Using structural equation modeling and expert elicitation to select nutrient criteria variables for south-central Florida lakes

To protect the nation's waterbodies from excessive impairments from pollution leading to eutrophication, the Clean Water Act requires states to establish water quality standards. These water quality standards are designed to protect the designated use, or water quality goal; however, they are indirectly measured and assessed using a water quality criterion. An alternative approach to develop nutrient criteria is the predictive approach (Reckhow et al. 2005), which determines the predictive variables by combining water quality data with assessments from multiple experts on the probability of designated use attainment using structural equation modeling (SEM). Our objective was to expand the predictive approach to include a region of waterbodies and to use multiple experts. To demonstrate these extensions, the approach was applied to lakes in south-central Florida using four experts to quantify attainment of a fish and wildlife designated use. Multiple models were built that related eutrophication processes to the designated use. Of the two plausible models, total phosphorus was the most predictive of the designated use followed by chlorophyll a. Using the model results, the risk of nonattainment of the designated use for these two predictive variables was calculated; to achieve high attainment (90% or more), total phosphorus should be <0.015 mg/L and chlorophyll a <5 μg/L. This study provides vital extensions to the previous approach through its use of multiple experts and a region of lakes, making the approach applicable to other regions of waterbodies and conclusions useful to inform policy.