencies, based on chlorophyll a (Chl a), in seven large south Florida lakes. Monthly data from 1995-2000 were sorted by lake and TP concentration. For each take, the sorted data were subdivided into five intervals of equal sample size, and the frequency of occurrence of Chl a >20,40, and 60 mug (.) L-1 was determined and plotted against the median TP in each interval. "Critical" TP concentrations, at which there was a rapid increase in bloom frequency, were examined by visual inspection of these plots. There was considerable variation among lakes in regard to relationship of bloom frequency to TP. For Chl a >20 mug (.) L-1, a rapid rise in frequency occurred at TP ranging from below 30 to greater than 40 mug (.) L-1. For Chl a >40 mug (.) L-1, a rapid rise in frequency began at TP concentrations ranging from 40 to above 70 mug (.) L-1. The variation between lakes was not related to lake water total or dissolved N:P, but it was strongly related to color. Color explained 86% of the variation between takes in terms of critical TP for increased frequency of Chl a >40 mug (.) L-1, which is defined as an algal bloom by the State of Florida. Color, which reduces light penetration, may be particularly important in these lakes because they typically have homogeneous water columns that do not allow algae to migrate to and sustain high biomass near the surface, where there is adequate light for net growth. The influence of color is probably less in lakes with stable thermal stratification. The simple method used to evaluate the seven lakes could be generally applied, making use of routine water quality monitoring data, as long as one could specify an appropriate Chl a concentration and frequency for protecting the major use of particular water resources.