A strategy for establishing numeric nutrient criteria for Florida lakes

Abstract

We used our knowledge of the factors that determine the nutrient concentrations in Florida lakes to develop a strategy for establishing numeric nutrient criteria for lakes in the State of Florida. Based on previous findings that natural factors were most important in determining the current nutrient concentrations in Florida lakes, we used current distributions of nutrients as a basis for setting criteria. We started with the US Environmental Protection Agency's Florida Lake Regions and grouped similar regions into 6 total phosphorus (TP) zones and 5 total nitrogen (TN) zones that were used to set numeric nutrient criteria for each zone. We propose criteria that will identify the lakes in each zone with the highest concentrations of TP and TN for subsequent investigation to determine the degree anthropogenic sources or natural backgrounds are responsible for their trophic status. To provide special protection to Florida's oligotrophic lakes, site-specific criteria are proposed, with oligotrophic lakes defined as those lakes that have (1) a long-term average of not more than 6 μg/L of chlorophyll and (2) less than 30% coverage by surface area of submersed aquatic macrophytes. The site-specific criteria for TP and TN will be the long-term (7 year) average concentrations. These proposed criteria are protective of the designated uses of Florida lakes and prevent the misclassification of many naturally eutrophic lakes as impaired when they are not undergoing cultural eutrophication. The proposed numeric standards can be implemented immediately in the State of Florida.